The Atom and Radioactive Decay: a full term of science
Ten ready-to-teach lessons for Year 9 Chemical sciences. Print this pack and the term is prepared: every lesson comes with a step-by-step plan, the questions to ask, a student worksheet, cut-out cards or exit tickets, an assessment kit and worked answers.
Start here: five minutes to Monday
- Skim the term at a glance on the next page.
- Print the lesson you need. Each lesson is three A4 sheets: plan, worksheet, and cards or tickets.
- Gather the few items under “You need” on the plan. Most come from the science prep room or the supermarket.
- Open the free interactive unit on your board or projector. Every plan tells you which picture to show and when.
- Teach straight from the plan. Timings, talk prompts, misconceptions and worked answers are all on the one page.
For any Year 9 science teacher
The pack suits a chemistry or physics specialist and a teacher working outside their own subject alike: each plan explains the idea in plain words, lists the misconceptions students bring, and gives worked answers. The big idea of the term: the model of the atom was built and rebuilt from evidence, from a solid ball to a tiny nucleus of protons and neutrons with electrons around it. Some nuclei are unstable; on their own they give out alpha, beta or gamma radiation and change, step by step, until they reach a stable atom. No one can say when one nucleus will decay, but half of a large sample decays in a fixed time, its half-life.
Two ways to run each lesson
Every lesson works as one 60-minute block, or as two 30-minute sessions; the split point is marked in every plan. Look ahead: Lesson 1 needs sealed mystery boxes made up beforehand, Lesson 3 needs a board on blocks with a hidden block under it, and Lesson 6 needs 100 dice for each group.
Safety and care in one look
No radioactive sources are used: decay is modelled with dice and studied through data. Smoke alarms are shown sealed and never opened. Balloons can be swapped for a plastic ruler for anyone with a latex allergy. Marbles stay on the bench or in a tray, so nobody slips on them.
The term at a glance
One lesson a week for a term. Each lesson stands on the ones before it, so run them in order where you can.
| # | Lesson | Students learn and do | You need (in short) |
|---|---|---|---|
| 1 | Models from hidden evidence | Work out what is inside sealed boxes without opening them, and put the atom’s story in order | Sealed mystery boxes |
| 2 | Thomson and the electron | Use static charge to show that atoms hold movable negative particles, and draw the plum pudding model | Balloons, a woollen cloth, scraps of paper |
| 3 | The gold-foil idea | Roll marbles at a hidden block to estimate its size, and connect it to the tiny nucleus | Marbles, a board, wooden blocks |
| 4 | Inside the atom | Count the protons, neutrons and electrons of eight atoms, and meet isotopes | The worksheet, a periodic table |
| 5 | Alpha, beta and gamma | Sort twelve facts about the three kinds of radiation from unstable nuclei | The radiation cards |
| 6 | Half-life with dice | Model radioactive decay by rolling 100 dice, and read a half-life from the graph | 100 dice for each group |
| 7 | Half-lives in the real world | Use the half-lives of five isotopes to work out what is left, and how old something is | The worksheet |
| 8 | The road to stable | Follow a natural decay chain step by step from radon-222 to stable lead-206 | The worksheet, a periodic table |
| 9 | Radiation at work | Match five uses of radiation to the kind used, and explain each choice | The worksheet |
| 10 | Show what we know | Date an old campfire and tell the atom’s story, sit the final check and play the class quiz | The task and check, the interactive unit |
How the sequence builds
Lessons 1 to 3 follow how the model of the atom was built from evidence: indirect clues, the electron and the gold-foil experiment. Lesson 4 counts the particles inside atoms and meets isotopes. Lessons 5 to 7 turn to unstable nuclei: the three kinds of radiation, half-life modelled with dice, and real half-lives. Lesson 8 follows a natural decay chain to a stable atom, Lesson 9 meets radiation at work, and Lesson 10 brings it together.
Curriculum links (Australian Curriculum V9)
The whole term teaches the Science Understanding descriptor AC9S9U06 quoted on the cover. The lessons also work these Science Inquiry and Human Endeavour descriptors:
Assessment in this pack
- Every plan ends with “Answers and look-fors”: what meeting the idea sounds like in a Year 9 voice, with working.
- The assessment sheet near the front has a class observation checklist and a three-level rubric.
- Lesson 10 is the summative pair: the old campfire task, plus the “Show what we know” check.
Materials for the whole term
One gathering session covers all ten lessons, and most of it is already in the science prep room or a games cupboard. Three things are worth doing ahead: make up the sealed mystery boxes for Lesson 1, set up the board and hidden block for Lesson 3, and collect 100 dice for each group for Lesson 6.
| Lesson | You need |
|---|---|
| 1 | six sealed mystery boxes (shoeboxes taped shut, each with a different object or a card wall inside), one for each group |
| 2 | for each pair: a balloon (or a plastic ruler), a woollen cloth or jumper, and small scraps of paper |
| 3 | for each group: a board about 50 cm wide resting on four blocks, a wooden block hidden under it, a tray to catch marbles, and 10 marbles |
| 4 | the worksheet and a periodic table for each pair |
| 5 | the radiation cards from this pack, one set per pair; scissors |
| 6 | for each group: 100 dice (or 100 small cubes with one face marked), a tray and a container |
| 7 | the worksheet only |
| 8 | the worksheet and a periodic table for each pair |
| 9 | the worksheet; a sealed smoke alarm to show, never opened |
| 10 | the task and the final check; the interactive unit open for the class quiz |
The one-trip list
- From the prep room: periodic tables, trays and containers.
- From a games cupboard or a maths room: dice, about 100 for each group, and marbles.
- From home or a cupboard: shoeboxes, tape, balloons, a woollen cloth, wooden blocks and a board about 50 cm wide.
- To show in Lesson 9: a sealed smoke alarm.
Safety and care in one look
- No radioactive sources are used; decay is modelled with dice and studied through data.
- A smoke alarm is shown sealed and never opened.
- A plastic ruler replaces a balloon for anyone with a latex allergy.
- Marbles stay in trays, so nobody slips on them.
Assessment without extra work
The term assesses itself. Every lesson plan ends with worked answers and look-fors, and Lesson 10 is the summative pair: the task, in which students date an old campfire with carbon-14 and explain the atom’s story, and the final check. This sheet is the place to note what you see along the way.
The three levels
| Idea | Working towards | At standard | Beyond |
|---|---|---|---|
| Models of the atom | names one model of the atom | describes how the model changed with the discovery of electrons, the nucleus, protons and neutrons | explains how a piece of evidence forced a model to change, and why models are still revised |
| Particles in the atom | names protons, neutrons and electrons | works out the particles in an atom from its atomic and mass numbers, and explains isotopes | links the stability of a nucleus to its balance of protons and neutrons |
| Radioactive decay | says some atoms give out radiation | describes alpha, beta and gamma radiation and what each does to the nucleus | follows a decay chain step by step to a stable atom |
| Half-life | says the amount goes down | reads a half-life from a graph and calculates the amount left | explains why one nucleus is unpredictable but a large sample is not |
| Working scientifically | records results | uses a model, such as dice, to collect and graph data | evaluates what a model shows and where it falls short |
Class observation checklist
| Name | Models | Particles | Decay | Half-life | Investigating |
|---|---|---|---|---|---|
A tick a lesson is plenty; the Lesson 10 task and check fill the gaps.
Word wall cards
Cut out the cards and build the wall as the words arrive: model and atom in Lesson 1; electron in Lesson 2; nucleus in Lesson 3; proton, neutron, atomic number, mass number and isotope in Lesson 4; unstable nucleus, radioactive decay, alpha particle, beta particle and gamma ray in Lesson 5; half-life in Lesson 6; background radiation in Lesson 7; decay chain and stable in Lesson 8.
a simplified picture that explains and predicts, and changes with new evidence
the smallest particle of an element
a tiny negative particle outside the nucleus
the tiny, dense centre of an atom, holding its protons and neutrons
a positive particle in the nucleus
a particle with no charge in the nucleus
the number of protons in an atom: it decides the element
the number of protons plus neutrons
atoms of one element with different numbers of neutrons
a nucleus that will, in time, give out radiation and change
an unstable nucleus giving out radiation and changing
two protons and two neutrons thrown out of a nucleus
a fast electron thrown out when a neutron becomes a proton
a high-energy electromagnetic wave given out by a nucleus
the time for half the unstable nuclei in a sample to decay
a series of decays that ends with a stable atom
a nucleus that does not decay
the low level of radiation around us all the time, from rocks, air and space
Dear families
This term in science, Year 9 studies the atom: how scientists worked out what is inside it, from a solid ball to a tiny nucleus of protons and neutrons with electrons around it, and how some atoms are unstable and change over time by natural radioactive decay until they become stable.
Along the way your student will work out what is inside sealed boxes without opening them, roll marbles at a hidden block, count the particles in atoms, roll 100 dice to model half-life, and follow a natural decay chain to stable lead. No radioactive materials are used in class.
Try this at home
- Rub a balloon on your hair and hold it near small scraps of paper. What has moved from your hair to the balloon?
- Toss 20 coins and remove the heads; toss the rest and repeat. How many tosses until about half are gone?
- Look for a smoke alarm at home (without opening it). What does its label say is inside?
- Ask an older relative what they remember being taught about atoms. How has the picture changed?
What to ask your scientist
- Why did the model of the atom keep changing?
- What is inside the nucleus of an atom?
- What is a half-life?
- How does an unstable atom end up stable?
A small note on care: no radioactive materials are used in class; decay is modelled with dice and studied through data.
Warm regards,
The Year 9 science team
Printed from the free seegongsik The Atom and Radioactive Decay teaching pack · seegongsik.com/au/y9/chemical/AC9S9U06/pack
Models from hidden evidence
Nobody has ever looked inside an atom the way we look inside a box. Scientists built their picture of the atom from indirect evidence: what happened when atoms were heated, charged or hit with tiny particles. Each picture was a model, the best explanation of the evidence so far, and each was changed or replaced when new evidence did not fit. Students practise the same thinking with sealed boxes: they tilt, shake and listen, draw a model of what is inside, and change it as the evidence grows. Then they put the story of the atomic model in order.
We are learning to
- build a model from indirect evidence, and change it when new evidence arrives,
- describe a scientific model as the best current explanation, not the final truth,
- put the main steps in the story of the atom in order.
Success criteria
- I can draw a model of what is inside a sealed box and give the evidence for each part.
- I can put eight events in the story of the atom in order.
You need
- six sealed mystery boxes (shoeboxes taped shut, each with a different object or a card wall inside), one for each group,
- the worksheet (next sheet), one per student, and the exit tickets (third sheet).
Lesson flow (about 60 minutes)
| 5 min | Too small to see Ask: “An atom is far too small to see, even with the best light microscope. How could anyone know what is inside one?” |
| 25 min | Mystery boxes Each group investigates a sealed box for five minutes, without opening it, and draws a first model in Part A. Boxes then move on once, and the new group improves the model with fresh evidence. Ask: “What evidence made you change your model? Could you ever be sure?” |
| 10 min | On the board Show the board (see the box). Add model and atom to the word wall. |
| 20 min | The atom’s story Students number the eight events in Part B and answer Part C. The boxes stay sealed: the point is the evidence, not the answer. |
Running two half-sessions instead? Do the opening and the boxes in Session A, and the board, the story and Part C in Session B.
Watch for these ideas
- “A model that was replaced was a mistake.” Each model explained the evidence of its time, and the next one built on it.
- “Scientists have seen inside atoms.” The structure of the atom was worked out from indirect evidence.
- “Today’s model is final.” It is the best so far; it can still be refined.
Make it easier, make it bigger
- Easier: give each group a list of possible objects to test their box against.
- Bigger: scientists publish their results so that others can test them. How did that help the model of the atom improve?
Answers and look-fors
- Part A: any model that matches the evidence, with each part backed by a test such as a sound, a roll or a stop.
- Part B, in the printed order: 6, 3, 8, 1, 4, 7, 2, 5. In time order: 1803, 1897, 1904, 1909, 1911, 1913, 1919, 1932.
- Part C: 1. We could not see inside, so we used clues: sounds, movement and weight. 2. When the evidence did not fit. 3. The electron: atoms were not solid, unbreakable balls after all. 4. A model is the best explanation of the evidence so far, and is changed when the evidence does not fit.
- Bigger: others repeated and checked the experiments, so mistakes were caught and good results were trusted and built on.
- Exit tickets: 1. A simplified picture that explains the evidence and can be changed. 2. New evidence that does not fit. 3. Dalton’s solid sphere.
Models from hidden evidence
A. Our mystery box
| What we did | What we noticed |
|---|---|
| Tilted it slowly | |
| Shook it gently | |
| Turned it over | |
| Weighed it in our hands |
B. The story of the atom
Number the events from 1 (the earliest) to 8 (the latest).
| Bohr places the electrons in fixed energy shells around the nucleus | |
| Thomson suggests the plum pudding model: electrons dotted through a positive atom | |
| Chadwick discovers the neutron, the neutral particle in the nucleus | |
| Dalton pictures each element as made of tiny solid atoms that cannot be split | |
| Geiger and Marsden fire alpha particles at gold foil; a few bounce back | |
| Rutherford identifies the proton in the nucleus | |
| Thomson discovers the electron, a tiny negative particle inside atoms | |
| Rutherford explains the result with a tiny, dense, positive nucleus |
C. Questions
Exit tickets
Cut out one ticket for each student.
1. What is a scientific model?
2. What makes scientists change a model?
3. Which was the earliest model of the atom?
1. What is a scientific model?
2. What makes scientists change a model?
3. Which was the earliest model of the atom?
1. What is a scientific model?
2. What makes scientists change a model?
3. Which was the earliest model of the atom?
1. What is a scientific model?
2. What makes scientists change a model?
3. Which was the earliest model of the atom?
Thomson and the electron
In 1897 J. J. Thomson showed that the glowing rays in a tube with almost all its air removed were streams of tiny particles with a negative charge, about 2,000 times lighter than a hydrogen atom, the lightest atom. They came out of atoms of every kind of metal he tried, so atoms were not solid, unbreakable balls after all: they had smaller parts inside. Atoms are neutral, so Thomson pictured the negative electrons dotted through a spread-out positive ball, the plum pudding model. Rubbing a balloon moves some electrons from one material to another, which shows in a simple way that atoms contain negative particles that can be moved.
We are learning to
- describe the discovery of the electron and how it changed the model of the atom,
- explain static charge as electrons moving from one material to another,
- draw and explain the plum pudding model.
Success criteria
- I can explain what happened to the balloon when it was rubbed, using electrons.
- I can draw the plum pudding model and say what evidence it explained.
You need
- for each pair: a balloon (or a plastic ruler), a woollen cloth or jumper, and small scraps of paper; a tap for the water test,
- the worksheet (next sheet), one per student, and the exit tickets (third sheet).
Care: offer a plastic ruler to anyone with a latex allergy; keep the water test well away from power points.
Lesson flow (about 60 minutes)
| 5 min | A shock from a door handle Ask: “After walking across carpet, you sometimes get a small shock from a metal handle. What is jumping?” |
| 20 min | Moving electrons Pairs try the four tests in Part A and record what happens. Ask: “The unrubbed balloon did nothing. What did rubbing change?” |
| 10 min | On the board Show the board (see the box). Add electron to the word wall. |
| 25 min | The plum pudding Students draw the plum pudding model in Part B and answer Part C. |
Running two half-sessions instead? Do the opening and the tests in Session A, and the board, the drawing and Part C in Session B.
Watch for these ideas
- “Rubbing makes new charge.” Rubbing only moves electrons from one material to the other.
- “Positive charges move to the balloon.” In solids it is the electrons that move; the positive charges stay put.
- “Thomson saw electrons.” He measured how the rays bent and worked out that they were tiny charged particles.
Make it easier, make it bigger
- Easier: do the paper test only, and draw the model with six electrons given.
- Bigger: Thomson found the same electrons in every metal he tested. Why was that such strong evidence that electrons are part of all atoms?
Answers and look-fors
- Part A: the rubbed balloon picks up paper, bends the water and sticks to the wall; the unrubbed balloon does nothing.
- Part B: a positive sphere with negative electrons dotted through it, the charges balancing so that the atom is neutral.
- Part C: 1. Electrons moved from the wool onto the balloon, giving it a negative charge. 2. Atoms contain tiny negative particles that can be moved. 3. Atoms are neutral, so the negative electrons must be balanced by positive charge. 4. It showed that atoms have parts, so they are not solid, unbreakable balls.
- Bigger: if every metal gives out identical particles, those particles must be a part shared by all atoms, not something special to one element.
- Exit tickets: 1. The electron. 2. Negative. 3. Positive charge spread through the atom, with electrons dotted through it.
Thomson and the electron
A. Moving electrons
| Test | What happened |
|---|---|
| Rub the balloon on wool, then hold it near scraps of paper | |
| Hold the rubbed balloon near a thin stream of tap water | |
| Hold the rubbed balloon against a wall | |
| Hold an unrubbed balloon near the paper |
B. The plum pudding model
Draw an atom with six electrons as Thomson pictured it. Label the electrons and the positive part, and show the charges.
C. Questions
Exit tickets
Cut out one ticket for each student.
1. Which particle did Thomson discover?
2. What charge does it carry?
3. Describe the plum pudding model in one sentence.
1. Which particle did Thomson discover?
2. What charge does it carry?
3. Describe the plum pudding model in one sentence.
1. Which particle did Thomson discover?
2. What charge does it carry?
3. Describe the plum pudding model in one sentence.
1. Which particle did Thomson discover?
2. What charge does it carry?
3. Describe the plum pudding model in one sentence.
The gold-foil idea
In 1909 Hans Geiger and Ernest Marsden, working with Ernest Rutherford, fired alpha particles at gold foil only about a thousand atoms thick. Almost all went straight through, but about 1 in 8,000 bounced back. Rutherford later called it the most incredible result of his life. If the positive charge were spread through the atom, as in the plum pudding, nothing could turn them back. So all the positive charge and nearly all the mass must sit in a tiny, dense nucleus, with the rest of the atom mostly empty space. Students meet the same kind of reasoning: they roll marbles under a board at a hidden block, and estimate its size from how often a marble is turned aside.
We are learning to
- describe the gold-foil experiment and what its result showed,
- use the share of hits to estimate the size of something hidden,
- explain why the nuclear model replaced the plum pudding model.
Success criteria
- I can estimate the width of the hidden block from our rolls.
- I can explain why a few particles bouncing back showed a tiny, dense nucleus.
You need
- for each group: a board about 50 cm wide resting on four blocks, a wooden block hidden under it, a tray to catch marbles, and 10 marbles,
- the worksheet (next sheet), one per student, and the exit tickets (third sheet).
Care: roll marbles gently and catch them in the tray, so that nobody slips on them.
Lesson flow (about 60 minutes)
| 5 min | A surprise Ask: “You throw tennis balls at a paper wall, and one in thousands bounces straight back. What must be behind the paper?” |
| 25 min | The hidden block Without looking under the board, groups roll marbles straight under it, spread evenly across its width, and count how many come out turned aside. They roll 100 times in all and record in Part A. Ask: “Why must the rolls be spread evenly across the whole board?” |
| 10 min | On the board Show the board (see the box). Add nucleus to the word wall. |
| 20 min | From marbles to atoms Students estimate the block’s width in Part B, then lift the board to check, and answer Part C. |
Running two half-sessions instead? Do the opening and the rolls in Session A, and the board, the estimate and Part C in Session B.
Watch for these ideas
- “Most particles bounced off the gold.” Almost all went straight through; only a very few bounced back.
- “The nucleus fills most of the atom.” The nucleus is tiny: the atom is about 100,000 times as wide.
- “Electrons turned the alpha particles back.” Electrons are far too light; only the heavy, positive nucleus could.
Make it easier, make it bigger
- Easier: use the example counts for the estimate, after a few rolls to see how it works.
- Bigger: if a nucleus were the size of a 1 cm marble, how wide would the whole atom be? Where in your town would its edge reach?
Answers and look-fors
- Part A, example: 12 of 100 rolls turned aside, a share of 12%.
- Part B: 12% of the 50 cm board = 6 cm; take away one marble width (1.5 cm), because a marble hits when its edge reaches the block: about 4.5 cm.
- Part C: 1. Most of the atom is empty space. 2. A tiny, dense, positive centre: the nucleus. 3. Spread-out positive charge could not turn a fast alpha particle back. 4. Like the gold foil, most marbles missed and a few were turned aside; the share of hits showed how small the target was.
- Bigger: about 100,000 cm, which is 1 km across.
- Exit tickets: 1. Most passed straight through. 2. A tiny, dense, positive nucleus. 3. Rutherford (with Geiger and Marsden).
The gold-foil idea
A. Rolling at a hidden block
| Count | Example group | Our group |
|---|---|---|
| Rolls | 100 | |
| Turned aside (hits) | 12 | |
| Went straight through | 88 |
B. How wide is the block?
The board is 50 cm wide and a marble is 1.5 cm wide. Width hit = share of hits × board width. Block width ≈ width hit − one marble width.
C. Questions
Exit tickets
Cut out one ticket for each student.
1. What happened to most of the alpha particles fired at the gold foil?
2. What did the few that bounced back show?
3. Whose team did the gold-foil experiment?
1. What happened to most of the alpha particles fired at the gold foil?
2. What did the few that bounced back show?
3. Whose team did the gold-foil experiment?
1. What happened to most of the alpha particles fired at the gold foil?
2. What did the few that bounced back show?
3. Whose team did the gold-foil experiment?
1. What happened to most of the alpha particles fired at the gold foil?
2. What did the few that bounced back show?
3. Whose team did the gold-foil experiment?
Inside the atom
After the nucleus came its parts. Rutherford identified the positive proton in 1919, and in 1932 James Chadwick discovered the neutron, a particle with no charge and about the same mass as a proton, which explained why atoms were heavier than their protons alone. The number of protons, the atomic number, decides the element: every carbon atom has 6. The mass number counts the protons and neutrons together. In a neutral atom the electrons equal the protons. Atoms of one element can have different numbers of neutrons: these are isotopes. Students work out the particles in eight atoms and three isotopes of carbon.
We are learning to
- describe the charge, mass and place of protons, neutrons and electrons,
- work out the particles in an atom from its atomic number and mass number,
- explain what isotopes are.
Success criteria
- I can complete the table for all eight atoms.
- I can explain how the three isotopes of carbon differ, and what they share.
You need
- a periodic table for each pair,
- the worksheet (next sheet), one per student, and the exit tickets (third sheet).
Lesson flow (about 60 minutes)
| 5 min | What makes gold gold? Ask: “Every atom has protons, neutrons and electrons. So what makes an atom of gold different from an atom of iron?” |
| 10 min | On the board Show the board (see the box). Add proton, neutron, atomic number, mass number and isotope to the word wall. |
| 25 min | Counting particles Work hydrogen and carbon together, then students complete Part A with the periodic table. Ask: “How do you get the neutrons from the two numbers?” |
| 20 min | Isotopes Students complete Part B and answer Part C. |
Running two half-sessions instead? Do the opening, the board and Part A in Session A, and Parts B and C in Session B.
Watch for these ideas
- “The mass number is the number of neutrons.” It is protons plus neutrons.
- “Isotopes are different elements.” They are the same element with different numbers of neutrons.
- “Electrons add a lot of mass.” An electron is about 2,000 times lighter than a proton; nearly all the mass is in the nucleus.
Make it easier, make it bigger
- Easier: complete the first four atoms only, with the rule written at the top: neutrons = mass number − atomic number.
- Bigger: uranium has 92 protons. Its most common isotope has a mass number of 238. How many neutrons does it have, and how does that compare with its protons?
Answers and look-fors
- Part A: hydrogen 1, 0, 1; helium 2, 2, 2; carbon 6, 6, 6; oxygen 8, 8, 8; sodium 11, 12, 11; aluminium 13, 14, 13; iron 26, 30, 26; gold 79, 118, 79 (protons, neutrons, electrons).
- Part B: carbon-12: 6 protons, 6 neutrons, 6 electrons; carbon-13: 6 protons, 7 neutrons, 6 electrons; carbon-14: 6 protons, 8 neutrons, 6 electrons.
- Part C: 1. The number of protons: carbon always has 6. 2. The number of neutrons, and so the mass number. 3. Protons and neutrons in the nucleus; electrons in shells around it. 4. The protons and neutrons; the electrons add almost nothing.
- Bigger: 238 − 92 = 146 neutrons, far more neutrons than protons; heavy nuclei like this are often unstable.
- Exit tickets: 1. 11 protons. 2. 12 neutrons. 3. The same element, with different numbers of neutrons.
Inside the atom
A. Counting particles
Protons = atomic number. Neutrons = mass number − atomic number. In a neutral atom, electrons = protons.
| Atom | Atomic number | Mass number | Protons | Neutrons | Electrons |
|---|---|---|---|---|---|
| Hydrogen (H) | 1 | 1 | |||
| Helium (He) | 2 | 4 | |||
| Carbon (C) | 6 | 12 | |||
| Oxygen (O) | 8 | 16 | |||
| Sodium (Na) | 11 | 23 | |||
| Aluminium (Al) | 13 | 27 | |||
| Iron (Fe) | 26 | 56 | |||
| Gold (Au) | 79 | 197 |
B. Three isotopes of carbon
| Isotope | Protons | Neutrons | Electrons |
|---|---|---|---|
| Carbon-12 | |||
| Carbon-13 | |||
| Carbon-14 |
C. Questions
Exit tickets
Cut out one ticket for each student.
1. Sodium has atomic number 11. How many protons does a sodium atom have?
2. Sodium’s mass number is 23. How many neutrons does it have?
3. What is an isotope?
1. Sodium has atomic number 11. How many protons does a sodium atom have?
2. Sodium’s mass number is 23. How many neutrons does it have?
3. What is an isotope?
1. Sodium has atomic number 11. How many protons does a sodium atom have?
2. Sodium’s mass number is 23. How many neutrons does it have?
3. What is an isotope?
1. Sodium has atomic number 11. How many protons does a sodium atom have?
2. Sodium’s mass number is 23. How many neutrons does it have?
3. What is an isotope?
Alpha, beta and gamma
Most nuclei are stable, but some have an unbalanced mix of protons and neutrons, or too much energy. These unstable nuclei change on their own, giving out radiation: this is natural radioactive decay. An alpha particle is two protons and two neutrons thrown out together; the nucleus loses two protons, so it becomes a different element. In beta decay a neutron turns into a proton and a fast electron is thrown out; the nucleus gains a proton and again becomes a different element. Gamma rays are high-energy electromagnetic waves, often given out along with alpha or beta, and they change no particles. The three differ in charge and in how easily they are stopped. Students sort twelve facts and build a summary table.
We are learning to
- describe alpha, beta and gamma radiation,
- compare their charge and how easily they are stopped,
- explain what each does to the nucleus that gives it out.
Success criteria
- I can sort the twelve facts and complete the summary table.
- I can explain why alpha and beta decay change the element, and gamma does not.
You need
- the radiation cards (third sheet), one set per pair, with scissors,
- the worksheet (next sheet), one per student.
No radioactive sources are used: radiation can damage living cells, which is why real sources are kept shielded and handled only by trained people.
Lesson flow (about 60 minutes)
| 5 min | Settling down Ask: “A ball balanced on top of a hill will roll down sooner or later. How might a nucleus be like that ball?” |
| 10 min | On the board Show the board (see the box). Add unstable nucleus, radioactive decay, alpha particle, beta particle and gamma ray to the word wall. |
| 25 min | Sort the facts Pairs cut out the cards, lay down the three grey heading cards and sort the facts under them. The piles are not the same size. They record the letters in Part A. Ask: “Which facts let you tell alpha and beta apart, when both are particles?” |
| 20 min | Summary Students complete the summary table in Part B and answer Part C. |
Running two half-sessions instead? Do the opening, the board and the sort in Session A, and Parts B and C in Session B.
Watch for these ideas
- “Radiation makes things radioactive.” Being exposed to radiation does not make an object give out radiation itself.
- “Beta particles are electrons from the shells.” They come from the nucleus, made when a neutron turns into a proton.
- “Gamma rays are particles.” They are electromagnetic waves, with no charge and no mass.
Make it easier, make it bigger
- Easier: give the summary table with the “What it is” row filled in.
- Bigger: alpha is stopped by paper, yet it is the most harmful if something giving out alpha is breathed in or swallowed. Suggest why.
Answers and look-fors
- Part A: alpha A, C, G, I, J; beta B, E, H, L; gamma D, F, K.
- Part B: alpha, two protons and two neutrons, positive, stopped by paper, loses two protons and two neutrons (a new element); beta, a fast electron, negative, stopped by a few millimetres of aluminium, gains a proton as a neutron changes (a new element); gamma, an electromagnetic wave, no charge, reduced by thick lead or concrete, gives out energy only (the same element).
- Part C: 1. The number of protons changes in alpha and beta decay, and the number of protons decides the element. 2. Gamma: it carries no charge and no particles. 3. Alpha: it is large and charged, so it gives up its energy within a few centimetres of air.
- Bigger: inside the body nothing shields the cells, and alpha gives all its energy to the cells close by.
Alpha, beta and gamma
A. Our sort
| Alpha | |
| Beta | |
| Gamma |
B. Summary
| Alpha | Beta | Gamma | |
|---|---|---|---|
| What it is | |||
| Charge | |||
| What stops it | |||
| What happens to the nucleus |
C. Questions
Alpha, beta or gamma?
Cut out the cards. Lay down the three grey heading cards, then sort the facts under them.
Teacher note: the piles are not the same size. The answer letters are in the plan.
Half-life with dice
Radioactive decay is random: there is no way to tell when one unstable nucleus will decay. Yet a large sample behaves very predictably, because each nucleus has the same chance of decaying in any second. The time for half of the unstable nuclei to decay is the half-life, and it is the same whether the sample starts large or small. Dice show this well. Each die is an unstable nucleus; each roll is one tick of time; a die that lands on six has decayed and is taken out. Nobody can predict any one die, but the count falls by about a sixth each roll, and halves in a steady number of rolls.
We are learning to
- model radioactive decay with dice, and graph the results,
- read a half-life from a decay graph,
- explain why one nucleus is unpredictable but a large sample is not.
Success criteria
- I can graph the dice left after each roll and read the half-life twice from the graph.
- I can say what the dice stand for, and one way the model is unlike real decay.
You need
- for each group: 100 dice (or 100 small cubes with one face marked), a tray and a container,
- the worksheet (next sheet), one per student, and the exit tickets (third sheet).
Lesson flow (about 60 minutes)
| 5 min | Which one next? Ask: “If I roll one die, can you tell me which roll will be the first six? If I roll a hundred, can you say how many sixes I will get?” |
| 25 min | Roll and remove Groups tip all 100 dice into the tray, take out every six, count what is left and record it in Part A. They repeat with the dice that are left, ten rolls in all. Ask: “Why do we take out the sixes, and never put them back?” |
| 10 min | On the board Show the board (see the box). Add half-life to the word wall. |
| 20 min | Reading the graph Students graph the counts in Part B, read the half-lives in Part C and answer Part D. |
Running two half-sessions instead? Do the opening and the rolls in Session A, and the board and Parts B to D in Session B.
Watch for these ideas
- “After two half-lives everything has decayed.” After two half-lives a quarter is left; the count halves again each half-life.
- “A nucleus that has not decayed for a long time is due to decay soon.” Like a die, it has the same chance every time.
- “Heating or squeezing a sample changes its half-life.” Nothing ordinary changes the half-life of an isotope.
Make it easier, make it bigger
- Easier: use the example counts for the graph after the first few rolls.
- Bigger: add every group’s counts together for the class. Why does the class curve look smoother than one group’s?
Answers and look-fors
- Part A, example: roll 0 100, roll 1 84, roll 2 69, roll 3 57, roll 4 49, roll 5 40, roll 6 34, roll 7 28, roll 8 22, roll 9 19, roll 10 16.
- Part B: a smooth curve through the points, falling steeply at first and then more gently.
- Part C: 100 to 50 at about roll 3.9; 50 to 25 at about roll 7.5, so another 3.6 rolls. The half-life is about 3.8 rolls, the mean of the two (a die has a one in six chance, which gives 3.8 rolls).
- Part D: 1. Each die is an unstable nucleus, each roll a tick of time, and each six a decay. 2. The chance for each die is the same every roll, so a fixed share decays each time. 3. No: any one die could last many rolls or go on the first. 4. Real samples have billions of nuclei, so their curves are smooth; and a decayed nucleus does not vanish, it becomes a different nucleus.
- Bigger: with more dice the random ups and downs even out, so the curve gets closer to the smooth halving curve.
- Exit tickets: 1. The time for half the unstable nuclei to decay. 2. 25. 3. No: decay is random for a single nucleus.
Half-life with dice
A. Dice left after each roll
| Roll | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Example | 100 | 84 | 69 | 57 | 49 | 40 | 34 | 28 | 22 | 19 | 16 |
| Ours | 100 |
B. Graph
C. Half-life
D. Questions
Exit tickets
Cut out one ticket for each student.
1. What is a half-life?
2. A sample has 100 unstable nuclei. How many are left after two half-lives?
3. Can you predict when one particular nucleus will decay?
1. What is a half-life?
2. A sample has 100 unstable nuclei. How many are left after two half-lives?
3. Can you predict when one particular nucleus will decay?
1. What is a half-life?
2. A sample has 100 unstable nuclei. How many are left after two half-lives?
3. Can you predict when one particular nucleus will decay?
1. What is a half-life?
2. A sample has 100 unstable nuclei. How many are left after two half-lives?
3. Can you predict when one particular nucleus will decay?
Half-lives in the real world
Every unstable isotope has its own half-life, from fractions of a second to billions of years, and that number decides how it can be used. Technetium-99m, supplied to Australian hospitals from the Lucas Heights research centre, has a half-life of about 6 hours: long enough to take a medical scan, short enough to fade from the body within a day or two. Carbon-14 has a half-life of about 5,730 years, so it can date things that were once alive, up to about 50,000 years old. Uranium-238, at 4.5 billion years, dates the oldest rocks. After one half-life half is left, after two a quarter, after three an eighth. Students use five isotopes to work out what is left, and how old something is.
We are learning to
- calculate the amount left after a whole number of half-lives,
- work out an age from the fraction of carbon-14 left,
- explain why each use needs an isotope with a suitable half-life.
Success criteria
- I can work out how much of a sample is left after any whole number of half-lives.
- I can explain why a medical scan needs a short half-life and dating old rocks a long one.
You need
- the worksheet (next sheet), one per student, and the exit tickets (third sheet).
Lesson flow (about 60 minutes)
| 5 min | Short or long? Ask: “A doctor wants a substance that shows up on a scan and then quickly fades away. Short half-life, or long?” |
| 10 min | On the board Show the board (see the box). Add background radiation to the word wall: radon, from some rocks and soils, is part of it. |
| 25 min | How much is left? Work the technetium question together as a halving table, then students complete Part B. Ask: “How many half-lives fit into the time? Then halve that many times.” |
| 20 min | The right isotope Students answer Part C, then share answers to question 2. |
Running two half-sessions instead? Do the opening, the board and Part B in Session A, and Part C in Session B.
Watch for these ideas
- “After four half-lives a quarter is left.” It halves four times: one sixteenth is left.
- “Carbon dating works on rocks.” It works on things that were once alive, such as wood, bone or charcoal.
- “A long half-life means a sample is dangerous for longer, so it must be stronger.” For the same number of nuclei, a longer half-life means fewer decays each second.
Make it easier, make it bigger
- Easier: give a halving table to fill in: 1, 1/2, 1/4, 1/8, 1/16.
- Bigger: the Earth is about 4.5 billion years old. About what share of the uranium-238 the Earth started with is left today?
Answers and look-fors
- Part B: 1. 24 ÷ 6 = 4 half-lives: 80 → 40 → 20 → 10 → 5 units. 2. 24 ÷ 8 = 3 half-lives, so 1/8 is left. 3. 1/4 is two halvings: 2 × 5,730 = 11,460 years.
- Part C: 1. A short half-life fades quickly after the scan, so the body gets little radiation; it gives out gamma, which escapes the body to the camera. 2. Carbon-14: the wood was once alive and its age is in the right range. 3. Its half-life is about the age of the Earth, so a measurable share is left in even the oldest rocks. 4. Radon: it comes from rocks and soils, not from people.
- Bigger: about half, as the Earth’s age is about one half-life of uranium-238.
- Exit tickets: 1. One eighth. 2. 20. 3. Something that was once alive.
Half-lives in the real world
A. Five isotopes
| Isotope | Half-life | Use |
|---|---|---|
| Technetium-99m | 6 hours | medical scans: it gives out gamma rays a camera outside the body can detect |
| Iodine-131 | 8 days | medical treatment of the thyroid gland |
| Radon-222 | 3.8 days | none: a natural gas that seeps from some rocks and soils |
| Carbon-14 | 5,730 years | working out the age of things that were once alive |
| Uranium-238 | 4.5 billion years | working out the age of rocks |
B. How much is left?
C. The right isotope for the job
Exit tickets
Cut out one ticket for each student.
1. What fraction of a sample is left after three half-lives?
2. A sample of 80 units has a half-life of 1 hour. How much is left after 2 hours?
3. What kind of object can carbon-14 date?
1. What fraction of a sample is left after three half-lives?
2. A sample of 80 units has a half-life of 1 hour. How much is left after 2 hours?
3. What kind of object can carbon-14 date?
1. What fraction of a sample is left after three half-lives?
2. A sample of 80 units has a half-life of 1 hour. How much is left after 2 hours?
3. What kind of object can carbon-14 date?
1. What fraction of a sample is left after three half-lives?
2. A sample of 80 units has a half-life of 1 hour. How much is left after 2 hours?
3. What kind of object can carbon-14 date?
The road to stable
A single decay does not always produce a stable atom. Uranium-238, found in many rocks, decays step by step through a chain of unstable nuclei, some lasting only fractions of a second and others for thousands of years, until it becomes lead-206, which is stable and decays no further. Each alpha decay takes away 2 protons and 2 neutrons, so the atomic number falls by 2 and the mass number by 4. Each beta decay turns a neutron into a proton, so the atomic number rises by 1 and the mass number stays the same. Students follow the last eight steps of the chain, from radon-222, a natural gas that seeps from some rocks, to stable lead-206.
We are learning to
- use the alpha and beta rules to find the nucleus after each decay,
- name each element from its atomic number,
- describe how natural decay leads, step by step, to a stable atom.
Success criteria
- I can complete every step of the chain and name each element.
- I can explain why the chain stops at lead-206.
You need
- a periodic table for each pair,
- the worksheet (next sheet), one per student, and the exit tickets (third sheet).
Lesson flow (about 60 minutes)
| 5 min | Downhill Ask: “A ball rolling down a rocky slope stops on a few ledges before it reaches the bottom. How is a decay chain like that?” |
| 10 min | On the board Show the board (see the box), then write the two rules. Add decay chain and stable to the word wall. |
| 25 min | Follow the chain Work the first step together: radon-222, atomic number 86, gives out an alpha particle. Students complete the table in Part A, using the periodic table for the names. Ask: “After a beta decay, why does the mass number not change?” |
| 20 min | Why it stops Students answer Part B, then share answers to question 4. |
Running two half-sessions instead? Do the opening, the board and the chain in Session A, and Part B in Session B.
Watch for these ideas
- “An alpha decay takes away 4 protons.” It takes away 2 protons and 2 neutrons: 4 particles in all.
- “Beta decay adds an electron, so the mass goes up.” A neutron becomes a proton; the mass number stays the same.
- “The chain ends when the sample runs out.” It ends at a stable nucleus, which does not decay at all.
Make it easier, make it bigger
- Easier: complete the atomic numbers first, then the mass numbers, then the names.
- Bigger: the full chain from uranium-238 to lead-206 has 8 alpha decays. How many beta decays must it have? Use the atomic numbers 92 and 82.
Answers and look-fors
- Part A: start: Radon-222 (86); 1. alpha: Polonium-218 (84); 2. alpha: Lead-214 (82); 3. beta: Bismuth-214 (83); 4. beta: Polonium-214 (84); 5. alpha: Lead-210 (82); 6. beta: Bismuth-210 (83); 7. beta: Polonium-210 (84); 8. alpha: Lead-206 (82).
- Part B: 1. 4 alpha and 4 beta decays. 2. 222 − 206 = 16: only alpha decays change the mass number, by 4 each, and 4 × 4 = 16. 3. Polonium, lead and bismuth each appear more than once, at different mass numbers: they are different isotopes. 4. Lead-206 is stable: it does not decay, so the chain ends there.
- Bigger: 8 alpha decays lower the atomic number by 16, from 92 to 76; to reach 82 it must rise by 6, so 6 beta decays.
- Exit tickets: 1. It falls by 2. 2. It rises by 1. 3. Lead-206.
The road to stable
Alpha decay: atomic number −2, mass number −4. Beta decay: atomic number +1, mass number the same.
A. The chain
| Step | Decay | Atomic number | Mass number | Element |
|---|---|---|---|---|
| start | — | 86 | 222 | Radon |
| 1 | alpha | |||
| 2 | alpha | |||
| 3 | beta | |||
| 4 | beta | |||
| 5 | alpha | |||
| 6 | beta | |||
| 7 | beta | |||
| 8 | alpha |
B. Questions
Exit tickets
Cut out one ticket for each student.
1. What happens to the atomic number in alpha decay?
2. What happens to the atomic number in beta decay?
3. Which stable atom ends the chain that starts with uranium-238?
1. What happens to the atomic number in alpha decay?
2. What happens to the atomic number in beta decay?
3. Which stable atom ends the chain that starts with uranium-238?
1. What happens to the atomic number in alpha decay?
2. What happens to the atomic number in beta decay?
3. Which stable atom ends the chain that starts with uranium-238?
1. What happens to the atomic number in alpha decay?
2. What happens to the atomic number in beta decay?
3. Which stable atom ends the chain that starts with uranium-238?
Radiation at work
The properties of each kind of radiation decide where it is useful. Alpha is stopped by a sheet of paper, so a tiny sealed alpha source in a smoke alarm works across a short gap, and smoke breaking that path sets off the alarm. Beta passes partly through thin materials, so the amount getting through can measure thickness. Gamma passes through the body and through sealed packets, so it is used for medical scans and to kill germs on medical equipment. Carbon-14, which gives out beta particles, dates things that were once alive. Students match five uses to the kind of radiation, and explain each choice from its properties.
We are learning to
- link a use of radiation to the properties of alpha, beta or gamma,
- explain why each use needs that kind of radiation,
- describe how sources are kept safe in use.
Success criteria
- I can match the five uses to the right radiation and give a reason for each.
- I can explain why a smoke alarm is safe while it stays sealed.
You need
- a sealed smoke alarm to show, never opened,
- the worksheet (next sheet), one per student, and the exit tickets (third sheet).
Safety: the smoke alarm stays sealed; its label says it holds a tiny source, and it is never opened or taken apart.
Lesson flow (about 60 minutes)
| 5 min | On the ceiling Hold up the smoke alarm and read its label. Ask: “Why would anyone put a radioactive source in a smoke alarm, and why is it safe there?” |
| 10 min | On the board Show the board (see the box), then recap what stops each kind of radiation. |
| 25 min | Match the uses Students match each use in Part A to alpha, beta or gamma and write the reason. Ask: “For each use, ask: does the radiation need to get through something, or be stopped by it?” |
| 20 min | Safe in use Students answer Part B, then share answers to question 3. |
Running two half-sessions instead? Do the opening, the board and the matching in Session A, and Part B in Session B.
Watch for these ideas
- “All radiation is dangerous, so it is never used.” It is used every day, kept safe by shielding, distance and short exposure.
- “Food or equipment treated with gamma becomes radioactive.” Gamma passes through; it does not make things radioactive.
- “Any radiation will do for any job.” Each job needs a particular range and a suitable half-life.
Make it easier, make it bigger
- Easier: give the three properties as prompts: stopped by paper, stopped by thin aluminium, passes through the body.
- Bigger: a medical tracer must be easy to detect but fade quickly. Explain why it should give out gamma and have a short half-life.
Answers and look-fors
- Part A, in the printed order: checking the thickness of paper in a factory, beta; a smoke alarm, alpha; dating a piece of ancient wood, beta; a tracer for a medical scan, gamma; killing germs on medical equipment, gamma.
- Reasons: a smoke alarm: alpha particles make the air inside conduct; smoke blocks them easily, and the alarm sounds; a tracer for a medical scan: gamma passes out through the body to a camera, and a short half-life means it soon fades; checking the thickness of paper in a factory: beta passes partly through paper, so the amount getting through shows the thickness; killing germs on medical equipment: gamma passes right through the sealed packet; dating a piece of ancient wood: carbon-14 gives out beta particles as it decays, at a known rate.
- Part B: 1. The alpha source is tiny and sealed, and alpha cannot pass through the alarm’s case or more than a few centimetres of air. 2. Shielding, keeping a distance and keeping the time short. 3. Gamma passes through the packet without making the equipment radioactive, killing germs inside the sealed packet.
- Bigger: gamma escapes the body to reach the camera outside, and a short half-life means the tracer has mostly decayed within a day or two.
- Exit tickets: 1. Alpha. 2. Gamma. 3. Shielding, distance or a short time.
Radiation at work
A. Which radiation, and why?
| Use | Alpha, beta or gamma? | Why that kind? |
|---|---|---|
| Checking the thickness of paper in a factory | ||
| A smoke alarm | ||
| Dating a piece of ancient wood | ||
| A tracer for a medical scan | ||
| Killing germs on medical equipment |
B. Questions
Exit tickets
Cut out one ticket for each student.
1. Which kind of radiation is used in a smoke alarm?
2. Which kind of radiation can pass through the body to a camera?
3. Name one way to stay safe near a radioactive source.
1. Which kind of radiation is used in a smoke alarm?
2. Which kind of radiation can pass through the body to a camera?
3. Name one way to stay safe near a radioactive source.
1. Which kind of radiation is used in a smoke alarm?
2. Which kind of radiation can pass through the body to a camera?
3. Name one way to stay safe near a radioactive source.
1. Which kind of radiation is used in a smoke alarm?
2. Which kind of radiation can pass through the body to a camera?
3. Name one way to stay safe near a radioactive source.
Show what we know
The term comes together. In the task, students date charcoal from an old campfire with carbon-14, follow one carbon-14 nucleus as it decays, and explain how the model of the atom was built from evidence. Then they sit the eight-question final check, and the class plays the unit quiz as a game on the board.
We are learning to
- use half-life to work out an age,
- apply the decay rules and the particle counts to a real isotope,
- explain how evidence changed the model of the atom.
Success criteria
- I can date the charcoal and show my working.
- I can answer the final check on my own.
You need
- the task (next sheet) and the final check (third sheet), one each per student; a periodic table,
- the interactive unit open on the board for the quiz; mini whiteboards if you have them.
Lesson flow (about 60 minutes)
| 5 min | The story so far Point to the word wall and recall the four models of the atom. Ask: “Which piece of evidence changed the model most?” |
| 25 min | The old campfire Students complete the task on their own, with a periodic table. |
| 20 min | Final check Students complete the final check on their own. |
| 10 min | Class quiz Run the unit quiz on the board as a class game (see the box). |
Running two half-sessions instead? Do the story and the task in Session A, and the final check and the quiz in Session B.
Watch for these ideas
- Students who take an eighth as three half-lives without checking: ask them to halve three times out loud.
- Students who say carbon-14 becomes carbon-13 when it decays: beta decay changes the element.
- Students who describe the old models as wrong: each fitted the evidence of its time.
Make it easier, make it bigger
- Easier: give a halving table (1, 1/2, 1/4, 1/8) on the task sheet.
- Bigger: carbon-14 dating stops working beyond about 50,000 years. Use the half-life to explain why.
Answers and look-fors
- Task: 1. 1 → 1/2 → 1/4 → 1/8 is 3 half-lives: 3 × 5,730 = 17,190 years. 2. The charcoal was once living wood, and the age is within carbon-14’s range; uranium-238’s half-life is far too long. 3. 6 protons, 8 neutrons, 6 electrons. 4. A neutron becomes a proton: atomic number 7, mass number 14, so nitrogen-14, which is stable. 5. For example, the gold-foil experiment: a few particles bouncing back showed a tiny, dense nucleus, replacing the plum pudding. 6. Decay is random for each nucleus; only the share of a large sample can be predicted.
- Final check: 1. The electron. 2. The atom is mostly empty space with a tiny, dense, positive nucleus. 3. Protons. 4. 17 − 8 = 9 neutrons. 5. Alpha. 6. Three half-lives: 120 → 60 → 30 → 15. 7. It rises by 1. 8. An unstable nucleus gives out alpha or beta radiation and becomes a different nucleus, step by step, until it reaches a stable one that does not decay.
- Bigger: after 50,000 years, almost nine half-lives, less than a three-hundredth of the carbon-14 is left, too little to measure reliably.
The old campfire
Charcoal from an old campfire is found buried in a riverbank. It has one eighth of the carbon-14 that living wood has. Carbon-14 has a half-life of about 5,730 years, and it decays by giving out a beta particle.