ACARA v9 CONTENT DESCRIPTION “explain how the model of the atom changed following the discovery of electrons, protons and neutrons and describe how natural radioactive decay results in stable atoms”
Builds on the idea that all matter is made of atoms. Here we open the atom up and follow how scientists worked out what is inside it, then ask what happens when a nucleus is unstable. The story moves from a simple ball to a nucleus surrounded by electron shells, and on to the quiet, natural way unstable atoms settle down.
A model that kept changing
A scientific model is the best current picture, and it changes when new evidence arrives. Dalton drew the atom as a solid sphere. Thomson found the electron and pictured those negative specks dotted through a positive atom. Rutherford fired particles at thin gold and found most went straight through, so the atom had to be mostly empty space with a tiny dense nucleus. Bohr then placed the electrons in set shells, and the discovery of protons and neutrons completed the nucleus.
How the model of the atom changed
Step through the history. Each new piece of evidence forced scientists to redraw the atom.
Dalton pictured each atom as a tiny solid ball that could not be broken down. It explained how elements combine in fixed ratios, but it gave the atom no inside parts.
The evidence that built the nucleus
The gold-foil experiment is a perfect example of evidence changing a model. If the atom were a soft pudding of spread-out charge, every particle should pass through with only a small nudge. Instead a few were turned sharply aside and the rare one bounced almost straight back. The only explanation is a small, heavy, positive centre. One careful experiment replaced the plum pudding with the nuclear atom.
The gold-foil idea
Fire positive particles at an atom. Aim at empty space, then aim near the centre, and see what the result reveals.
Almost every particle sails straight through, which tells you the atom is mostly empty space. The old plum-pudding atom, with charge spread everywhere, could not explain this.
The three subatomic particles
An atom is built from three particles. Protons carry positive charge and neutrons carry no charge; both sit in the central nucleus and give the atom almost all of its mass. Electrons carry negative charge and move in shells around the nucleus. In a neutral atom the number of protons equals the number of electrons, so the charges cancel. The number of protons is what makes an atom one element rather than another.
Inside an atom
Protons and neutrons pack into the nucleus; electrons sit in shells. Switch element and read the counts.
The gold circles are protons and the grey circles are neutrons, both held in the tiny central nucleus. The blue circles are electrons out in the shells. A neutral atom has equal protons and electrons, so hydrogen has 1 of each, with 0 neutrons adding mass but no charge.
When a nucleus is unstable
Most atoms are stable and last unchanged. Some have a nucleus holding more energy than it can keep, and these are radioactive. Without any push from outside, such a nucleus releases radiation and changes into a more stable one. Alpha decay sends out a clump of two protons and two neutrons. Beta decay turns a neutron into a proton and sends out a fast electron. Gamma rays carry off leftover energy. Each release moves the nucleus closer to stability.
One natural decay event
An unstable nucleus has too much energy to hold together. Choose a decay type, then let it happen.
This nucleus is unstable, carrying more energy than it can hold. Nothing forces it from outside; on its own it will eventually let radiation go. Press Decay.
Half-life and the road to stability
We cannot say when one particular nucleus will decay, but a large sample follows a clear pattern. The half-life is the time for half of the unstable nuclei to decay. After one half-life half are left, after two a quarter, after three an eighth, and so on. Step after step the unstable count falls toward nothing while the stable atoms build up. This steady halving is why half-life is so useful, from carbon dating old wood to medical tracers and smoke detectors, all of which rely on the science of natural decay.
Half-life: halving toward stable
Advance one half-life at a time. Each step, exactly half of the unstable nuclei left have decayed to stable.
A half-life is the time it takes for half of the unstable nuclei to decay. Starting from 64, after one half-life 32 are left, after two 16, and so on. The count never quite reaches zero in one jump, but step by step almost all of the sample has decayed into stable atoms.
Why this matters
Following how the atomic model changed shows science at work: a picture is held until evidence forces a better one. Knowing where protons, neutrons and electrons sit lets you read the periodic table and predict how elements behave. Understanding natural decay and half-life explains how unstable atoms become stable and underpins safe, everyday uses such as dating ancient remains and tracing how the body works.
Quick self-check
1. Thomson discovered electrons. How did that change the model of the atom?
2. In the gold-foil experiment, most particles passed straight through but a few bounced sharply back. This is evidence that...
3. In a neutral atom, the protons and neutrons are found in the...
4. What happens during natural radioactive decay of an unstable nucleus?
5. A sample starts with 80 unstable nuclei. After one half-life, how many are still unstable?