grade 8 Atomic Models

Atomic Models

If you keep splitting matter, you reach the atom, the smallest unit that still keeps a substance's properties. An atom has a nucleus of protons and neutrons, with electrons orbiting outside. Models from Dalton to Bohr were revised by experiment, and losing or gaining electrons makes ions. Switch among Dalton, Thomson, Rutherford, and Bohr and the structure and ion formation appear.

What is an Atom?

⚛️ Splitting Matter Endlessly?

①Cut paper repeatedly → eventually it loses paper-like properties

②The smallest unit retaining a substance's properties = atom

③Atoms are too small to see directly (~10⁻¹⁰ m)

Evolution of Atomic Models

💡 Why the Models Changed

①Dalton: indivisible — collapsed when electrons were discovered

②Thomson: electrons in positive dough — revised by alpha-particle scattering

③Rutherford: nucleus + orbiting electrons — needed energy explanation

④Bohr: electrons exist only at specific orbits (energy levels)

Structure of an Atom

Atomic Composition

Atom = nucleus (protons + neutrons) + electrons

Proton count = electron count → electrically neutral

Atomic Number & Mass Number

Atomic # = protons; Mass # = protons + neutrons

Same atomic number = same element; different mass number = isotope

Formation of Ions

Cation

Atom − electron → cation (+)

Sodium (Na): loses 1 electron → Na⁺

Anion

Atom + electron → anion (−)

Chlorine (Cl): gains 1 electron → Cl⁻

💡 Why Ions Form

①Atoms try to fill their outermost electron shell

②Metals (Na, Mg): lose electrons → cations (easier to lose)

③Non-metals (Cl, O): gain electrons → anions (easier to gain)

Summary

Atom Core

Atom = nucleus (p + n) + electrons / protons = atomic #

Models progressed: Dalton → Thomson → Rutherford → Bohr

🎯 Exam Key Points

①Dalton (solid sphere) → Thomson (plum pudding) → Rutherford (planet) → Bohr (orbits)

②Nucleus = protons (+) + neutrons (0); electrons (−) outside

③Proton # = electron # → neutral atom

④Cation: lost electrons (+); anion: gained electrons (−)

⑤Atomic # = protons = electrons (in a neutral atom)

Examples and Unit Test

Example 1

An atom is made of 11 protons, 11 electrons, and 12 neutrons. What are its atomic number and mass number?

The atomic number equals the number of protons, and the mass number is the sum of protons and neutrons.

There are 11 protons, so the atomic number is 11, and the mass number is 11 + 12 = 23.

▸ Atomic number 11, mass number 23

Atomic number = number of protons, mass number = protons + neutrons.

Example 2

When a sodium atom (Na) loses one electron, what particle does it become, and what is its charge?

When an atom loses an electron it becomes a positive ion (cation); when it gains one it becomes a negative ion (anion).

When sodium loses one electron it becomes a positively charged sodium ion, Na⁺.

▸ A cation, Na⁺ (positive charge)

Losing electrons makes a positive ion; gaining electrons makes a negative ion.

Unit-test style

Which statement about atomic structure and ions is correct?

①The atomic number equals the number of neutrons

②An atom has equal numbers of protons and electrons, so it is electrically neutral

③When an atom gains an electron it becomes a cation

④The mass number is the sum of the numbers of electrons and protons

⑤The nucleus is made only of electrons

▸ ② An atom has equal numbers of protons and electrons, so it is electrically neutral

An atom has the same number of positive protons and negative electrons, so overall it is neutral.

The atomic number equals the number of protons (①), gaining an electron makes an anion (③), the mass number is protons plus neutrons (④), and the nucleus is made of protons and neutrons (⑤).

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Chemical Equations

Oxidation & Reduction

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