The big idea: A coin feels completely solid, yet almost all of every atom inside it is empty space around a tiny central nucleus, with the electrons spread far around it.
The nucleus holds two kinds of particle, called nucleons:
- protons — positive charge - neutrons — no charge
The electrons (negative) orbit far outside, in the mostly-empty space around the nucleus.
| Particle | Where it is | Charge | Relative mass |
|---|---|---|---|
| Proton | in the nucleus | +1 (positive) | 1 |
| Neutron | in the nucleus | 0 (neutral) | 1 |
| Electron | around the nucleus | −1 (negative) | ≈ 1/2000 (negligible) |
Two words to know: Nucleon = a particle in the nucleus → a proton or a neutron.
Nuclide = a specific type of nucleus, set by how many protons and neutrons it has (e.g. carbon-14 is one nuclide, carbon-12 is another).
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Every nuclide is written with two numbers in front of the element symbol. The top number is the nucleon number; the bottom number is the proton number.
From those two numbers you can count every particle. The neutron count is the difference between the two numbers:
- proton (atomic) number — the bottom number; defines the element
- nucleon (mass) number — the top number; protons + neutrons
- number of neutrons in the nucleus
A neutral atom has as many electrons as protons. But an ion has lost or gained electrons, so its electron count is shifted by its charge:
- proton number (number of protons)
- the ion's charge in units of e (e.g. +2 for a 2+ ion, −1 for a 1− ion)
Watch the charge sign: A 2+ ion has lost 2 electrons, so it has Z − 2 electrons (q = +2).
A 1− ion has gained 1 electron, so it has Z + 1 electrons (q = −1).
The charge only changes the electrons — the proton and neutron counts are fixed by A and Z.
An aluminium ion is written as a nuclide with nucleon number A = 27, proton number Z = 13, and overall charge 3+. How many protons, neutrons and electrons does it have?
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How we know the nucleus is there: Geiger and Marsden fired alpha particles (small, fast, positively charged) at a very thin gold foil and watched where they went.
('Alpha particle' = a tiny positive particle — 2 protons + 2 neutrons — given off by some radioactive sources.)
What was observed
- Almost all alpha particles passed straight through, barely bent
- A small number were deflected through large angles
- A very few (about 1 in 8000) bounced straight back
What it told us
- The atom is mostly empty space (so most pass through)
- The positive charge + nearly all the mass sit in a tiny, dense core
- That core (the nucleus) is concentrated and positive — it repels a head-on alpha right back
The logic in one line: Most pass through → atom is mostly empty.
A few bounce back hard → all the positive charge and mass is squeezed into a tiny central nucleus.
This replaced the old 'positive pudding with electrons dotted through it' picture.
How this is tested — both halves of this micro show up:
Paper 1A
- A one-step count — from a nuclide or an ion, pick the number of protons, neutrons or electrons.
- Write or identify a nuclide symbol.
Paper 2
- Describe the alpha-scattering observations.
- Outline how they were interpreted to give the nuclear model.
The classic trap: Changing the proton or neutron count when an atom becomes an ion — only the electron count changes.
In the alpha-particle scattering experiment, a beam of alpha particles is fired at a thin metal foil. (a) Describe two observations made about the paths of the alpha particles. (b) Outline how these observations led to the nuclear model of the atom.
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