Atomic structure, for the exam.

Part 1Size, scale and what's inside

An atom is unimaginably small. Its radius is about 1 × 10⁻¹⁰ m (0.1 nanometres). You could line up about a million of them across the width of a human hair.

Nearly all of that tiny atom is empty space. In the middle sits the nucleus, made of protons and neutrons. The nucleus is where almost all the mass is concentrated, yet its radius is around 10 000 times smaller than the atom — less than 1/10 000 of the atomic radius. Around the nucleus, electrons orbit at different distances.

Atoms have no overall charge — they are neutral. That's because every atom has the same number of protons as electrons, so the +1 and −1 charges cancel exactly. The number of protons (the atomic number) is what makes an atom a particular element.

Part 2Electron energy levels

Electrons don't sit just anywhere — they occupy energy levels (shells) at set distances from the nucleus. If an atom absorbs electromagnetic radiation, an electron can jump to a higher energy level (further out). When it falls back, the atom emits radiation again.

Part 1The story so far

The model of the atom is a great example of how science changes when new experimental evidence appears. It happened in stages:

Before the electron — atoms were thought to be tiny solid spheres that couldn't be divided. Then the electron was discovered, which changed everything: atoms must contain even smaller charged particles.

The plum pudding model — this suggested the atom was a ball of positive charge with negative electrons embedded in it, like fruit in a pudding. There was no nucleus in this model.

Part 2The alpha-scattering experiment

To test the plum pudding model, alpha particles (positive) were fired at a very thin sheet of gold foil. If plum pudding were right, the alpha particles should all pass almost straight through with only tiny deflections.

But that's not what happened. The results were:

Most alpha particles passed straight through — so the atom is mostly empty space. Some were deflected — so the centre must be positively charged (it repelled the positive alphas). A few bounced almost straight back — so the centre must be very small, very dense and hold most of the mass.

This led to the nuclear model: a tiny, dense, positive nucleus at the centre, with electrons orbiting it. Later, Niels Bohr corrected the model by proposing that electrons orbit at specific distances (energy levels), and the nucleus was found to contain positive protons. Later still, neutrons were discovered in the nucleus.

Part 1Atomic number and mass number

Every element is defined by its atomic number — the number of protons. Change the number of protons and you've changed the element. The two numbers written by a symbol tell you what's in the nucleus:

The atomic number (bottom) = number of protons. The mass number (top) = number of protons + neutrons. So neutrons = mass number − atomic number. In a neutral atom, the number of electrons equals the atomic number.

Part 2Isotopes vs ions

These two words sound similar but mean very different things.

Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons. They have the same atomic number but a different mass number. Because they have the same number of protons and electrons, isotopes of an element react in exactly the same way chemically.

An ion is an atom (or group of atoms) that has lost or gained electrons, so it now has an overall charge. Losing electrons makes a positive ion; gaining electrons makes a negative ion. Crucially, forming an ion does not change the nucleus — the number of protons and neutrons stays the same.

Part 1Unstable nuclei and the four radiations

Some atoms have unstable nuclei. To become more stable they give out radiation — this is radioactive decay. Decay is completely random: you can't predict which nucleus will decay next, or when. The activity of a source is the rate at which nuclei decay, measured in becquerels (Bq), where 1 Bq = 1 decay per second. We measure it with a Geiger–Müller tube as a count-rate (counts per second).

There are four kinds of nuclear radiation you need to know:

Part 2Penetration and ionising power

The radiations differ in how far they travel and how strongly they knock electrons off atoms (how ionising they are). There's a neat rule: the more ionising a radiation is, the less penetrating it is, because it gives up its energy quickly.

Alpha is the most ionising but least penetrating — stopped by a sheet of paper or a few cm of air. Beta is moderately ionising and penetrating — stopped by a few mm of aluminium. Gamma is the least ionising but most penetrating — only reduced by thick lead or metres of concrete.

Part 1The two decays

When a nucleus decays it can change. In a nuclear equation, the totals must balance: the mass numbers (top) on each side must be equal, and the atomic numbers (bottom) on each side must be equal.

Alpha decay emits an alpha particle (a helium nucleus, ⁴₂He). The nucleus loses 2 protons and 2 neutrons, so the mass number falls by 4 and the atomic number falls by 2. Because the number of protons changes, it becomes a different element.

Beta decay emits a beta particle (an electron, written ⁰₋₁e). Inside the nucleus a neutron turns into a proton. So the mass number does not change, but the atomic number rises by 1 — again, a new element.

Part 2Writing and balancing the equations

You write a decay like a chemistry equation, with the parent nucleus on the left and the products on the right. The alpha particle is written as a helium nucleus; the beta particle as an electron.

Part 1What half-life means

Because decay is random, we can't say when one nucleus will decay — but with billions of nuclei we can describe the average behaviour. The half-life is the time taken for the number of unstable (radioactive) nuclei in a sample to halve. It is also the time for the activity (count-rate) to fall to half its starting value.

After each half-life, the activity halves again: from 100% it goes to 50%, then 25%, then 12.5%, and so on. It never quite reaches zero.

Part 2Contamination vs irradiation

Two words that examiners deliberately test, because they're easy to confuse.

Irradiation means being exposed to radiation from a source outside the body. The object or person does not become radioactive — once the source is removed or shielded, the exposure stops.

Contamination is when radioactive atoms get onto or into an object (e.g. dust on the skin, or breathing in a radioactive gas). Now the source is with the person and keeps emitting radiation until it's removed or has decayed — which can be a serious hazard.