The nuclear atom

Mostly empty space

• If an atom were the size of a stadium, its nucleus 原子核 would be a pea in the centre.
• Almost all the atom is empty space — a surprising fact proved by a clever experiment.
• Let's build up the nuclear model 核模型 of the atom.

The nuclear atom

• An atom has a tiny central nucleus (positive) with electrons (negative) moving around it.
• The whole atom is neutral 中性的 — equal positive and negative charge.
• Almost all the mass is in the nucleus, which is tiny compared with the atom.
• Lose electrons → a positive ion; gain electrons → a negative ion.

A nuclear power station; the large towers release waste heat as steam

The alpha-scattering experiment

• Fast, positive alpha particles α粒子 were fired at very thin gold foil 金箔.
• Most went straight through → the atom is mostly empty space.
• A few were deflected → the nucleus is positive (it repels the alpha particles).
• A very few bounced back → the nucleus is tiny, dense and heavy.

Inside the nucleus

• The nucleus is made of nucleons 核子: protons 质子 (charge $+1$) and neutrons 中子 (charge $0$).
• Proton number $Z$ = number of protons; nucleon number $A$ = protons + neutrons.
• So the number of neutrons is $A - Z$.
• We write a nucleus as $^{A}_{Z}\text{X}$ — for example $^{14}_{\ 6}\text{C}$ has $6$ protons and $14 - 6 = 8$ neutrons.

The nuclear model of the atom

Isotopes 同位素

• Isotopes are atoms of the same element (same $Z$) with different numbers of neutrons (different $A$).
• They behave the same in chemistry but differently in the nucleus.
• Example: $^{12}_{\ 6}\text{C}$ and $^{14}_{\ 6}\text{C}$ are both carbon.

Isotopes have the same number of protons but different numbers of neutrons — the same element with a different mass

You've got it

Write from 10 neutrons

• A nucleus has proton number 8 and nucleon number 18.
• Find the number of neutrons. Say what removing an electron does.