Atomic structure

Protons, neutrons and electrons, and what the numbers on the periodic table mean.

What it is

An atom has a nucleus of protons and neutrons with electrons in shells around it. The atomic number is the number of protons and defines the element. The mass number is protons plus neutrons.

Where it applies

GCSE chemistry and combined science.

How it works

Isotopes are atoms of the same element with different numbers of neutrons — same chemistry, different mass. Relative atomic mass is the weighted average across isotopes, which is why it is rarely a whole number.

Electrons occupy shells holding 2, then 8, then 8, and the outer shell governs how the element reacts.

How to use it in your work

Protons define the element, electrons define the reactions, neutrons define the isotope. That sentence answers a lot of questions.

For electronic structure, write it as 2,8,1 and check the total equals the atomic number.

Where to get help

Your teacher, your board's specification for the depth required at your tier, and Bitesize for the diagrams. The subject song runs the topics in order.