# Structure of the atom

## The nuclear model

Atoms contain three sub-atomic particles called , and .

The protons and neutrons are found in the at the centre of the atom. The nucleus is very much smaller than the atom as a whole. The electrons are arranged in shells around the nucleus.

## Properties of sub-atomic particles

ParticleRelative massRelative charge
Proton1+1
Neutron10
Electron$\frac{1}{1836}$-1

The number of electrons in an atom is always the same as the number of protons, so atoms are electrically overall.

Atoms can lose or gain electrons. When they do, they form charged particles called ions:

• if an atom loses one or more electrons, it becomes a positively charged ion
• if an atom gains one or more electrons, it becomes a negatively charged ion