The structure of an atom 

Atoms are very small, having a radius of about 1 × 10-10 metres.

The radius of a nucleus is less than 1/10 000 of the radius of an atom. Most of the mass of an atom is concentrated in the nucleus.

The basic structure of an atom is a positively charged nucleus composed of both protons and neutrons surrounded by negatively charged electrons.

Subatoms & their properties

Proton - relative mass = 1 charge = +1

Neutron - relative mass = 1 charge = 0

The electrons are arranged at different distances from the nucleus (different energy levels). The electron arrangements may change with the absorption of electromagnetic radiation (move further from the nucleus; a higher energy level) or by the emission of electromagnetic radiation (move closer to the nucleus; a lower energy level).