Atoms, as you should know from GCSE, consist of:

• A positively-charged nucleus, containing protons and neutrons.
• Negatively-charged electron shells surrounding the nucleus at different levels, that contain electrons.

Constituents

Constituent	Location	Weight / kg	Relative charge (in terms of $ -e $)	Charge (C)
Proton	Nucleus	$ 1.67 \times 10^-27 kg $	$ 1 $	$ +1.60 \times 10^-19 $
Neutron	Nucleus	$ 1.67 \times 10^-27 kg $	$ 0 $	$ +0 $
Electron	Electron shells	$ 9.11 \times 10^-31 kg $	$ -1 $	$ -1.60 \times 10^-19 C $

Isotopes

Isotopes are atoms with the same number of protons but differing number of neutrons, therefore of the same element. The notation used consists of:

• The number of nucleons $ A $ is written in superscript.
• The symbol of the element $ X $ is written to the right.
• The number of protons $ Z $ is written in subscript.

Specific charge

Specific charge is the charge per unit of mass, and is calculated with:

$$C (Ckg^-1) = \frac{Q (C)}{m (kg)}$$