Constituents of the Atom

Constituents of the Atom

An atoms is made up of:

  • protons
  • neutrons
  • electrons

The protons and neutrons are located in the nucleus on atom. The nucleus itself is surrounded by empty space in which the electrons are based.

Proton, neutron and electron data

Particle Relative mass Relative charge Charge / C Mass / kg
Protons 1 +1 +1.6 x 10-19 1.67 x 10-27
Neutrons 1 Neutral 0 1.67 x 10-27
Electrons 0.0005 -1 -1.6 x 10-19 9.11 x 10-31

Atoms and isotopes

An atom is described by the number of protons it contains. This is called the proton number (Z). The number of neutrons or neutrons a nucleus contains is known as the nucleon number (A). The nucleon number can be worked out by the following equation:

nucleon number = number of protons + number of neutrons

The example below demonstrates how these numbers are displayed:


Atoms which share the same number of protons but possess different neutron numbers are called isotopes. Isotopes are atoms of the same element. For example:

are isotopes.

Specific charge

When you divide the charge (Q) of either a particle of an atom by the mass (m) of the same particle or atom then you get the specific charge. This is measured in coulombs per kilogram (C kg-1):

specific charge = Q / m