Structure of the atom

Protons, neutrons and electrons

Atoms are very small, they have a radius of around 1×10-10 metres.

The modern view of the atom is of a positively-charged nucleus containing protons and neutrons with smaller electrons orbiting outside the nucleus.

The bohr atomic model, three neutrons and three protons in the centre with two enegy levels, the first energy level has two electrons and the second has one electron.

Each particle has its own charge and its own mass.

Relative charge Relative mass
Neutron 0 1
Electron -1 Close to 0 (1/2,000)

Mass number and atomic number

Protons and neutrons are the heaviest particles in an atom and, as a result, they make up most of the mass of the atom. The mass of electrons is often not considered to be significant.

The number of protons is what defines the element, ie an atom with six protons in its nucleus will always be carbon, and uranium will always have 92 protons.

The total number of protons and neutrons is called the mass number and the number of protons is called the atomic number.

Mass number can also be called ‘nucleon number’. Atomic number can also be called ‘proton number’.

In a neutral atom, the number of electrons is always the same as the number of protons. If the atom becomes ionised however, the number of electrons will change. An ion is an atom that has lost or gained one or more electrons.

Using atomic symbols

Mass number and atomic number are two important pieces of information about an atom.

An atom can be represented using the symbol notation:



  • Z is the mass number
  • A is the atomic number
  • X is the symbol

For example, chlorine (Cl) can be shown as:

Chlorine atom with mass number 35 and atomic number 17.

This symbol shows that chlorine has 35 particles in the nucleus (protons and neutrons), 17 of which are protons. It also tells us that chlorine has 18 neutrons (35 - 17) and, as the number of electrons and protons are equal in a neutral atom, chlorine also has 17 electrons.