Structure of the atom

Protons, neutrons and electrons

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

Small molecules are at least ten times larger.

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

The radius of the nucleus is very much smaller than that of the atom.

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 chargeRelative mass
Electron-1close 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.

In a neutral atom, the number of electrons is always the same as the number of protons. However, the number of electrons will change if the atom becomes ionised.