Structure of the atom

Nucleus and shells

An atom has a central nucleus surrounded by electrons arranged in areas called shells.

Carbon atom with the nucleus and an electron labelledThe structure of a carbon atom, not drawn to scale

The nucleus is very small compared to the atom as a whole:

  • as electrons are even smaller, most of an atom is empty space
  • the diameter of a nucleus can be 100,000 times less than the diameter of an atom

Subatomic particles

The nuclei of all atoms contain subatomic particles called protons and most also contain neutrons.

Structure of a carbon atom, with a proton, neutron and electron labelled

The masses of subatomic particles are very tiny. Instead of writing their actual masses in kilograms, we often use their relative masses. The relative mass of a proton is 1, and a particle with a relative mass smaller than 1 has less mass.

The mass of electrons is very small compared to protons and neutrons. Since a nucleus contains protons and neutrons, most of the mass of an atom is concentrated in its nucleus.

Protons and electrons have opposite electrical charges.

Subatomic particleRelative massRelative charge
Proton1+1
Neutron10
Electron \frac{1}{1835}-1
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It may help to remember that Protons are Positive, and Neutrons are Neutral.