Structure of the atom

Nucleus and shells

An atom has a central nucleus. This is surrounded by electrons arranged in shells.

The nucleus is tiny compared to the atom as a whole:

  • the radius of an atom is about 0.1 nm (1 × 10-10 m)
  • the radius of a nucleus (1 × 10-14 m) is less than \(\frac{1}{10,000}\) of the radius of an atom

For comparison, the radius of a typical bacterium is 1 × 10-6 m and the radius of a human hair is about 1 × 10-4 m.

Subatomic particles

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

Structure of a carbon atom, with a proton, neutron and electron labelledThe structure of a carbon atom, not drawn to scale

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.

Subatomic particleRelative massRelative charge
ElectronVery small-1

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

Protons and electrons have electrical charges that are equal and opposite.

Remember that Protons are Positive, and Neutrons are Neutral.