Structure of the atom

The typical size of atoms and small molecules is around 1 × 10-10 m (0.1 nm). For example:

diameter of a hydrogen atom, H1.0 × 10-10 m
width of a hydrogen molecule, H22.9 × 10-10 m
length of an H-H bond0.74 × 10-10 m

Nucleus and shells

An atom consists of:

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

Compared to an atom as a whole:

  • the radius of the nucleus is much smaller than the radius of the atom
  • the nucleus contains almost all of the mass

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 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.

Protons and electrons have opposite electrical charges.

Subatomic particleRelative massRelative charge
Proton1+1
Neutron10
Electron0.0005-1
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It may help to remember that Protons are Positive, and Neutrons are Neutral.