Structure of the atom

Nucleus and shells

The current model of an atom has a positively charged central nucleus surrounded by negatively charged electrons arranged in shells.

Carbon atom with the nucleus and an electron labelledA carbon atom, diagram not 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 radius of a nucleus can be 100,000 times less than the radius of an atom
  • most of the mass of an atom is in the nucleus

Subatomic particles

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

The masses of subatomic particles are very tiny. Instead of writing their actual masses in kilograms, their relative masses are used. 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
Electron0.0005-1
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It may help to remember that Protons are Positive, and Neutrons are Neutral.