Structure of the atom

Different scientists investigated the structure of the atom after Rutherford's new model was put forward. By the 1930s they had found that:

  • atoms can be broken down into something simpler (protons, neutrons and electrons)
  • atoms of an element have identical numbers of protons and electrons, but can have different numbers of neutrons
  • atoms of different elements have different numbers of protons and electrons

Nucleus and shells

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

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

Size of the atom and the nucleus

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

  • the diameter of a nucleus can be 100,000 times less than the diameter of an atom
  • if an atom was the size of a large football stadium, the nucleus would be the size of a football

Atoms are too small to be seen clearly, even with very powerful microscopes:

  • the diameter of an atom is typically around 0.1 nm or 1 × 10-10 m
  • the thickness of a piece of paper is typically around 0.05 mm or 5 × 10-5 m
  • therefore, a piece of paper is about half a million atoms thick

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 labelled

The nucleus of the simplest atom - hydrogen - contains only one proton and no neutrons.

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
Electron \frac{\textup1}{\textup2000}-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.

It may help to remember that Protons are Positive, and Neutrons are Neutral.