Atomic structure - AQA
Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons in shells. The numbers of subatomic particles in an atom can be calculated from its atomic number and mass number.
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
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.
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 particle | Relative mass | Relative charge |
|---|---|---|
| Proton | 1 | +1 |
| Neutron | 1 | 0 |
| Electron | Very 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.