# Structure of the atom

## Nucleus and shells

An atom has a central . This is surrounded by 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 . The nuclei of most atoms also contain .

The structure of a carbon atom, not drawn to scale

The of subatomic particles are very tiny. Instead of writing their actual masses in kilograms, we often use their . 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
Proton1+1
Neutron10
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.