Lesson 1.6 · AQA Combined Science

Inside the atom

Meet the three particles: protons, neutrons and electrons.

01 · A closer look

Three particles inside an atom

Protons and neutrons are packed into the nucleus, the small centre of the atom. Electrons occupy shells around it.

A helium atom with 2 protons, 2 neutrons and 2 electrons. The particles and nucleus are enlarged to make them visible.

02 · Learn the differences

Relative means compared with the other particles. A proton and a neutron have about the same mass.

Subatomic particles
Particle	Location	Charge	Relative mass
Proton	Nucleus	+1	1
Neutron	Nucleus	0	1
Electron	Shells	−1	Very small about 1/1836

Remember: proton = positive; neutron = neutral; electron = negative.

03 · Charges balance

An atom has equal numbers of protons and electrons. Their opposite charges cancel out, so the atom has no overall charge.

2 protons: +22 electrons: −2Overall charge: 0

Neutrons have no charge, so they do not affect the overall charge.

04 · Tiny, with an even tinier centre

Most of an atom is empty space. Almost all its mass is in the nucleus, because electrons have very little mass.

An atom has a radius of about 0.1 nanometres (0.1 nm), or 1 × 10⁻¹⁰ m.

The nucleus has a radius less than 1/10,000 of the atom’s radius. It is much smaller than it looks in our diagram.

Nucleus: protons and neutrons; almost all the mass.

Shells: negative electrons.

Neutral atom: equal numbers of protons and electrons.