Protons, neutrons and electrons

By the end of this lesson

By the end you can describe what's inside an atom, give the relative mass and charge of a proton, neutron and electron, and count all three in any atom or ion.

An atom is mostly nothing

An atom is mostly empty space.

At the centre is a nucleus: very small, very dense, made of protons and neutrons.
The electrons are found in shells in the empty space around the nucleus.
The nucleus is around 10 000 to 100 000 times smaller across than the whole atom.

If an atom were a football stadium, the nucleus would be a pea on the centre spot. Everything else is electrons and a lot of nothing.

The three particles

particle	where it is	relative mass	relative charge
proton	nucleus	1	+1
neutron	nucleus	1	0
electron	shells around the nucleus	1/1836	−1

The electron's mass is tiny (you'll also see 1/1840), but it is not zero.

Where the mass and charge are

Mass: protons and neutrons are each about 1836 times heavier than an electron, so almost all of the atom's mass is in the nucleus.
Positive charge: all of it is in the nucleus (the protons).
Negative charge: spread through the space around the nucleus (the electrons).
A neutral atom has equal numbers of protons and electrons, so the charges cancel.

Definition

Atomic number (proton number), Z

The number of protons in the nucleus of an atom.

Definition

Mass number (nucleon number), A

The total number of protons and neutrons (nucleons) in the nucleus of an atom.

Quick check

Multiple choiceQuestion 1 · Recall

Multiple choiceQuestion 2 · Recall

Fill in the blanksQuestion 3 · Recall

Answer the 3 questions above to continue.