First things first atoms are the building blocks of matter – we’re clear on that.

However, even atoms are made up of stuff, and the smaller stuff is known as sub-atomic particles.

In this section it’s these sub-atomic particles that we’re going to focus on. Sub-atomic particles are made up of protons, neutrons and electrons which are all held together by strong nuclear forces.

So far so good!

Unsurprisingly the mass of these particles is very very small – and I mean tiny!

Both protons and neutrons have a mass of one atomic mass unit. They are all together in the nucleus, but they cannot repel one another because of the strong nuclear force exerted by the protons and the neutrons.

You may or may not know but atoms have a neutral charge and this is because they carry the same number of protons and electrons to cancel each other out.

Protons carry a single positive charge and the electrons carry a single negative charge, so in the neutral atom there are always the same number of protons and electrons.

Electrons have a single negative charge and only weigh approximately 0.005 atomic mass unit. So if you thought neutrons and protons were small – electrons are even smaller!

Ions are formed when atoms either lose or gain electrons to attain a full outer shell. Non-metals gain electrons and turn into negative ions. Metal atoms lose electrons and become positive ions.
Atoms also have this thing called isotopes. These are atoms of a specific element and have a specific number of neutrons.

In other words, isotopes are atoms of an element that have a differing number of neutrons in the nucleus, but the same number of protons, wso they have the same atomic number but differing atomic mass numbers.

Most elements have several isotopes, some of which are stable, and others that spontaneously break apart releasing radioactivity.

To find the mass number of an atom you simply add together the number of protons and electrons.

For instance is an atom has seven neutrons and seven protons, its mass number (A) is 14.

To find the atomic number we look at the protons in an atom.

For example look at chlorine, it has 17 protons and so it has an atomic number (Z) of 17.

You can also calculate the number of protons, electrons and neutrons. Let’s use sodium as an example.

The atomic number, which is the number of protons, is 11. The number of protons is equal to the number of electrons. So sodium has 11 protons and 11 electrons. The mass number is 23. The number of neutrons = mass number – atomic number. The number of neutrons = 23 – 11 = 12.