Atomic Number, Electron Number, Mass Number and Isotopes

Back to Chemistry Grade 9/10

The video below is a basic chemistry tutorial that covers the below concepts. It aims to meet the GCSE (high school grade 9/10) requirements in Unit 1 Chemistry, and some Unit 2 Chemistry, as well as Science A Unit 2

Check out the bottom of this page for associated resources.

Atomic number vs Mass Number

We know that in their ground state, atoms have the same number of electrons as protons - but how many is that? Well, you can work it out by looking at the atomic number, and the mass number.

The atomic number is the number of protons in an elements' atom; it's what defines that atom as being from that element. For example,

The mass number is the number of neutrons + protons in an atom i.e. the number of subatomic particles in its nucleus. It is not to be confused with the mass of an atom (more on that here).

How many protons, electrons and neutrons are there in an atom?


The atomic number tells us how many protons there are in an atom.


In its ground state, atoms have the same number of protons as electrons; so the atomic number will also tell us how many protons there are.


To work out the number of neutrons in an atom, you need both its mass number and atomic number; then you can do some simple algebra:

NB:The video goes through three different examples, using the periodic table.

What are isotopes?

Although the atomic number will be the same for all atoms of a certain element (e.g. all carbon atoms will have 6 protons, and therefore an atomic number of 6), the neutron number can vary. This means that their mass number can vary.

Atoms that have the same atomic number but different mass number are called isotopes. The video above goes through an example of two isotopes. If you'd like to work through some more exercises, check out this page. It contains another video with more examples, as well as a worksheet for you to work through in your own time to consolidate your understanding.

Make notes

Want to make notes as you follow along? Click on this image to download the document:

If you'd like a document of chemical element symbols with their names, sorted by atomic number, click on this image:

If you'd like a copy of the periodic table with atomic numbers and mass numbers, click here: