An atom is the smallest chemically indivisible part of an element - that means you can't split it by chemical (or physical) means into anything smaller - it takes a nuclear reaction to do it!

All of the different types of atom known to exist in the universe are listed in the periodic table.

Here is a simple diagram of the atom - you should KNOW this... including the sizes! Click here to find out how we know that the atom is like this.

At KS4 you do not need to know the mass of the particles in the atom in kiliogrammes. You just need to be able to compare the masses - to know that the mass of the proton is roughly the same as that of the neutron but that you would need almost 2000 electrons to have the same mass of electrons as that of a proton or neutron!

The above diagram is not to scale. If it was drawn to scale and you had a nucleus of 1 cm diameter in the centre of your screen you would need a screen of 1km diameter to show the full graphic! Most of the atom is empty space!.

For Chemists among you here is an interactive illustration of the orbital shapes of electrons.

Click here to find out more about the nucleus and click here for an overview of the atom.