The structure of the atom

As you know atoms are very small but every atom has even smaller particles inside it.
All atoms consist of two parts:
(a) a central heavy NUCLEUS that contains
(i) PROTONS – particles with a unit positive charge
(ii) NEUTRONS - neutral particles with a mass slightly greater than that of a proton
(b) ELECTRONS orbiting the nucleus. These are particles with a negative charge, equal and opposite to that of a proton. They have a mass about 1/1860 of that of a proton.

We really ought to explain what we mean by light and heavy here.
You would need 1000 million million million million protons or neutrons to have a mass of one kilogram. This may seem a huge number but you would need almost 2000 times as many electrons to have the same mass!

The diagram below shows a simplified picture of the structure of an atom.

schoolphysics atom orbital model animation

To see an animation of the motion of electrons around a nucleus click on the animation link.

Atoms are also very small. We can think of an atom as a tiny sphere about a hundredth of a millionth of a centimetre in diameter.
This means that 10 thousand million atoms could be laid side by side along a one metre rule!

The number of PROTONS in the nucleus tells us what element the atom is:
If there are six it is carbon
If there are eight it is oxygen
If there are twenty-six it is iron
If there are ninety-two it is uranium

For normal atoms the number of protons is balanced by an equal number of orbiting electrons - this makes the atom as a whole neutral.

There are also neutrons in the nucleus of all atoms except the simplest form of hydrogen.


We use a special way of writing the structure of a nucleus so that is easy to see what the proton and nucleon numbers are.

This is how to write down the structure of carbon and uranium:

This means that in the nucleus of a carbon atom there are 6 protons and 6 neutrons making 12 particles (or nucleons). In a uranium atom there are 92 protons and 146 neutrons making 238 nuclear particles (nucleons).

You will find a list of the structure of some of the more common elements in the data section.

The diagrams below show the structure of a few of the lighter atoms.