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You need to know about the structure of an atom and the particles that make it up.

All atoms are made up of the same 3 basic particles:

  • protons
  • electrons
  • neutrons.

The only difference between one atom and the next is the number of these particles in the atom. That is enough to make things as different as gold and oxygen.

Neutrons and protons are heavy in comparison to electrons. In fact, a neutron or a proton weighs about 2000 times as much as an electron!

The other thing to remember is that protons have a positive charge, electrons have a negative charge and neutrons have no charge at all.

An atom is not a solid thing. In fact, quite the opposite. Atoms are nearly completely empty.

Protons and neutrons are tightly clumped together in the middle, in the nucleus, while electrons spin around them. This diagram gives you an idea of what they look like, but it is not to scale:

Atomic structure

To give you an idea of the proportions, imagine a full size football stadium. The nucleus would be equivalent to the size of an ant in the middle, with the electrons whizzing around the outskirts.

The central part of the atom is called the nucleus. That's where you find all the protons and neutrons.

As we said above, protons and neutrons are heavy compared to electrons, so you can see that all the mass is concentrated in the middle of the atom. Also, as all the protons are in the nucleus of the atom, the nucleus has a positive charge.

The electrons (negatively charged) orbit around the outside of the atom.

So what have we learnt so far?

An atom is made up of mostly empty space.

Protons have a positive charge and a lot of mass

Neutrons are neutral but are as heavy as protons.


To make sure you understand, here is a quick test. See if you can identify the correct sub-atomic particle in the table below. Then mark your answer:

Copyright S-cool

Scientists used to think that atoms were solid. They thought that they were a bit like plum puddings - a light sponge pudding with bits of plum mixed into it.

Atomic structure

This was a good model because:

  1. Atoms occupied the correct amount of space (it was the space filled by the sponge).
  2. The mass of the atom was about right (because although the sponge filled the whole of the atom, it was very low density).
  3. The charge on the positive sponge was balanced by the negative plums so that overall the atom was neutral.

Then a man called Ernest Rutherford performed a now famous experiment. He fired an alpha particle (α - particle) at a thin sheet of gold foil.

α - particles are fast moving, small, dense, positively charged particles, and all predictions said that they should smash through the soft spongy 'plum pudding' atoms almost unaffected.

And most of them did! So far so good.

But Rutherford noticed that some α - particles were deflected through big angles. Some even bounced straight back.

That seemed impossible - there shouldn't be anything dense enough in the 'plum pudding' atom to make the α - particle bounce back.

So this model was scientifically proven to be incorrect

The diagram below shows what happens to some of the α -particles as they approach a nucleus.

Copyright S-cool

These observations allowed Rutherford to work out what was inside the atom.

The Table below shows each of the observations Rutherford made to determine the structure of the atom:

Observation Deduction
Alpha particle bounces back Shows there must be small concentrated masses in the atom that were dense enough to make the alpha particles rebound. This is the evidence for the nucleus.
Alpha particle changing direction The alpha particle is deflected by the gold nucleus as they both have a positive charge. This showed that the nucleus of an atom is positively charged.
Alpha partcles pass straight through Many alpha particles passed straight through the gold showing that most of the atom is made up of space.

The model of the atom that Rutherford came up with is called the 'nuclear model'. It is the model of the atom that we still use today.