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

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.

Atoms are neutral particles - they have no net charge

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

The total number of NEUTRONS and PROTONS in a nucleus is called the NUCLEON NUMBER or MASS NUMBER
The number of PROTONS in the nucleus is called the PROTON NUMBER or ATOMIC NUMBER
The number of NEUTRONS in the nucleus is called the NEUTRON NUMBER

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.