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Earlier we spoke about everything been made up of particles, from a lump of gold to our entire body! Now we are going to give these particles a name - atoms.

Single atoms are far too small to be seen even with the most powerful microscope. However, in spite of their tiny size, we know an awful lot about atoms.

Scientists have found that every atom contains a nucleus, and a cloud of particles called electrons that rotate around the nucleus in the centre of the atom.


We have already said that atoms consist of a nucleus surrounded by a cloud of electrons whizzing around it. However, the nucleus itself is composed of two particles, neutrons and protons.

All particles are light so scientists measure their masses in atomic mass unit instead of grams.

Protons and electrons carry a charge; neutrons do not.

Particle in atom: Mass: Charge:
proton 1 unit positive (+1)
neutron 1 unit neutral
electron negligible negative (-1)


A magnesium atom:

The different energy levels for the electrons are called energy shells. Each shell can hold a limited number of electrons.


The first shell can hold 2 electrons.

The second shell can hold 8 electrons.

The third shell can hold a maximum of 8 electrons.

A magnesium atom holds 2 electrons in the third shell only. This is not a full shell or octet.

The magnesium atom is often written as 2,8,2 - this is known as its electronic configuration.

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The above table supplies the reader with plenty of facts about individual atoms belonging to all the known elements. This table is known as the PeriodicTable.

Each element is given a symbol, a sort of shorthand version of its name, for example Sodium has the symbol Na. Next to the symbol are two numbers, the top number (the larger of the two), is known as the mass number, the lower number (smallest of the two) is the proton number.

The periodic table is arranged in order of proton number, a hydrogen atom having the smallest!

Look at the magnesium atom diagram again. It has 12 protons. This fact is used to identify an atom since it is specific to them.

For example: only a magnesium atom has 12 protons, only a sodium atom has 11 protons!

The magnesium atom has 12 electrons. The number of electrons for an atom always equals the number of protons. This means that their opposite and equal charge cancel one another out. Atoms are neutral!

The electrons in an atom have negligible mass. So the mass number only takes into account the number of protons and neutrons.

The mass no. = the no. of protons + the no. of neutrons.

Since we already know the number of protons, as given by the proton number, we can calculate the number of neutrons.

For a magnesium atom:

Mass number = 24

Proton number =12

24 = 12 + the number of neutrons

Therefore, number of neutrons in a magnesium atom = 24 - 12 = 12 neutrons.

The atoms of an element are not always the same! Although they may contain the same number of protons their neutron numbers may differ from atom to atom.


Isotopes are atoms of the same element, with the same number of protons but different numbers of neutrons.