Before we go any further we have to understand what makes up the matter around us – the computer in front of me, the air around us and even ourselves. We are all made up of atoms.

The word atom means “indivisible” or “that which cannot be split”. You don’t really want to be saying such things to these scientists. What did they do – they went and split it.

So what are atoms made up of?

	name	charge	mass	position
	Protons	+ve	1 atomic unit (au)	Inside the nucleus
	Neutrons	No Charge	1 atomic unit (au)	Inside the nucleus
	Electron	-ve	Very small	Orbits around the nucleus

The protons and neutrons make up what is called the nucleus of the atom. The electrons orbit around the nucleus. The mass of the electron is very much smaller than that of the proton and neutron and we shall ignore it through the rest of this talk. The mass of the neutron and proton are about the same – about 1.66�10^-27 Kg (if you do not understand this notation see here). For convenience masses are usually stated in atomic mass units (amu) which is just the number of neutrons and protons i.e. eight protons and eight neutrons would weigh 16 atomic mass units.

To be electrically neutral an atom has to have the same number of electrons as protons.

The number of protons (and therefore electrons) in an atom determine its chemical properties. Atoms with different number of protons are different chemical elements and have different chemical properties. For example the simplest element – Hydrogen – with one proton is a gas. The element with three protons – Lithium – is a reactive metal.

Every element is given a chemical symbol – usually one or two letters. For example Hydrogen is H, Helium is He, Carbon is C and Uranium is U.

Atomic Number

The number of protons in an atom is called the atomic number of that atom. Every element has a unique atomic number.

Atomic Mass

The atomic mass is the number of neutrons plus the number of protons. Because it is possible to have the same element with different numbers of neutrons then the atomic mass is not unique to that element. Elements with different numbers of neutrons in the nucleus are called isotopes.

Sometimes isotopes are written as follows:

In reality there is no need to put the atomic number since that is already specified by the chemical symbol. Hydrogen always has atomic number 1, uranium always has atomic number 92 etc.

Let us look at a couple of examples
Atomic number is 1 (therefore Hydrogen with 1 proton) and the atomic mass (neutrons plus protons) is 3. Therefore the number of neutrons is 3- 1 = 2. This is not the isotope of hydrogen normally found – we will come onto that later.

Atomic number is 92 (therefore Uranium with 92 protons) and the atomic mass (neutrons plus protons) is 238. Therefore the number of neutrons is 238 – 92 = 146

Isotopes are often refered to just by the element name followed by the atomic mass. For example Uranium-238 or Carbon-14.

Atoms and Molecules

Atoms can join together to form molecules. For example an oxygen atom and combine with two hydrogen atoms to produce H2O – water. How atoms combine and the properties of the resultant molecules depends on the number of electrons (and hence protons) and is what is studied in chemistry. For the moment we are going to forget about the electrons and chemistry and concentrate on the nucleus.

Atoms and molecules are extremely small. For example there are about 167,000,000,000,000,000,000,000 (1.67×10²³ if you don’t understand what this means see here) molecules in a teaspoonful of water. If everyone in the world started counting one each second then it would take three quarter of a million years to count them all.

Wikipedia