Glenn Walsh, of King's High School, asks :-

Why are the elements numbered?

Jim Simpson, a chemist at the University of Otago, responded.

Each pure element contains only one type of atom. Atoms are incredibly small, less than one millionth of a centimetre across, and are made up of three sorts of even smaller particles.

Protons, which have a positive electrical charge, and neutrons, which are not charged, make up the nucleus, or centre, of the atom. Around the nucleus orbit electrons. These have a negative charge, equal but opposite to that of the proton.

The atoms of all elements are electrically neutral, therefore the number of protons in the nucleus of an atom of the element must be equal to the number of electrons around the nucleus of the element. The number of each is given a special name - it is called the atomic number of the element. This number therefore tells us the number of protons and also the number of oppositely charged electrons that exist in a particular element.

When atoms of elements react to form compounds, it is the outer electrons which determine their chemical properties. Hence the atomic number of an element helps us to classify the element in terms of its chemical properties.

It is interesting to note that new heavy elements can be manufactured. In November and December of 1994 the most recent, radioactive, 'artificial' elements were made by scientists working in Germany. These have atomic numbers 110 and 111 respectively, but have yet to be given names.