Atoms - Electronic Structure

There are different ways of numbering the groups of the periodic table. Most UK exam boards still follow the convention that the number of electrons in the outer shell = number of group. If you study chemistry to higher levels, you will find that this is no longer the case other than for groups 1 and 2.

Atoms contain protons, neutrons and electrons.The electrons orbit the nucleus in discrete shells which can also be referred to as energy levels. The electrons are held in place by the postive charge of the nucleus. How these electrons are arranged is very important to their chemical properties - chemical reactions are simply an exchange or sharing of electrons. You need to be able to draw the electronic structures for the first 20 elements of the Periodic Table and will have noticed that the elements in an individual group all contain the same number of electrons in the outer shell.

Each electron shell can contain only a specific number of electrons. The first shell can contain up to two electrons at most, the second shell can contain up to eight electrons at most, as can the third. You don't need to know the maximum values for the other shells as the situation is a bit complicated and you only need to go up to the 20^th element, calcium.

During chemical reactions, electrons are either swapped or shared in order to give them a full outer shell. That makes them very stable. When they are swapped, the two atoms involved end up with an slight electrical charge. This is because they still have the same number of protons as they started with but end up with either more or fewer electrons. Atoms that lose electrons end up with less negative charge than at the start and vice-versa.

When working out what is likely to happen to an atom during bonding, there are a few general rules. These are not hard and fast but will help in most cases at high school. You need to look at what is likely to be the easiest thing for that atom to do to gain a full outer shell of electrons. Metals lose electrons as they have one, two or three electrons in their outer shell. Non metals will either share electrons with other non metals or gain electrons when reacting chemically with metals. The carbon group elements have to either gain or lose four electrons so they generally don't bond with metals, but share electrons with other non metal elements.