Before you can write chemical equations you need to understand the idea of valencies and groups. For this we will need to use our old friend the periodic table of elements:

 Each vertical column is known as a group – see above. Each group has elements with similar chemical properties. The number of outer electrons is equal to the group number – for example group one has one outer electron. Every element wants to have a stable (full) outer electron energy level which is eight electrons.

In order to do this the atoms must gain or loose electrons dependant on which group they are in. Groups 1-3 lose electrons, groups 5-7 gain electrons and group four can either gain or lose electrons.     

                                                                                                               However, each group loses a different number of electrons. Let’s think about two examples: sodium and oxygen. Sodium is in group one so its outer energy level consist of one electron (no. of outer electrons= group number). The full electron energy arrangement for sodium is 2,8,1. Sodium wants to get a stable electron energy level so it looses on electron to leave it with electron arrangement 2,8 which is stable. Oxygen (2,6) wants to get a stable electron arrangement of 2,8 so it must gain two electrons.

 The number of electrons gained or lost makes up our valency table: 

We need to also consider charges. An atom has a neutral (no) charge and electrons are negatively charged. If your neutral atom gains a negative electron it becomes negative and if it loses electrons it becomes positive. The more electrons gained or lost the greater the positive or negative charge.