Question

Molecular formula of Sulphur is S8, but why in chemical reaction sulphur is written as S and not S8

Answer 1

This is because one molecule of sodium contains 8 atoms. However, a molecule is the smallest unit which can participate in a Chemical Reaction. Hence we only write the valencies (or the combining capacity) of an element in a Chemical Reaction and not the number of atoms it contains. 

Hope this helps. Correct me if I am wrong.