What is meant by covalency (valence)? explain with example. And oxygen has covalency of 2 while sulphur has covalency up to 6. Why this is so as both belong to the same group?
The valence of certain element is the number that shows how many monovalent atoms (e.g. hydrogen) it can bind or replace. The essence of this concept is connected with structure of atomic orbitals and electrons on it. The max amount of electrons located on the external atomic orbital is 8. Different elements have different amount of electrons there, which are paired or unpaired, so it determines the chemical properties and their valences. Oxygen has two unpaired electrons on the external orbital; hence, its valence is 2. The difference between oxygen and sulfur is in their belonging to the different periods of periodic table. Oxygen is located in 2nd period it means that oxygen has only 2 orbitals (s- and p-orbital) and sulfur belongs to 3rd period and has the additional d-orbital. In case of sulfur, its paired electrons can shift to d-orbital and become unpaired. In such way, the amount of unpaired electrons on the external orbital increase and the valence rise to 6.