Question

Why is eight such a good number in covalent bonds?

Answer 1

Octect rule:
Atoms tendency of the elements in the periodic system of the elements, is to complete their last levels of energy with 8 electrones, since they obtain the electronic configuration of a noble gas, located at the right of the periodic table, they are inert, so it's very difficult for them to react with other elemetns, though they're electrochemically stable, since they accomplish Lewis' law o the Octect rule.