"when determining the formula for a compound, it is important to know the valence numbers of each element. sup

Answer 1

Yes. Lets take water for instance. Why is the formula for water H2O? Hydrogen has a valence of +1 while Oxygen has a valence of -2. So Oxygen has room for 2 hydrogens. The valence level is the outer most level of the atom. It is here where they react. The octet rule which states an atom is looking to satisfy its valence level by either filling it to the max with 8 electrons or rid it of the ones it does not want in order to become stable. Hydrogen would have to pick up 7 which is not that easy so it gives up the one it has. Oxygen on the other hand will pick up 2 instead of giving up the 6 it has.

Answer 2

it is beneficial