Can somebody please help me with B and C if you can’t do them both just to be will give brain thanks

Plz choose one of the compounds from the table and explain how you know the numbers of atoms in your formula. is it possible for two different compounds to be made from the exact same two elements? why or why not? with a limited number of elements (less than 120 are known), does this mean we also have a small number of compounds or do we have a large number of compounds in this world?

Which other elements contain the same number of outer electrons as sodium

Ineed ! i have no clue how to do this problem.