Question

what is valency of any atom and what is molecules

Answer 1

The electron present in the outermost orbit of an atom is called valence electron. Ex Na (atomic no.-11) 2,8,1 so valence is 1
Molecule - the simplest structure unit of an element or compound
Or a tiny piece of anything

Answer 2

A molecule is a group of more than 1 atoms of same kind or different.
Valency is the no. of electrons that an atom can gain or loose during a chemical reaction.