Classically speaking, an atom is the smallest, indivisible particle of matter. Although this is a simplistic measure, it is more or less true. 

A molecule is a combination of atoms, forming simple combinations as small as two atoms, or as large as many hundreds. 

An ion can be an atom that has a charge associated with it. A cation is an atom that has one less electron in its shell, giving it a positive charge ( the positive proton is the culprit here), whereas an anion is a negatively charged atom, usually having an extra electron. (electrons have a negative charge). 

However, an ion may also refer to a molecule, but simplistically, refer to an ion as a particle that has a electrical charge to it, being either negative or positive. 

helium has  a completely filled electronic or valnace shell .Thus it has valency 0