Atom means 'indivisible'. John Dalton, an English chemist put forward a theory to describe the nature of the atom - An atom is the smallest particle of an element which can take part in a chemical reaction.
MOLECULES
A molecule is the smallest particle of a substance that can normally exist alone and still retain the chemical properties of that substance.
ATOMICITY OF AN ELEMENT
This is the number of atoms in each molecule of an element.
IONS
An Ion is any atom or group of atoms which possesses an electric charge.
