Molecular Weight:
The weight of a molecule, a molecular weight is the total mass of the individual atoms. Thus, it is fairly easy to calculate the mass of any molecule if its formula is known (which is, the elements and the number of each which make up the molecule). Remember that the words mass and weight are used interchangeably within chemistry.
Molecules of a Compound
Two atoms of variant elements held together through a bond form a compound. The molecule is the basic particle of a chemical compound. A few instances of this type of molecule involve hydrogen chloride (HCl), water (H2O), methane (C4H ), and ammonia (NH3).