molecular mass

Molecular mass, also called molecular weight, is the total mass of a molecule. It is calculated by adding the atomic masses of all the atoms that make up the molecule, based on their position in the periodic table. For example, a water molecule (H₂O) has two hydrogen atoms and one oxygen atom; by summing their atomic masses, we get the molecular mass. This measurement is usually expressed in atomic mass units (amu) or unified mass units, and it helps chemists understand the size, quantity, and reactions of molecules.