The number of electron or protons present in any atom is called ‘Atomic Number’ of that atom. It is denoted by Z. For example:
Atomic Number of Na is 11, Atomic Number of Ca Is 20, Atomic Number of O is 8
The sum of Number of protons and number of neutrons present in the nucleus of an atom is called “Mass Number 1 of that atom. It is denoted by A.
Na has 11 protons and 12 neutrons then
Atomic Mass Number = No. of Protons + No. of neutrons =11 + 12
Atomic Mass Number = 23 a.m.u
The algebraic sum of Atomic mass of all the atoms of each element present in a molecule is called ‘Molecular Mass’ of that molecule.
2(23) + 1(16) + 3(16) = 46 + 12 + 48 = 106 a.m.u
4(39) + 1 (56) + 6 (12) + 6 (14) = 156 + 56 + 72 + 84 = 368 a.m.u
In crystalline substances atoms do not aggregate into molecules but are bonded in a network structure like NaCl, MgBr2, BaSO2 etc. For these types of compounds a formula mass is assigned, that can be defined as:
‘The sum of the atomic masses as given in the simplest formula of a crystalline
substance is called Formula Mass.
For example: NaCI à 1 : 1 à 23 + 35.5 = 58.5 a.m.u