Atomic Masses

Atomic mass: It is the average relative mass of atom of element as compared with an atom of carbon –12 isotope taken as 12.

Atomic mass = average mass of an atom/1/12×Mass of an atom of C¹²

Average atomic mass: If an elements exists in two isotopes having atomic masses ‘a’ and ‘b’ in the ratio   m : n, then average atomic mass =  (m×a)+(n×b)/m+n Since the atomic mass is a ratio, it has no units and is expressed in amu, 1 amu = 1.66×10²⁴g. One atomic mass unit (amu) is equal to (1/12)th of the mass of an atom of carbon-12 isotope.

Gram atomic mass (GAM): Atomic mass of an element expressed in grams is called Gram atomic mass or gram atom or mole atom.

(i) Number of gram atoms = Mass of an element/GAM

(ii) Mass of an element in gm. = No. of gm. atom × GAM

(iii) Number of atoms in 1 GAM = 6.02 × 10²³ 

∴ Number of atoms in a given substance

= No. of  GAM  6.02 × 10²³ = (Mass/GAM) × 6.02 × 10²³ 

(iv) Number of atoms in 1gm of element = 6.02×1023/Atomic mass

(v) Mass of one atom of the element (in gm.) = GAM/6.02×10²³