Atomic and molecular masses:-

The mass of an atom is very-very small before 1961, Hydrogen was taken as standard by assigning arbitrarily its mass 1( without units)

(because hydrogen is lightest element) and other elements were given masses relative to it, but it does not react with all the elements and due to many other reasons. In 1961 a meeting at most abundant isotope of carbon C-12 taken as standard and it has been assigned exactly 12 amu(atomic mass unit) and

1 amu is = 1/12 x mass 1 C-12 atom

And 1 amu = 1.660 x 10^-24g

Mass of oxygen is 15.995 amu

Today amu is replaced by ‘u’ (unified mass)

Average atomic mass:- as all the elements have many isotopes (the atoms of same element having similar atomic no. but different mass no). in different % age in nature, so the average atomic masses are taken eg. Carbon has 3 isotopes in different abundance like:-

Isotope	% age	Atomic
12C	98.892	12
13C	1.108	13.0033
14C	2 x 10-10	14.0031

 

similarly average atomic masses of other elements are also calculated.

“the elements with fractional atomic masses are supposed to exist as isotopes.”

Molecular mass:- “ it is defined as sum of atomic masses of the elements present in a molecular”

Its obtained by multiplying the atomic mass of each element by the number of its atoms and adding then together.

eg. Mass of Methane (CH₄)

= 12.11 + 4 x 1.008 = 16.043

Molecular mass of glucose (C₆ H₁₂ O₆)

6 x 12.011 + 12 x 1.008 + 6 x 16 = 180.162 u

Formula mass:-  some substances do not exist as discrete molecules like NaCl. In such cases formula mass is calculated, eg. Formula mass of NaCl is =23.0 u + 35.5 u = 58.5 u.

Gram atomic mass is the quantity of element whose mass is grams is numerically equal its relative atomic mass eg.

Mass of an atom of Oxygen = 16 u.

Mass of gram atom of Oxygen = 16 g.