139.

What do you understand by:
(i) Atomic Mass?
(ii) Gram atomic mass(gram-atom)?

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Atomic Mass. The atomic mass of an element is defined as the average relative mass of the atom as compared to the mass of the atom of carbon (C¹²) taken as 12. In other words, atomic mass is a number which tells us as to how many times an atom of the element is heavier than  It is therefore, called relative atomic mass.
Thus, Relative atomic mass

The term relative used in the above definition means that the atomic masses are not actual masses of the atoms but are obtained by comparison with carbon.
The term average is used in the definition because atoms of the same element may have different atomic masses (isotopes) and hence an average value is taken. For example, there are two isotopes of chlorine having atomic masses 35 and 37 respectively. These isotopes occur in chlorine in the ratio 3:1, thus, Average atomic mass of chlorine


(ii) Gram atomic mass(gram-atom). It is the quantity of an element whose mass in grams is numerically equal to its atomic mass on amu scale. In other words, atomic mass of an element in grams is called gram atomic mass. It is also called gram atom. For example,

Atomic mass of hydrogen (1 gram atom of hydrogen) = 1·008 g
Atomic mass of oxygen (1 gram atom of oxygen) = 16 g
Atomic mass of sodium (1 gram atom of sodium) = 23 g
Number of gram atoms and mass in grams of an element are related by the following relation:
No. of gram-atoms =