Notes On Atomic and Molecular Masses - CBSE Class 11 Chemistry
The atomic mass of an element is the number of times an atom of that element is heavier than an atom of Carbon -12.

One atomic mass unit is defined as a mass equal to 1/12th  the mass of Carbon -12 atom. Average atomic mass of an isotope is the sum of abundance of each isotope multiplied by precise atomic mass.

Ex: Average atomic mass of Chlorine.
Isotope Atomic Mass Relative Abundance (%)
  35Cl      34.97           0.755
  37Cl      36.97           0.245

                                          Chlorine Isotopes

Average Atomic Mass = Sum of abundance of each isotope multiplied by precise atomic mass
Average Atomic Mass = 34.97 x 0.755 + 36.97 x 0.245 = 35.46 u

Fractional abundance of an isotope is the fraction of total number of atoms that is comprised of that particular isotope. Average atomic mass can also be calculated as the sum of the products of fractional abundance of the isotopes and their corresponding mass numbers.

When the atomic mass of an element is expressed in grams, it is called gram atomic mass. Gram atomic mass is also called one gram atom of the element. The molecular mass of substance (element or compound) is the number of times the molecule of the substance is heavier than 1/12th the mass of an atom of Carbon -12.

The molecular mass of any element or compound can be calculated by adding the atomic mass of all the atoms present in one molecule of that element or compound.

Ex:
Molecular mass of H2O = (2 * Atomic mass of Hydrogen) + Atomic mass of Oxygen
= (2*1.008) + 16.0
= 18.02 u

When the molecular mass is expressed in grams, it is known as gram molecular mass. Gram molecular mass is also called 1gram molecule of the substance. Formula mass is the sum of atomic masses of atoms present in one formula unit.

Ex: Formula mass of Sodium chloride (NaCl) is calculated as
Formula mass = Atomic mass of Sodium + Atomic mass of Cl
= 23.0 u + 35.5 u
= 58.5 u

When the formula mass is expressed in grams, it is known as gram formula mass.

Summary

The atomic mass of an element is the number of times an atom of that element is heavier than an atom of Carbon -12.

One atomic mass unit is defined as a mass equal to 1/12th  the mass of Carbon -12 atom. Average atomic mass of an isotope is the sum of abundance of each isotope multiplied by precise atomic mass.

Ex: Average atomic mass of Chlorine.
Isotope Atomic Mass Relative Abundance (%)
  35Cl      34.97           0.755
  37Cl      36.97           0.245

                                          Chlorine Isotopes

Average Atomic Mass = Sum of abundance of each isotope multiplied by precise atomic mass
Average Atomic Mass = 34.97 x 0.755 + 36.97 x 0.245 = 35.46 u

Fractional abundance of an isotope is the fraction of total number of atoms that is comprised of that particular isotope. Average atomic mass can also be calculated as the sum of the products of fractional abundance of the isotopes and their corresponding mass numbers.

When the atomic mass of an element is expressed in grams, it is called gram atomic mass. Gram atomic mass is also called one gram atom of the element. The molecular mass of substance (element or compound) is the number of times the molecule of the substance is heavier than 1/12th the mass of an atom of Carbon -12.

The molecular mass of any element or compound can be calculated by adding the atomic mass of all the atoms present in one molecule of that element or compound.

Ex:
Molecular mass of H2O = (2 * Atomic mass of Hydrogen) + Atomic mass of Oxygen
= (2*1.008) + 16.0
= 18.02 u

When the molecular mass is expressed in grams, it is known as gram molecular mass. Gram molecular mass is also called 1gram molecule of the substance. Formula mass is the sum of atomic masses of atoms present in one formula unit.

Ex: Formula mass of Sodium chloride (NaCl) is calculated as
Formula mass = Atomic mass of Sodium + Atomic mass of Cl
= 23.0 u + 35.5 u
= 58.5 u

When the formula mass is expressed in grams, it is known as gram formula mass.

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