This document discusses isotopes, atomic mass, and molecular mass. It defines isotopes as atoms of the same element that have the same number of protons and electrons but different numbers of neutrons. Atomic mass is determined relative to carbon-12, which is assigned a mass of 12 atomic mass units. Molecular mass is calculated as the sum of the atomic masses of each atom in the molecular formula. It represents the mass of one molecule relative to 1/12 the mass of one carbon-12 atom.

1. Standard/ Class/ Grade XI Chemistry
Chapter 1 Basic Concepts
Gurudatta K Wagh, gkwagh@gmail.com
Isotopes, Atomic mass, Molecular mass
Isotopes
Atoms of the same elements having same atomic number
containing same number of protons and electrons but
different number of neutrons
Have different mass numbers
Hydrogen – Three isotopes, H-1 (protium 1H, 1 proton and
no neutron, 99.98 %), H-2 (1 proton and 1 neutron)
(deuterium 2H), H-3 (tritium 3H) (1 proton and 2 neutrons),
observed atomic mass 1.008 u

2. Neon-20 (19.9924 u), Ne-22 (21.9914 u), Ne-21 (20.9940
u). Average atomic mass of Ne = 20.1707 u
Observed atomic mass is the average atomic mass taking
into consideration the natural abundance (per cent
occurrence) of the isotopes

3. Atomic mass
Masses of atoms of elements is determined relative to mass
of a standard
Carbon-12 is chosen as standard
Atomic mass unit (amu) or unified mass (u) of carbon is 12
Masses of other atoms are determined relative to the mass
of an atom of carbon-12
Carbon-12 (98.89 %), C-13, C-14 are the three isotopes of
carbon

4. Atomic mass unit (amu)
or unified mass (u) of
carbon is 12
Masses of other atoms
are determined
relative to the mass of
an atom of Carbon-12

5. Molecular/ molar mass
Relative mass compared to mass of standard atom of
Carbon-12
Ratio of mass of one molecule of a substance to 1/12th of
mass of one atom of Carbon-12
Algebraic sum of atomic masses of constituent atoms
Unitless quantity
Molar mass expressed in gram is gram molar mass
1 gram molar mass = 1 gram molecule = 1 mole = 1 gram
atom

6. It is calculated as the sum of
the atomic mass of each
constituent atom multiplied
by the number of atoms of
that element in
the molecular formula
Molecular mass or
molecular weight is the
mass of a molecule. The
adjective 'relative' is
omitted as it is universally
assumed that atomic and
molecular masses are
relative to the mass of 12C