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1.BASIC CONCEPTS eLearn.Punjab

1.2 RELATIVE ATOMIC MASS

Relative atomic mass is the mass of an atom of an element as compared to the mass of an

atom of carbon taken as 12.

The unit used to express the relative atomic mass is called atomic mass unit (amu) and it is 1/12

th of the mass of one carbon atom, On carbon -12 scale, the relative atomic mass of 162C is 12.0000

amu and the relative atomic mass of 1 H is 1.008 amu. The masses of the atoms are extremely
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small. We-don’t have any balance to weigh such an extremely small mass, that is why we use the

relative atomic mass unit scale.

The relative atomic masses of some elements are given in the following Table (1.1).

Table (1.1) Relative atomic masses of a few elements

Element Relative Atomic Mass Element Relative Atomic Mass
(amu) (amu)

H 1.008 Cl 35.453

O 15.9994 Cu 63.546

Ne 20.1797 U 238.0289

These element have atomic masses in fractions and will be explained in the following article on
isotopes.

1.3 ISOTOPES

In Dalton’s atomic theory, all the atoms of an element were considered alike in all the properties
including their masses. Later on, it was discovered that atoms of the same element can possess
different masses but same atomic numbers. Such atoms of an element are called isotopes.
So isotopes are different kind of atoms of the same element having same atomic number, but
different atomic masses. The isotopes of an element possess same chemical properties and same
position in the periodic table. This phenomenon of isotopy was first discovered by Soddy. Isotopes
have same number of protons and electrons but they differ in the number of neutrons present in
their nuclei.

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