Question

Assertion: Atomic Mass of certain elements are expressed in fraction taking in account the availability of its various isotopes.
Reason: Isotopes of an element have the same atomic mass but different atomic number.
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Answer 1

Explanation:

Atoms of different elements have different masses because different atoms have a different number of subatomic particles i.e neutrons, protons and electrons.

Atomic mass of an element cannot be same as the mass of a single atom of that element because by investigation of mass spectrometry it was found that many naturally occurring elements exist in the form of two or more isotopes. Isotopes are the atoms of the same elements having the same atomic number containing the same number of protons but the different number of neutrons and because of this, it possesses different mass number. So, the observed atomic mass of the atom of an element is the average atomic mass of the elements. Average of masses of a different isotope of the same element is taken and then the atomic mass of the element is decided.

A formula to calculate average atomic mass:-

Example:-

Lithium exists in nature in the form of two isotopes Li−6 and Li−7 with atomic masses 6.0151u and 7.0160u and the percentages 7.59 and 92.41 respectively.

Solution:- Average atomic mass of Li

[Atomic mass of Li−6× percentage + Atomic mass of Li−7× percentage]/100

=

100

6.015u×7.59+7.0160u×92.41

=6.939u

Hence, the atomic mass of an element is not the same as the mass of a single atom of that element; Atomic mass is average of isotopic mass o atoms.