Physics Class 12 NCERT Solutions: Chapter 13 Nuclei Part 1

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Average atomic mass Definition

Average Atomic Mass Definition

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Q: 1. (A) Two stable isotopes of lithium and have respective abundances of and. These isotopes have masses and , respectively. Find the atomic mass of lithium.

(b) Boron has two stable isotopes, and. Their respective masses are and, and the atomic mass of boron is. Find the abundances of and .


(A) Mass of lithium isotope ,

Mass of lithium isotope ,

Abundance of

Abundance of ,

The atomic mass of lithium atom is given as:

(B) Mass of boron isotope ,

Mass of boron isotope,

Abundance o, ,

Abundance on,

The atomic mass of boron atom is given as:


Hence, the abundance of is and that of is

Q: 2. The three stable isotopes of neon: have respective abundances of . The atomic masses of the three isotopes are and, respectively. Obtain the average atomic mass of neon.


Atomic mass of,

Abundance of,

Atomic mass of,

Abundance of,

Atomic mass of,

Abundance of,

The average atomic mass of neon is given as:

Q: 3. Obtain the binding energy of a nitrogen nucleus.



Atomic mass of nitrogen,

A nucleus of nitrogen contains protons and neutrons.

Hence, the mass defect of this nucleus,


Mass of a proton,

Mass of a neutron,


Hence, the binding energy of the nucleus is given as:


Speed of light

Hence, the binding energy of a nitrogen nucleus is.