Physics Class 12 NCERT Solutions: Chapter 13 Nuclei Part 1 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Average Atomic Mass Definition

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 .