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 .

Answer:

(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:

And

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.

Answer:

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.

Given

Answer:

Atomic mass of nitrogen,

A nucleus of nitrogen contains protons and neutrons.

Hence, the mass defect of this nucleus,

Where,

Mass of a proton,

Mass of a neutron,

But

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

Where,

Speed of light

Hence, the binding energy of a nitrogen nucleus is.