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Atoms & Nuclei

JEE Main level, single correct, medium

159 of 239

Question 159 of 239

Which of the following pair of nuclei are isobars of the element?

Show answer and solution

Answer: (A) ${ }_{1}^{3} \mathrm{H}$ and ${ }_{2}^{3} \mathrm{He}$

Isobars are atoms of different chemical elements that have the same number of nucleons (mass number

A

), but different atomic numbers (

Z

).
In the given options:
(1)

_1^3\text{H}

and

_2^3\text{He}

: Both have the same mass number

A = 3

. Since they have different atomic numbers (

Z=1

and

Z=2

), they are isobars.
(2)

_1^2\text{H}

and

_1^3\text{H}

: These have the same atomic number (

Z=1

) but different mass numbers. They are isotopes.
(3)

_{80}^{198}\text{Hg}

and

_{79}^{197}\text{Au}

: These have different mass numbers (

198

and

197

).
(4)

_{92}^{236}\text{U}

and

_{92}^{238}\text{U}

: These have the same atomic number (

Z=92

) but different mass numbers. They are isotopes.
Thus, the pair

_1^3\text{H}

and

_2^3\text{He}

are isobars.

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