Question

Which of the following pairs are isotopes?

• A. ${}_{84}{}^{}\mathrm{X}^{209}\mathrm{and}{}_{84}{}^{}\mathrm{X}^{210}$
• B. ${}_{90}{}^{}\mathrm{X}^{232}\mathrm{and}{}_{91}{}^{}\mathrm{X}^{232}$

Answer 1

The correct option is A

${}_{84}{}^{}\mathrm{X}^{209}\mathrm{and}{}_{84}{}^{}\mathrm{X}^{210}$

Explanation of correct option

ISOTOPES:

• The isotopes of an element have the same atomic number but different atomic masses.
• The difference in their masses is due to the presence of a different number of neutrons.

Example: ${}_{84}{}^{}\mathrm{X}^{209}\mathrm{and}{}_{84}{}^{}\mathrm{X}^{210}$ has the same atomic number 84 but different atomic mass 209 and 210 respectively.

Explanation of Incorrect option

ISOBARS:
The atoms of different elements with different atomic numbers, which have the same mass number are called isobars.

Example: ${}_{90}{}^{}\mathrm{X}^{232}\mathrm{and}{}_{91}{}^{}\mathrm{X}^{232}$ has the same atomic mass 232 but different atomic numbers 90 and 91 respectively.

Hence, Option(A) is the correct option.