Question

Structures of nuclei of three atoms A, B, and C are given below: A has 90 protons and 146 neutrons. B has 92 protons and 146 neutrons. C has 90 protons and 148 neutrons. Based on the above data, which of these atoms are isotopes and which are isobars?

• A. A and C are isotopes; B and C are isobars
• B. A and B are isotopes; A and C are isobars
• C. B and C are isobars; A and B are isotopes
• D. A and C are isotopes; A and B are isobars

Answer 1

The correct option is A

A and C are isotopes; B and C are isobars

The mass number of A = number of protons + number of neutrons in A = 90+146=236

The mass number of B= number of protons + number of neutrons in B = 92+146 =238

The mass number of C= number of protons + number of neutrons in C = 90+148=238

So, A,B and C can be represented as:

${}_{90}^{236}A,{}_{92}^{238}B,{}_{90}^{238}C$

${}_{90}^{236}Aand{}_{90}^{238}C$ are isotopes as they have same atomic number but different atomic mass/mass number
${}_{92}^{238}B,{}_{90}^{238}C$ are isobars as they have same atomic mass/mass number but different atomic numbers.