Question

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

• 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

Isotopes are the atoms which have the same atomic number but different mass number. This is because they contain different number of neutrons.

Atomic number = number of protons

The representation of the given atoms is mentioned below.

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

Here, A and C are isotopes because they have:
- same atomic number (90), i.e. same number of protons.
- different mass numbers (236 and 238) as number of neutrons are different.