Question

Which one of the following is a pair of isotones?

• A. \( ^{40}_{18}\mathrm{K} \) and \( ^{40}_{19}\mathrm{Ar} \)
• B. \( ^{13}_{7}\mathrm{N} \) and \( ^{14}_{7}\mathrm{N} \)
• C. \( ^{37}_{17}\mathrm{Cl} \) and \( ^{40}_{18}\mathrm{K} \)
• D. \( ^{198}_{80}\mathrm{Hg} \) and \( ^{197}_{79}\mathrm{Au} \)

Hint:

Always remember: {Isotones = same number of neutrons}. Calculate neutrons using (Mass number − Atomic number) to quickly identify them.

Answer 1

The Correct Option is D

Step 1: Understanding isotones.
Isotones are atoms of different elements that have the same number of neutrons but different atomic numbers. The number of neutrons is calculated as:
\[ \text{Number of neutrons} = \text{Mass number} - \text{Atomic number} \]
Step 2: Checking each option.
(A) \( ^{40}_{18}\mathrm{K} \) and \( ^{40}_{19}\mathrm{Ar} \):
Neutrons in K = \( 40 - 18 = 22 \), neutrons in Ar = \( 40 - 19 = 21 \).
Since the number of neutrons is different, they are not isotones.
(B) \( ^{13}_{7}\mathrm{N} \) and \( ^{14}_{7}\mathrm{N} \):
Neutrons in first N = \( 13 - 7 = 6 \), neutrons in second N = \( 14 - 7 = 7 \).
These are isotopes, not isotones.
(C) \( ^{37}_{17}\mathrm{Cl} \) and \( ^{40}_{18}\mathrm{K} \):
Neutrons in Cl = \( 37 - 17 = 20 \), neutrons in K = \( 40 - 18 = 22 \).
Neutron numbers are different, so they are not isotones.
(D) \( ^{198}_{80}\mathrm{Hg} \) and \( ^{197}_{79}\mathrm{Au} \):
Neutrons in Hg = \( 198 - 80 = 118 \), neutrons in Au = \( 197 - 79 = 118 \).
Both have the same number of neutrons but different atomic numbers, so they are isotones.
Step 3: Conclusion.
The correct pair of isotones is (D) \( ^{198}_{80}\mathrm{Hg} \) and \( ^{197}_{79}\mathrm{Au} \).