How many of the following nuclides are isotones of \({}^{198}_{80}\mathrm{Hg}\)?
\({}^{197}_{79}\mathrm{Au}\), \({}^{198}_{78}\mathrm{Pt}\), \({}^{199}_{80}\mathrm{Hg}\), \({}^{196}_{78}\mathrm{Pt}\), \({}^{198}_{79}\mathrm{Au}\), \({}^{199}_{81}\mathrm{Tl}\)
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Three
Isotones have the same neutron number \( N \) but different atomic numbers. The neutron number of \( {}^{198}_{80}\mathrm{Hg} \) is \( 198-80=118 \).
The nuclides that are isotones of \( {}^{198}_{80}\mathrm{Hg} \):
\( {}^{197}_{79}\mathrm{Au} \) — \( 197-79=118 \) neutrons
\( {}^{196}_{78}\mathrm{Pt} \) — \( 196-78=118 \) neutrons
\( {}^{199}_{81}\mathrm{Tl} \) — \( 199-81=118 \) neutrons
The nuclides that are not:
\( {}^{198}_{78}\mathrm{Pt} \) — 120 neutrons; it has the same mass number, so it is an isobar
\( {}^{199}_{80}\mathrm{Hg} \) — 119 neutrons; it has the same atomic number, so it is an isotope
\( {}^{198}_{79}\mathrm{Au} \) — 119 neutrons
Number of isotones of \( {}^{198}_{80}\mathrm{Hg} \) = 3
NCERT (2026-27):
Atomic Masses and Composition of Nucleus – “… Nuclides with same neutron number N but different atomic number Z, for example 19880Hg and 19779Au, are called isotones.”