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Asked in GUJCET 2025 · Composition, isotopes and isobars
Idea: isotones have the same neutron number N=A-Z but different atomic number, so work out N for both members of each pair.
¹⁹⁸₈₀Hg: N=198-80=118. ¹⁹⁷₇₉Au: N=197-79=118. Equal neutron numbers, and Z differs (80 and 79) — isotones.
³₁H and ³₂He: N=2 and N=1, same A — isobars.
²¹⁴₈₂Pb and ²¹⁴₈₃Bi: N=132 and N=131, same A — isobars.
¹²₆C and ¹⁴₆C: same Z — isotopes.
So the isotone pair is ¹⁹⁸₈₀Hg and ¹⁹⁷₇₉Au.
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