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Asked in GUJCET 2026 · Composition, isotopes and isobars
Idea: compare Z, A and N=A-Z for the two nuclides and see which of the three is shared.
¹⁹⁷₇₉Au: Z=79, A=197, so N=197-79=118.
¹⁹⁸₈₀Hg: Z=80, A=198, so N=198-80=118.
The atomic numbers differ and the mass numbers differ, but the neutron number is the same, 118.
Nuclides with the same neutron number and different atomic number are called isotones.
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