The elements of which of the following pairs are isobars?

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¹₁H and ³₁H

¹₁H and ²₁H

¹²₆C and ¹⁴₆C

⁴⁰₁₈Ar and ⁴⁰₂₀Ca

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UPSC CDS-1 – 2020

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Isobars are atoms of different chemical elements that have the same mass number (number of protons + neutrons) but different atomic numbers (number of protons).

In option D, ⁴⁰₁₈Ar (Argon) has an atomic number (Z) of 18 and a mass number (A) of 40. ⁴⁰₂₀Ca (Calcium) has an atomic number (Z) of 20 and a mass number (A) of 40. Since they have different atomic numbers but the same mass number, they are isobars.

Options A, B, and C show pairs of isotopes, which are atoms of the same element (same atomic number) but different mass numbers.

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