Which of the following is a pair of isobars?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 4020Ca and 4018Ar
Isobars are atoms of different elements with the same mass number but different atomic numbers. Calcium (atomic number 20) and argon (atomic number 18) both have mass number 40, so they are isobars.
Why the other options are incorrect
Option (1) 126C and 168O — Incorrect. The mass numbers are 12 and 16, so the two atoms are not isobars.
Option (2) 136C and 146C — Incorrect. Both are carbon with different mass numbers; they are isotopes.
Option (4) None of these — Incorrect. Option (3) is a pair of isobars.