Isobars have:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Same mass number
Isobars are atoms of different elements that have the same mass number (the total number of protons and neutrons) but different atomic numbers. For example, argon-40 (18 protons), potassium-40 (19 protons) and calcium-40 (20 protons) all have mass number 40. Because their atomic numbers differ, isobars are different elements with different chemical properties.
Why the other options are incorrect
Option (1) Same number of protons — Incorrect. Atoms with the same number of protons are atoms of one element; if their mass numbers differ, they are isotopes.
Option (2) Same atomic number — Incorrect. The atomic number is the number of protons, so this too describes isotopes such as carbon-12 and carbon-14.
Option (3) Same number of electrons — Incorrect. Species with the same number of electrons are called isoelectronic, such as Na+, Ne and F− (10 electrons each).