Isobars of elements have same:

Correct — C, Mass numbers. Isobars are atoms of different elements that happen to have the same mass number. Mass number A is the total count of protons and neutrons in the nucleus, so two nuclei can arrive at the same A by different routes: argon-40 has 18 protons and 22 neutrons, calcium-40 has 20 protons and 20 neutrons, and both total 40. Because they are different elements, their atomic numbers must differ — that is what makes them different elements — and since a neutral atom has as many electrons as protons, their electron counts differ too. Their neutron counts differ as well, in the opposite direction from the protons, which is exactly how the totals come out equal. So mass number is the only quantity the definition holds fixed. The word itself is the memory aid: iso- meaning same, and -bar from the Greek for weight. Same weight, different element.

• (a)Atomic numbers. — This is the definition of isotopes, not isobars. Atomic number is what identifies an element, so two atoms with the same atomic number are the same element — and isobars are by definition different elements.
• (b)Number of electrons. — A neutral atom has as many electrons as protons, so equal electron counts would mean equal atomic numbers — the same element again. This is the isotope condition restated.
• (d)Number of neutrons. — Atoms with the same neutron number but different proton number are isotones, a third category. In isobars the neutron counts differ precisely so as to compensate for the differing proton counts.

Three related terms describe atoms that share one nuclear quantity. Isotopes share the atomic number Z — same element, different mass, like carbon-12 and carbon-14. Isobars share the mass number A but are different elements, like argon-40 and calcium-40. Isotones share the neutron number N. Since A equals Z plus N, fixing any one of the three leaves the other two free, which is why exactly three such categories exist.

This is a definition question, and the reliable way to answer it is to notice which options are secretly the same. Options (a) and (b) both amount to 'same number of protons' — atomic number is the proton count, and in a neutral atom the electron count equals it. Two options that say the same thing cannot both be the answer to a single-correct question, so both can be discarded at once. That leaves mass number and neutron number, and the etymology settles it: -bar means weight, so isobars share the weight, which in nuclear terms is the mass number. This trick of spotting equivalent options is worth carrying into other single-correct items; the paper cannot key two options that mean the same thing. The standard illustration is worth memorising as a pair, because CDS has used it before: argon-40 and calcium-40, with 18 and 20 protons respectively.

• Isobars are atoms of different elements having the same mass number A, the total of protons and neutrons.
• Argon-40 (18 protons, 22 neutrons) and calcium-40 (20 protons, 20 neutrons) are the standard example of isobars.
• Isotopes share the atomic number Z — the same element with different mass numbers, such as carbon-12 and carbon-14.
• Isotones share the neutron number N.
• A = Z + N, which is why fixing any one of the three quantities defines exactly one of the three categories.
• In a neutral atom the number of electrons equals the number of protons, so equal electron counts imply the same element.

• Reading 'isobar' as the meteorological line of equal pressure; the nuclear sense is what is being asked.
• Confusing isobars with isotopes — the commonest error, since isotopes are the more familiar term.
• Forgetting that different elements must have different atomic numbers, which rules out two of the four options immediately.

As a straight definition item like this one, or by giving two nuclides in symbol form and asking which pair are isobars — the form CDS used in 2020.

• practice — not a real PYQ

  Atoms having the same number of neutrons but different atomic numbers are called

  • (a)isotopes
  • (b)isobars
  • (c)isotones
  • (d)isomers

  Answer(c) isotones — they share the neutron number N, while isotopes share Z and isobars share A.

• practice — not a real PYQ

  Which one of the following pairs are isobars?

  • (a)Carbon-12 and carbon-14
  • (b)Argon-40 and calcium-40
  • (c)Hydrogen-1 and hydrogen-2
  • (d)Sodium-23 and magnesium-24

  Answer(b) Argon-40 and calcium-40 — different elements, 18 and 20 protons, but the same mass number of 40.