CDS (I) 2020 General Knowledge · Question 2 of 120

Correct — D, 40¹⁸Ar and 40²⁰Ca. Isobars are atoms of different elements that happen to share a mass number while differing in atomic number. Argon has 18 protons and calcium 20, so they are different elements; both nuclides here carry 40 nucleons in total, so their mass numbers match. That is precisely the pair NCERT uses to introduce the word, noting that 'the number of protons in these atoms is different, but the mass number of both these elements is 40'. The other three pairs all keep the atomic number fixed and change the mass number, which is the definition of isotopes, not isobars.

• (a)1¹H and 1³H — Protium and tritium — both hydrogen, atomic number 1, with mass numbers 1 and 3. Same element, different mass number, so these are isotopes.
• (b)1¹H and 1²H — Protium and deuterium, again both hydrogen. Isotopes of the same element cannot be isobars, because isobars must belong to different elements.
• (c)12⁶C and 14⁶C — Carbon-12 and carbon-14 share the atomic number 6 and differ in mass number, so they are isotopes. The mass numbers 12 and 14 are unequal, which rules out an isobaric pair on its own.

Atomic number Z counts the protons and fixes which element an atom is. Mass number A counts protons plus neutrons. Isotopes hold Z constant and let A vary — hydrogen's protium, deuterium and tritium, or carbon-12 and carbon-14. Isobars do the opposite: A is the same and Z differs, so the two atoms are different elements that weigh nearly the same.

Every option in this item comes off a single page of NCERT Class 9. Its isotope examples are hydrogen's three species, carbon-12 with carbon-14 and chlorine-35 with chlorine-37; its isobar example is argon-40 with calcium-40. So the paper is testing whether you can tell the chapter's two look-alike terms apart, and the fastest route is to compare the element symbols first — if the two symbols are the same letter, the pair cannot be isobars. One point about how the bank prints these nuclides: the booklet stacks the mass number above the atomic number to the left of the symbol, and that two-level layout cannot survive in a single line of text, so the digits appear side by side here. Read them as argon-40 and calcium-40, protium and tritium, protium and deuterium, carbon-12 and carbon-14 — the nuclides are unambiguous even where the flattened order is not.

• Isobars have the same mass number but different atomic numbers; isotopes have the same atomic number but different mass numbers.
• Argon-40 and calcium-40 are NCERT's own worked example of isobars — 18 protons against 20, both with 40 nucleons.
• Hydrogen's three isotopes are protium, deuterium and tritium, with mass numbers 1, 2 and 3 and atomic number 1 throughout.
• Chemical properties follow the atomic number, so isotopes of an element behave alike chemically while their physical properties differ.
• Chlorine's average atomic mass of 35.5 u comes from its two isotopes of mass 35 u and 37 u mixing in a 3:1 ratio.

• Reading any pair of similar-looking nuclides as isobars without checking whether the element symbols differ.
• Assuming isobars must be neighbours in the periodic table — argon and calcium are two places apart.
• Mixing up this sense of isobar with the meteorological one, a line of equal pressure on a weather map.

Usually as 'which pair is isobars/isotopes', or as a one-line definition item asking what isobars share.

• practice — not a real PYQ

  Two atoms have the same number of neutrons but different numbers of protons. Such atoms are called

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

  Answer(c) isotones — the neutron number is what is held constant; isotopes fix the proton number and isobars fix the mass number.

• practice — not a real PYQ

  The number of neutrons in the nucleus of an atom of argon-40 (atomic number 18) is

  • (a)18
  • (b)20
  • (c)22
  • (d)40

  Answer(c) 22 — neutrons equal mass number minus atomic number, so 40 − 18 = 22.