CDS (II) 2024 General Knowledge · Question 12 of 120

Correct — D, ³⁵₁₇C and ³⁷₁₇C. Isotopes are atoms of the same element with the same atomic number but different mass numbers, so the subscript must match in both members of the pair and the superscript must differ. Only the fourth option satisfies that with the right numbers: both carry the subscript 17, which is chlorine's atomic number, and their mass numbers are 35 and 37, which are chlorine's two naturally occurring stable forms. Chlorine-35 makes up roughly three-quarters of natural chlorine and chlorine-37 the rest, and averaging the two in that proportion is what gives chlorine its familiar relative atomic mass of about 35.5 — a value that puzzles students until they learn that it is a weighted average of two whole-numbered nuclides rather than the mass of any single atom. One printed detail must be read past: the paper prints the element symbol as C in all four options where it should read Cl. That is a misprint in the booklet itself, and the subscript 17 shows chlorine is meant, since carbon's atomic number is 6.

• (a)³⁵₁₇C and ³⁶₁₇C — The pair is at least self-consistent, since both have atomic number 17, but mass 36 is not one of chlorine's two natural forms. Chlorine-36 exists only as a rare radioactive trace nuclide; the second stable one is mass 37.
• (b)³⁴₁₇C and ³⁶₁₈C — The two subscripts differ, 17 and 18, so these are atoms of two different elements and cannot be isotopes of anything at all. Atomic number 18 is argon. Both mass numbers are wrong for chlorine as well.
• (c)³⁵₁₇C and ³⁷₁₈C — The trap option. The mass numbers 35 and 37 are right, but the second subscript has been changed to 18, so the pair names one chlorine atom and one argon atom. Different atomic numbers with different mass numbers make them neither isotopes nor isobars.

An atom is identified by its atomic number, the number of protons, written as a subscript before the symbol; its mass number, the protons plus neutrons, is written as a superscript. Isotopes are atoms of the same element — same atomic number — with different mass numbers, so they differ only in neutron count. Isobars are the opposite pairing: different elements, different atomic numbers, but the same mass number.

Every question in this family is answered by looking only at the two numbers. For isotopes the subscripts must agree and the superscripts must differ; for isobars the superscripts must agree and the subscripts must differ. Here the distractors are built by breaking one of those rules at a time, which is why scanning the subscripts first eliminates two options at a glance. The extra care this particular item demands is with its printing. The symbol is set as C throughout, where chlorine's symbol is Cl. Reading the numbers rather than the letter protects a candidate from that: an atomic number of 17 cannot belong to carbon. Chemically, this pair is why chlorine's relative atomic mass is a fraction — about three parts of mass 35 to one part of mass 37 averages to roughly 35.5.

• Isotopes have the same atomic number and different mass numbers; isobars have the same mass number and different atomic numbers.
• Chlorine has atomic number 17, and its two naturally occurring stable nuclides are chlorine-35 and chlorine-37.
• Natural chlorine is roughly three parts chlorine-35 to one part chlorine-37, which averages to a relative atomic mass of about 35.5.
• NCERT's standard isotope examples are protium, deuterium and tritium, carbon-12 and carbon-14, and chlorine-35 and chlorine-37.
• Atomic number 18 is argon, which is why an option with subscript 18 cannot be a chlorine nuclide.

• Checking mass numbers before atomic numbers; a wrong subscript kills an option instantly.
• Assuming any pair of nuclides with the right masses is an isotope pair — the elements must match too.
• Being thrown by a printed symbol; the numbers, not the letters, identify the atom.

Usually as an identify-the-pair item on isotopes or isobars, or as a fractional-atomic-mass calculation; chlorine and hydrogen supply most of the worked examples.

• practice — not a real PYQ

  Two atoms have the same mass number but different atomic numbers. They are called

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

  Answer(b) isobars — the mass numbers agree while the proton counts differ, as with argon-40 and calcium-40.

• practice — not a real PYQ

  The relative atomic mass of chlorine is about 35.5 rather than a whole number because

  • (a)a chlorine atom contains half a neutron
  • (b)natural chlorine is a mixture of chlorine-35 and chlorine-37
  • (c)chlorine molecules contain two atoms
  • (d)chlorine is radioactive

  Answer(b) natural chlorine is a mixture of chlorine-35 and chlorine-37 — roughly three parts to one, and the average of the two in that ratio is about 35.5.