Home / Topic / Isotopes and isobars
Topic Isotopes and isobars
Asked 3 times in CDS over 7 years (2018–2025), peaking in 2025.
Key insights
- Most tested in CDS — 3 of 3 PYQs
- Most asked in 2025
- Recurs across 3 years (2018–2025)
Across exams
CDS · 3How the concept evolved
Rising — more than half its appearances (3 of 3) are since 2016.
Related concepts
Ideas UPSC tests alongside this one — keep exploring.
Sample questions
You've seen the pattern — here are a few of the actual PYQs.
CDS2025
Isobars of elements have same :
- A Atomic numbers.
- B Number of electrons.
- C Mass numbers. ✓
- D Number of neutrons.
✓ Correct answer: (C)
Science & Technology › Physics & Chemistry
CDS2020
The elements of which of the following pairs are isobars?
- A ₁¹H and ₁²H
- B ₁¹H and ₁³H
- C ₆¹²C and ₆¹⁴C
- D ₁₈⁴⁰Ar and ₂₀⁴⁰Ca ✓
✓ Correct answer: (D)
Science & Technology › Physics & Chemistry
CDS2018
Which one of the following elements will be an isobar of calcium if the atomic number of calcium is 20 and its mass number is 40?
- A Element with 20 protons and 18 neutrons
- B Element with 18 protons and 19 neutrons
- C Element with 20 protons and 19 neutrons
- D Element with 18 protons and 22 neutrons ✓
✓ Correct answer: (D)
Science & Technology › Physics & Chemistry
Study how UPSC repeats — not just the papers
₹199 / 1 year · Founding Member
Full access unlocks the three things no PYQ PDF gives you:
① Cross-exam learning — see the same idea recur across CSE, CAPF, CDS and NDA.
② Topic evolution — how each idea is framed, year by year.
③ Question intelligence — every PYQ + answer, linked to related concepts and similar questions.
e.g. Round Table Conference: CDS 2017 → CSE 2017 → … → CAPF 2024
15 days free, no card required — then ₹199 for 1 year.
Start your 15-day free trial →