JEE Main 2021 · 27 July · evening · Single correct · +4 / −1
The number of neutrons and electrons, respectively, present in the radioactive isotope of hydrogen is :
- (a) 1 and 1
- (b) 3 and 1
- (c) 2 and 1
- (d) 2 and 2
Answer and solution
Answer (c) 2 and 1
Solution
Chemistry ma’am · 3 steps
Only tritium is radioactive, and its mass number 3 hides one proton and two neutrons.
- Pick the isotope. Hydrogen has three isotopes: protium ¹₁H, deuterium ²₁H and tritium ³₁H. Protium and deuterium are stable. Only tritium is radioactive; it is a β-emitter.
neutrons = A - Z = 3 - 1 = 2
- Count electrons. A neutral atom has as many electrons as protons. Every hydrogen isotope has Z = 1, so tritium has 1 electron.
- So. Tritium has 2 neutrons and 1 electron, which is option (c).
The idea to keep. Isotopes differ only in neutrons. The proton and electron counts stay fixed by Z, so get neutrons from A - Z and electrons from Z.
Answer from the NTA final answer key. Solution by PrepWiser.