Hydrogen-1, deuterium and tritium are the isotopes of hydrogen. The number of which of the following differs among them?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Nucleons
Option (1) Nucleons — Correct. Isotopes have the same number of protons but different numbers of neutrons. So their nucleon numbers (mass numbers) differ. They are 1 in hydrogen-1, 2 in deuterium and 3 in tritium.
Option (2) Protons — Incorrect. All three are isotopes of hydrogen, so each nucleus has one proton (\( Z=1 \)).
Option (3) Electrons — Incorrect. A neutral atom has as many electrons as protons, so each atom has one electron. This same electronic structure gives the isotopes the same chemical behaviour.
NCERT (2026-27):
Atomic Masses and Composition of Nucleus – “… One also uses the term nucleon for a proton or a neutron. Thus the number of nucleons in an atom is its mass number A.”
Atomic Masses and Composition of Nucleus – “… The nuclei of isotopes of a given element contain the same number of protons, but differ from each other in their number of neutrons. Deuterium, \( {}^{2}_{1}\mathrm{H} \), which is an isotope of hydrogen, contains one proton and one neutron. Its other isotope tritium, \( {}^{3}_{1}\mathrm{H} \), contains one proton and two neutrons.”