Choose the correct statements from below:
A. Nuclides with same atomic number but different number of neutrons are called isotopes.
B. Nuclides with same mass number are known as isotopes.
C. Nuclides with same number of neutrons are called isobars.
D. More than 99.9% mass of atom is concentrated in nucleus.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A and D Only
Nuclides are classified by their atomic number Z (protons), mass number A (protons plus neutrons) and neutron number N.
(A) Nuclides with same atomic number but different number of neutrons are called isotopes — Correct. Isotopes have the same Z but different N and hence different A, for example \( {}^{1}_{1}\mathrm{H} \), \( {}^{2}_{1}\mathrm{H} \) and \( {}^{3}_{1}\mathrm{H} \).
(B) Nuclides with same mass number are known as isotopes — Incorrect. Nuclides with the same mass number A are isobars, for example \( {}^{3}_{1}\mathrm{H} \) and \( {}^{3}_{2}\mathrm{He} \).
(C) Nuclides with same number of neutrons are called isobars — Incorrect. Nuclides with the same number of neutrons are isotones.
(D) More than 99.9% mass of atom is concentrated in nucleus — Correct. The mass of an electron is only about \( 1/1836 \) of the mass of a proton, so even in hydrogen the electron contributes about 0.05% of the mass of the atom, and the share of the nucleus is higher still in heavier atoms.
NCERT (2026-27):
Summary – “… More than 99.9% mass of the atom is concentrated in the nucleus. … Nuclides with the same atomic number Z, but different neutron number N are called isotopes. Nuclides with the same A are isobars and those with the same N are isotones.”