Atoms of the same element that have the same chemical properties but differ in mass exist. Which of the following are these called?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Isotopes
Option (1) Isotones — Incorrect. Isotones have the same neutron number but different atomic numbers, so they are atoms of different elements.
Option (2) Isobars — Incorrect. Isobars have the same mass number but different atomic numbers, so they too belong to different elements and differ in chemical properties.
Option (3) Isotopes — Correct. Atoms of the same element have the same atomic number and the same electronic structure, so they behave the same way chemically. They differ in mass because their nuclei hold different numbers of neutrons.
Option (4) Mirror nuclei — Incorrect. In a pair of mirror nuclei, the proton number of one equals the neutron number of the other. So the two have different atomic numbers and are different elements.
NCERT (2026-27):
Atomic Masses and Composition of Nucleus – “… The measurement of atomic masses reveals the existence of different types of atoms of the same element, which exhibit the same chemical properties, but differ in mass. Such atomic species of the same element differing in mass are called isotopes.”