What are atoms of the same element that have different numbers of neutrons called?

• A. Ions
• B. Isotopes
• C. Molecules
• D. Allotropes

Answer: (B)

Atoms of the same element can have different numbers of neutrons, which changes their mass but not their identity as that element. This variation gives rise to isotopes. They share the same number of protons, so the element’s chemical behavior stays essentially the same, but the different neutron count can affect stability and physical properties like mass and sometimes rate of radioactive decay.

Why this fits best: isotopes are defined by having the same atomic number (same element) but different mass numbers due to varying neutrons, which is exactly what is described here.

Why the others don’t fit: ions are atoms that have gained or lost electrons and carry a charge, not about neutron count. Molecules are groups of atoms bonded together, not about variation within a single element. Allotropes are different structural forms of the same element (like graphite and diamond) with the same atoms arranged differently, not different numbers of neutrons.