Isotopes are atoms of the same element with different numbers of what?

• A. Protons
• B. Neutrons
• C. Electrons
• D. Mass number

Answer: (B)

Isotopes are atoms of the same element that differ in the number of neutrons in the nucleus. The element is defined by its protons, so having the same number of protons keeps it the same element, while changing neutrons changes the mass and some nuclear properties without altering the chemical identity. Electrons can vary to form ions, but that doesn’t create different isotopes. The total mass number changes with neutron count, but the key difference that defines isotopes is the neutron number.