Which statement defines isotopes?

• A. An element with a different number of neutrons.
• B. An element with a different number of protons.
• C. An element with a different electron arrangement.
• D. An element with a different chemical symbol.

Answer: (A)

Isotopes are atoms of the same element that differ in the number of neutrons. Because they share the same number of protons, they are the same element, but the different neutron count changes their mass. That’s why the defining feature is a different number of neutrons. For example, carbon-12 and carbon-14 have the same element (carbon) and the same number of protons, but carbon-12 has six neutrons while carbon-14 has eight. If the number of protons changed, you would have a different element entirely, not an isotope; changing the electron arrangement describes ions rather than isotopes, and the chemical symbol remains the same for isotopes of the same element.