Isotopes differ in what property?

• A. Number of protons
• B. Number of neutrons
• C. Number of electrons
• D. Element symbol

Answer: (B)

Atoms of the same element can have different isotopes because they share the same number of protons (which defines the element) and the same number of electrons (in a neutral atom), but they differ in the number of neutrons. The property that changes among isotopes is the neutron count. Neutrons add to the mass of the nucleus without changing the electric charge, so isotopes have nearly identical chemical behavior but different total masses and nuclear stability. For example, carbon-12 and carbon-14 are both carbon because they have six protons, but carbon-12 has six neutrons and carbon-14 has eight neutrons. Changing the proton count would create a different element, and changing the electron count would alter charge or ionization, not define a different isotope. The element symbol remains the same because the identity of the element is set by the number of protons, while the isotope designation reflects a different neutron number.