An isotope differs from its most common form by which characteristic?

• A. Different number of neutrons than protons
• B. Different number of protons than electrons
• C. Different total number of electrons
• D. Different number of protons and neutrons

Answer: (A)

Isotopes are variants of the same element that share the same number of protons, but have different numbers of neutrons. This changes the mass while leaving the chemical identity (and most properties) largely the same. So an isotope differs from its most common form by having a different number of neutrons, not by changing the number of protons. For example, carbon-12 and carbon-14 both have 6 protons, but 6 vs 8 neutrons, giving different mass numbers.

If you altered the number of protons, you’d be a different element altogether. Changing the total number of electrons would affect the charge or ionization state, not the isotope form of the same element. Changing both protons and neutrons would again describe a different species rather than a simple isotope.