Question

elements that differ in their ______ are known as isotopes.

multiple choice

proton number

electron number

atomic number

type of bonds

neutron number

Explanation:

Isotopes are atoms of the same element (same proton/atomic number) with different neutron numbers (and thus different mass numbers). Proton number (same for same element), electron number (varies in ions), and type of bonds (related to chemical bonding, not isotopes) don't define isotopes. So the correct factor is neutron number.

Answer:

E. neutron number