Isotopes of an element have different mass numbers. Complete the sentence to explain why.  Because they have different numbers of:neutronsprotonselectrons

Isotopes of an element have different mass numbers because they have different numbers of neutrons.

Explanation: An atom is made up of protons, neutrons, and electrons. The number of protons in an atom determines the atomic number and thus the element. For example, all atoms with 6 protons are carbon atoms. The mass number of an atom is determined by the sum of protons and neutrons.

Isotopes are atoms of the same element (same number of protons) but with different numbers of neutrons. Therefore, they have the same atomic number but different mass numbers. For example, Carbon-12 and Carbon-14 are both isotopes of carbon, but Carbon-12 has 6 neutrons while Carbon-14 has 8 neutrons.

The number of electrons in a neutral atom is equal to the number of protons, so it does not affect the mass number. Therefore, isotopes of an element have different mass numbers because they have different numbers of neutrons.