Question

most sodium atoms have an atomic mass of 23, but a few have atomic masses of 22 or 24. how do these isotopes differ from one another?
a. in the number of protons
b. in the number of electrons
c. in the number of neutrons
d. in the number of ions

Explanation:

Isotopes are atoms of the same element (so same number of protons, and in neutral state, same number of electrons) but different atomic masses. Atomic mass is the sum of protons and neutrons. Since protons are the same (as they are same element), the difference must be in neutrons. Ions are charged atoms (due to gain or loss of electrons), which is not relevant here.

Answer:

C. In the number of neutrons