What isotopes of the same elements have differing numbers of?

Hint: We must have to know that the number of protons is equal to the atomic mass of an element, whereas number of neutrons is the difference between mass number and number of protons. In an atom, protons and neutrons are placed inside the nucleus whereas electrons move around the nucleus in circular orbits.

Complete step by step answer:
Before explaining this question we must know about isotopes. Isotopes are nothing but small variants of any chemical element in which there is a difference in the number of neutrons but the number of protons will be the same in each atom.
Isotopes of carbon can be as follows:
\[^{12}{C_6}{,^{13}}{C_6}{,^{14}}{C_6}\]
\[^{12}{C_6}\]:
Atomic number = \[6\] (No. of protons)
Mass number =\[12\] (No. of neutrons + no. of protons)
\[^{13}{C_6}\]:
Atomic number = \[6\] (No. of protons)
Mass number =\[13\] (No. of neutrons + no. of protons)
Carbon-\[12\] has the same number of protons and neutrons whereas carbon-\[13\] has a different number of neutrons than protons.
We must have to know that the number of electrons and the number of protons are the same, that is the atomic number of an element protons lies inside the nucleus whereas an electron revolves around the circular path known as orbits. And neutrons are also inside the nucleus.

Note:
We have to remember that the \[^{12}{C_6}{,^{13}}{C_6}{,^{14}}{C_6}\] these are different isotopes of carbon which has same atomic number or number of protons but different mass number or number of nucleons. Protons carry positive charge, electrons carry negative charge whereas neutron is a neutral species that has zero or no charge. Number of neutrons is different in isotopes.