The element nickel has five naturally occurring isotopes. Which of the following describes the relationship of these isotopes?

Different mass, same atomic number

Explanation: Elements are made of similar atoms, however the atoms may have different number of neutrons but the same number of protons in their nucleus. Such atoms that have equal number of protons but different number of neutrons are known as isotopes. Difference in the number of neutrons makes isotopes to have different mass numbers. Atomic number is the number of protons in the nucleus of an atom, therefore, isotopes have the same atomic number.