Which of the following is a characteristic of the MOST common isotope of an atom?

The number of protons in an atom is its atomic number, and it determines what element the atom is. For example, all carbon atoms have 6 protons, which means that their atomic number is 6. The number of neutrons in an atom can vary, and this is what creates isotopes. Isotopes of an element have the same number of protons, but different numbers of neutrons.