Every atom of a given element is defined by its atomic number (Z), which equals the number of protons in the nucleus. In a neutral atom, the number of electrons also equals Z, since the negative charge of the electrons must balance the positive charge of the protons. Isotopes are atoms of the same element that therefore have identical Z (same number of protons and, in the neutral state, the same number of electrons and the same electron configuration), but they differ in the number of neutrons present in the nucleus.

The mass number of an atom is given by:

A = Z + N

where Z is the number of protons and N is the number of neutrons. Since Z is fixed for a given element but N can vary, isotopes of the same element end up with different mass numbers, and consequently different atomic masses, even though they occupy the same position in the periodic table and display essentially the same chemical behaviour (because chemical properties are governed mainly by the number and arrangement of electrons, which is unchanged).

A classic illustration is hydrogen, which has three naturally occurring isotopes:

• Protium (¹H): 1 proton, 0 neutrons, mass number 1
• Deuterium (²H or D): 1 proton, 1 neutron, mass number 2
• Tritium (³H or T): 1 proton, 2 neutrons, mass number 3 (radioactive)

Other well-known examples include the isotopes of carbon (¹²C, ¹³C, ¹⁴C) used in radiometric dating, and the isotopes of uranium (²³⁵U, ²³⁸U) relevant to nuclear technology.

The remaining options can be ruled out as follows: the number of electrons in the outer shell is determined by the number of protons (via electrical neutrality) and does not change between isotopes; the number of protons is, by definition, identical for isotopes of the same element (a change in proton number would create a different element altogether, i.e. an isotope of a different atom, not an isotope of the same one); and "positive ions in orbit" is not a meaningful description of atomic structure — electrons, not ions, occupy orbitals/shells around the nucleus. Thus, the correct distinguishing feature of isotopes is the differing number of neutrons inside the nucleus.