The atomic number does not determine the number of neutrons in an atomic core. As a result, the number of neutrons within an atom can vary. Then atoms that have the same atomic number may differ in atomic mass. Atoms of the same element that differ in atomic mass are called isotopes.
Mainly with the heavier atoms that have a higher atomic number, the number of neutrons within the core may exceed the number of protons.
Isotopes of the same element are often found in nature alternately or in mixtures.
An example: chlorine has an atomic number of 17, which basically means that all chlorine atoms contain 17 protons within their core. There are two isotopes. Three-quarters of the chlorine atoms found in nature contain 18 neutrons and one quarter contains 20 neutrons. The mass numbers of these isotopes are 17 + 18 = 35 and 17 + 20 = 37. The isotopes are written as follows: 35Cl and 37Cl.
When isotopes are noted this way the number of protons and neutrons does not have to be mentioned separately, because the symbol of chlorine within the periodic chart (Cl) is set on the seventeenth place. This already indicates the number of protons, so that one can always calculate the number of neutrons easily by means of the mass number.
A great number of isotopes is not stable. They will fall apart during radioactive decay processes. Isotopes that are radioactive are called radioisotopes.