An isotope is a variant on an element that has a different atomic mass number, but with the same atomic number.  Except for the commonest form of hydrogen — which has only a proton — every atomic nucleus in normal matter is made of both protons and neutrons. Isotopes of a given element have the same number of protons, but different numbers of neutrons. They have essentially the same chemical properties, but differ slightly in their physical characteristics, such as melting point and boiling point. Some isotopes are unstable and tend to decay into other elements, giving off subatomic particles or radiation; these are radioactive and are known as radioisotopes.

When scientists refer to a particular isotope of an element, the mass number, or the number of protons plus the number of neutrons, appears at the top left, next to the symbol for the element. For example, the form of hydrogen that has a proton and a neutron is written as 2 H. Similarly, 235 U and 238 U are two different isotopes of uranium. These are also commonly written as uranium-235 and uranium-238.

Expressing atomic mass number and atomic number for any element 

 Stable Carbon 

  And if we needed to determine the number of neutrons in carbon we subtract the atomic number from the atomic mass number

12 nucleons - 6 protons = 6 neutrons 

 Unstable (radioactive) Carbon 

 And if we needed to determine the number of protons in carbon 14  we subtract the atomic number from the atomic number 

14 nucleons - 6 protons = 8 neutrons

an isotope