Isotopes are variants of the same chemical element with an equal number of protons in the nucleus but a different number of neutrons, resulting in a different atomic mass.
An element is determined by the unique number of protons it has, also known as its atomic number. For example, the atomic number of carbon is 6 because it always has six protons, even in isotopic form.
The abundant form of carbon has 6 protons and 6 neutrons which, when added together, give its mass number of 12. It is sometimes called carbon-12 to distinguish it from two other naturally-occurring isotopes, carbon-13 and carbon-14. Carbon-13 has 6 protons and 8 neutrons, and an atomic mass of 13, while carbon-14 has 6 protons and 8 neutrons, and an atomic mass of 14.
While isotopes of the same element have similar chemical properties, they may have different physical characteristics, including stability and radioactivity.