Elements are defined by the number of protons in the atomic nucleus. For example, an atom with 6 protons must be carbon, and an atom with 92 protons must be uranium.
In addition to protons, the atoms of nearly every element also contain neutrons. The mass of a neutron is almost identical to that of a proton. Both are formally given an atomic mass number, A, of 1.
The simplest, commonest form of hydrogen has a nucleus that consists of a single proton; it is the only atom with no neutrons: its mass number is 1.
A rarer form of hydrogen known as deuterium has one proton and one neutron: its mass number is 2.
A third form of hydrogen known as tritium has one proton and two neutrons: its mass number is 3.
When an element's atoms have different numbers of neutrons they are said to be isotopes of that element. Three isotopes of hydrogen have been described above.
Carbon atoms exist naturally with 6, 7 or 8 neutrons. Since each atom of carbon has 6 protons, the isotopes must have atomic mass numbers of 12, 13 and 14. (Since atomic mass number = protons + neutrons)
These isotopes are called carbon-12, carbon-13 and carbon-14. Alternatively, they may be written 12C, 13C and 14C.
Carbon-12 and carbon-13 are stable. Carbon-14 is unstable, decaying with a half-life of about 5,700 years. It is produced in Earth's atmosphere by cosmic ray bombardment of nitrogen-14.
Uranium has three naturally occurring isotopes. These are uranium-234, uranium-235, and uranium-238. Since each atom of uranium has 92 protons, the isotopes must have 142, 143 and 146 neutrons respectively.
Chemical and Physical Properties of Isotopes
We have seen that isotopes differ in mass number. The mass number has very little bearing on chemical reactions; therefore the reactivity and chemical reactions of an element's different isotopes are almost identical.
The physical properties of atoms, however, do depend on mass. This enables isotopes to be separated from one another by methods such as diffusion and fractional distillation.
The stability of any atom's nucleus depends on the ratio of protons to neutrons. Many isotopes have a ratio of protons to neutrons that makes them unstable and, as a result, they are radioactive.