Elements are defined by the number of protons in an atom's 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 every element (except the simplest form of hydrogen) also contain neutrons. When an element's atoms have different numbers of neutrons they are said to be isotopes of that element.

Isotopes Example 1

Carbon atoms exist naturally with 6, 7 or 8 neutrons. Since each atom of carbon has 6 protons, the isotopes must have atomic masses of 12, 13 and 14. (Since atomic mass = mass protons + mass 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.

Isotopes Example 2

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. Atomic mass 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.

Atomic Stability

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 renders them unstable and, as a result, they are radioactive.

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