An atom is determined by the number of protons in its nucleus and because of this all atoms of an element must have the same number of protons, they do not however have to have the same number of neutrons. Atoms that have the same atomic number but a different number of neutrons are known as Isotopes. An isotope is still the original element, just altered a little.
Carbon serves as a good example because it has three naturally occurring isotopes, 12C, 13C and 14C, each with a progressively larger number of neutrons, the first with six and so on. Each element that occurs in nature is a mixture of its isotopes. For example that graphite that makes up a lead pencil is made up of the three previously mentioned carbon isotopes.
Most isotopes do not occur with equal density. For the carbon example the first one, 12C, occurs nearly 98% of time and the other two occur only in the two percent. The percentage of atoms that occur as a given isotope is referred to as the natural abundance of that isotope. Some elements, such as fluorine, have only one naturally occurring isotope, whereas some have several isotopes.
The chemical behavior of an atom is largely determined by its electronic structure, which does not differ between isotopes, and therefore different isotopes exhibit nearly identical chemical behavior. The exception to this rule is known as the kinetic isotope effect. What occurs under this effect is that isotopes that are heavier, and therefore have larger masses, may react more slowly than isotopes that have lighter masses.
Isotopes have certain uses in science. One technique, called Isotope analysis which determines the relative abundance of the isotopes in a sample. This can used for things like detecting food products that have been altered. As well as identifying the origin of meteorites, such as the one that came from Mars. Another use is the ability to date things like fossils and other archaeological finds. This can be done using the half-life information of carbon isotopes. This is possible because isotopes will eventually lose their extra neutrons and revert back to the number in the original atom. The process by which that occurs is known as radioactive decay. This decay happens quite regularly, like clockwork, and will happen over a few thousand years. Some elements will decay over such a long time while others will decay in a matter of minutes.