Isotopes are atoms which are all of the same element, but differ in terms of the number of neutrons in their nucleus. Although because they are the same elements they will generally be identical in terms of basic physical properties, two significant differences are that isotopes will have different weights (because of the different number of neutrons), and that in some cases certain isotopes of an element but not others will be dangerously radioactive.
The term originated in 1913 with Glasgow chemist Frederick Soddy and physician Margaret Todd, who noticed that seemingly identical types of chemicals could have measurably different weights.
IT'S ALL IN THE NUCLEUS
All atoms contain three basic components: positively charged protons, neutrally charged neutrons, and negatively charged electrons. The protons and neutrons are closely packed together in the core, or nucleus, of the atom. What element, or basic chemical, an atom is, will be determined by the number of protons: for example, hydrogen has just one proton in its nucleus, but oxygen has eight.
In the same way, how many neutrons an atom has will determine what isotope it is. All atoms of the same element have the same number of protons, but they will not all have the same number of neutrons. For example, all oxygen atoms have 8 protons in their nucleus. About 99% of these also have eight neutrons; however, very small numbers of oxygen atoms have 9 neutrons, or even 10 neutrons.
REFERRING TO ISOTOPES
Isotopes can be distinguished by referring to their atomic mass number - in other words, the total number of protons and neutrons in their nucleus. For example, a carbon atom always has six protons. Sometimes it also has six neutrons, and is called carbon-12; other carbon atoms have seven neutrons, and are referred to as carbon-13.
In general, there are no essential differences in the behaviour of different isotopes of the same compound. However, certain minor differences can occur because of the differing weights: the lighter ones will usually react more quickly than their heavier counterparts. The difference is usually slight enough as to be effectively irrelevant; however, this is not the case with extremely light elements, like hydrogen and helium. In the case of hydrogen, for example, a hydrogen atom with no neutrons will weigh just one-third of the weight of an hydrogen atom with two neutrons. For this reason, hydrogen is special in that different names have been assigned to its different isotopes. Protium, which contains no neutrons, is most common; deuterium, which contains one neutron, is relatively uncommon (and is used to make "heavy water"); and tritium, which contains two neutrons is extremely rare, as well as being radioactive.
SIGNIFICANCE OF ISOTOPES
Only certain isotopes can actually be formed for a given element: that is, their atoms are only stable with specific numbers of neutrons in the nucleus. A hydrogen atom can form stably with one, two, or three neutrons; however, it cannot form with four. Either one neutron will simply fly away, or the entire nucleus will fly apart under the strain, unable to hold its mass together.
This becomes especially significant because of the middle range - those isotopes which can form, but are not stable. This is the basis for what is more commonly understood as radioactivity. Radioactive chemicals are simply those in which the nuclei of the atoms are not stable, and, as a result, they are starting to fly apart, ejecting subatomic components at high speed. Radiation refers to this resulting stream of subatomic particles, and is dangerous because when they collide with other atoms (such as the atoms in your body), it can cause those atoms to break apart as well.
Certain extremely heavy elements, like plutonium, are never stable: they will always break apart, at varying speeds, until there is nothing left of the sample. By the same token, certain elements are pretty much always stable, at least in naturally occurring isotopes. Most significant to many scientists are the so-called radioisotopes: isotopes which exist or can be created and are unstable, or radioactive, even while other isotopes of the same chemical are stable and perfectly safe. For example, hydrogen with one neutron is stable, but hydrogen with two neutrons (called tritium) is radioactive. Tritium has a half life of about ten years.