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Isotope - Atoms with the same number of protons but different number of neutrons are called isotopes. By changing the number of neutrons, isotopes still maintain the same overall neutrality and hence the chemical behavior remains unchange.
Some elements have several isotopes while others may have only one isotope. For example, there is only one isotope for the element cesium while
the element calcium can have six isotopes found in nature. The simplest isotopes being those of hydrogen, show schematically below:
Protium
 
Deuterium
Hydrogen is the simplest form of an atom and the ordinary isotope (called protium) consists of only one proton and one electron (diagram on left).
Almost all hydrogen atoms found in nature are consisted of this form. The remaining, about 0.015%, are the hydrogen isotope with a neutron (diagram on right). It is given a
special name deuterium. Another isotope, called tritium, has two neutrons in the nucleus. This isotope is only found in traces in nature. However, tritium be produced in a nuclear reactor.
Decay (radioactive) - Some isotopes are unstable, especially those with a lot of neutrons compare with the number of protons in the nucleus. These isotopes tend to eject some particles, in the form of
radiation, until a stable nucleus is produced. Such ejection process is called the radioactive decay. Isotopes that undergo radioactive decay are called radioisotopes or radionuclides. The decay process is not a chemical process, neither can it be controlled.
It occurs spontaneously and at random. Radioactive decay often poses health risk, especially those with intense radiation, as it penetrates the body and destroys biological cells.
Half-life - A quantity used to measure the stability of a radioisotope. It is the time taken for half of the radioisotope in a sample to undergo radioactive decay. Stable radioisotopes may take eons to decay while the unstable ones may disappear in fractions of a second!
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