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Isotope - Atoms with the same number of protons but different number of neutrons are called isotopes. Changing the number of neutrons still maintain the overall neutrality of an atom and hence the chemical behavior remains unchange.
Some elements have a number of 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 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. It can, however, 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 at the nucleus. These isotopes tend to eject some particles, in the form of
radiation, until a stable nucleus is produced. The ejection process is called the radioactive decay. Isotopes that undergo radioactive decay are called radioisotope or radionuclide. The decay process is not a chemical process, neither can it be controlled.
It will occur 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 a few seconds!
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