The atomic bomb -also called the A-Bomb- is a destructive weapon, marked by its great explosive power. It works on the principle of nuclear fission, in which uranium-235 or plutonium-239 is used to undergo fission. When a neutron strikes the nucleus of an atom of uranium-235 or plutonium-239, the nucleus fissions or splits.During such process, a great deal of gamma rays and thermal energy are produced, in addition to two or more neutrons that set out to split the surrounding nuclei, forming what is referred to as a chain reaction.
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The hydrogen bomb (mankind's most destructive weapon -also known as the H-Bomb or thermonuclear bomb) works on the principle of nuclear fusion, where isotopes of hydrogen (namely Deuterium and tritium) combine or fuse under extremely high temperatures to form helium. Hydrogen atoms are "ideal candidates" here for their very light nuclei that carry very weak positive charges serve well in the process of fusion. The conversion of mass into energy in the hydrogen bomb is actually an application of Albert Einstein's famous formula E = mc2. As mentioned before, Hydrogen nuclei fuse only under extremely high temperatures, accordingly, the hydrogen bomb requires tremendous heat in order to detonate. Such heat is attained by the explosion of an atomic bomb, i.e: it takes an atomic bomb just to initiate the process of fusion.