Hydrogen is the lightest element that can initiate nuclear fusion. The two hydrogen isotopes, deuterium and tritium react more efficiently and yield more energy when fused than do regular hydrogen atoms. The energy that occurs throguh nuclear fusion comes from the binding energy of nuclear matter. This is essentially the a measure of the efficiency with which the nucleons are held together. The binding energy comes from the difference in the mass of the protons and the neutrons of the atom bound together and there masses considered separately. Through experimentation is had been determined that the binding energy per nucleon is a maximum of 1.4*10^-12 Joules for a element with an atomic mass number of about 60. The energy that is created is only released if the final mass total is less than the initial mass. If the final mass where greater is would actually absorb energy instead of create it.
There are two types of nuclear fusion reactions. In one reaction the number of protons and neutrons doesn't change. This one is used more for pratical energy production. The other reaction is the key to the burning of stars and involves the conversion of the protons and neutrons. The reaction between deuterium and tritium is and example of a practical fusion reaction as it produces Helium and a neutron.
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The reaction that occurs in stars is a process in which two hydrogen atoms combine to form deuterium. In this case two protons are combined to form one proton and one neutron bound together as deuterium.