In simple words, Nuclear Fusion is the combination of two lighter elements to form a heavier element. Most of you might have heard the term “nuclei” instead of elements. This is so, because, for atoms to fuse, the temperatures and pressures need to be high enough that atoms are forced so close to each other that the Strong-nuclear force (the force that binds the nucleus together) overcomes the Electromagnetic force. Such a state of matter is called a Plasma. In the plasma state, the electrons are free to roam and are not bound to their nuclei. Thus, instead of talking about fusion of the element as a whole, we can say that two nuclei have fused together. (As far as I know, electrons don’t fuse together).
You might be wondering at this moment, “I have heard such reactions produce devastating amounts of energy. Why’s that?”. The reason is, because of the nature of the particles that fuse together (the lighter nuclei) and the product. The sum of the masses of the lighter nuclei is slightly more than the mass of the heavier particle made. Now, according to a dude by the name of Einstein, Energy= Mass*(speed of light)^2. So, the net energy created in the process from one atom is equal to the change in mass times the square of the speed of light, which a’int much when its just one atom fusing.
However, just a gram of Hydrogen atoms contains approximately 6.022*10^23 atoms! Even if such small amounts of Hydrogen is forced to fuse, it would release a considerable amount of energy! This form of energy is the driving force in stars. The Sun, which is basically three quarters Hydrogen, is about 2*10^30 kg in mass! Even with such a staggering amount of Hydrogen, the Sun is classified as a medium-sized G-type star. Some stars are tens to hundreds of times more massive than the sun!
A simple Nuclear Fusion reaction is (with just the mass number; I am assuming you are familiar with atomic numbers and molecular symbols)
1H +1H= 2H + Energy
2H+2H= 3He (which is formed by the radio-active decay of 3H)+ 1 Neutron +Energy
3He+3He= 4He+1H+1H+Energy
4 1H= 4He+Energy
where,
H= Hydrogen (1)
He= Helium (2)
2H= Deuterium (heavy hydrogen)
Please note that these reactions also produce other sub-atomic particles such as Neutrinos and Positrons, but I have not included those.
Nuclear Fission is just the opposite of a Nuclear Fusion reaction. A heavy nucleus is bombarded with a neutron of varying energy (in terms of the needs), which produces an unstable nucleus which quickly disintegrates into two or more new elements and results in the release of energy. Nuclear Fission is much less powerful than Nuclear Fusion. This is because, the difference in masses is lesser than that of the Nuclear Fusion reaction.
Nuclear Fission of a heavier nuclei into two or more lighter nuclei doesn’t always produce the same lighter nuclei! There are different combinations and the occurrence of certain elements has some probability. In the case of Nuclear Fission, the most widely used element is Uranium, and is followed by Plutonium. The power source of the Voyager spacecrafts as well as the New Horizons spacecraft is based on the radioactive decay of an isotope of Plutonium,
Two combinations of different elements formed by the splitting of Uranium is given below:
235U+ n (slow moving neutron)= 236U
236U= 141Ba+92Kr+3n
or,
235U+ n (slow moving neutron)= 236U
236U= 140Cs+92Rb
where,
U= Uranium (92)
Cs= Cesium (55)
Rb= Rubidium (37)
Ba= Barium (56)
Kr= Krypton (36)
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