Expert: Osman Kemal Kadiroglu - 12/3/2000

Question
hi~ we are doing different kinds of reactions at school right now.. and my topic is to find out "the difference between chemical and nuclear reactions." If you could help me answer my question, it would be greatly appreciated :)...thanks for reading my question ~jennW~  

Answer
Chemical reactions are the result of rearrangement of electrons in the orbits of an atom. Nuclear reactions are generated within the nucleus of an atom. If you burn coal you join an atom of carbon from the coal with two atoms of oxygen from the air. What accually happens is that the outer electron orbits of a carbon atom and two oxygen atoms start to share electrons. By rearranging the electron orbit you have changed the energy state of the three atoms by making a molecule. The difference in the energy states is the energy you get from this chemical reaction. Usually one generates few electronvolts of energy from chemical reactions. (An electronvolt is a small unit of energy)

Nuclear reactions happen at the nucleus of an atom. If you are able to send a subatomic particle, say a neutron, to the nucleus of, say uranium atom, you may start a nuclear reaction. Let us assume that we have uranium 235 isotope, and a slow neutron is send straight into it. Since neutrons do not have electrical charges, it will penetrate the atoms outer and inner electron shells and enters into the nucleus. This neutron will meet with 92 protons and 142 neutrons in the nucleus of U235. Now the nucleus is 144 neutrons and 92 protons vibrating within a very very small volume. Depending on some nucleonic proporties this vibrating nucleus can reject one neutron out. This reaction is called a scattering reaction. Here the outgoing neutron can have various values of energy. If there is an energy exchange with the nucleus then this nuclear reaction is called an inelestic scattering. Then the nucleus being left at higer energy level will eject gamma rays and reach to its minimum energy level, which is called the ground state of the nucleus.

If the vibrating nucleus vibrates violently then it is possible that this nucleus splits into two. Imagine a liquid drop, like a rain drop hitting a solid object and the splitting into two or three parts. U235 by admitting a slow neutron into it will split into two major parts and few small subnuclear particles. This is called the fission reaction and this reaction powers nuclear reactors. U235 nucleus splits into two fission fragments, which are new nuclei like Barium, Cesium, Iodine,etc., and 2 or 3 very fast moving neutrons.

Or U235 nucleus after vibrating for a while slows down and beconmes U236 nucleus by emmiting few gamma rays to reach its ground state. Or, it could change into some other nucleus by emmiting alpha particles (helium nucleus), or beta particles (electrons). These are some of the main nuclear reactions.

The energy of a nuclear reaction is much higher than a chemical reaction. When you burn coal you get around 4 eV per rection whereas a fission reaction produces 200 million eV (MeV).