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Atomic Structure
Atoms consist of a nucleus of protons and neutrons surrounded by a cloud of orbiting electrons. In nuclear reactions it is the nucleus that is of importance. Protons are positively charged particles, and the number of protons in the nucleus denotes the element. For example, all atoms of carbon have six protons while all atoms of nitrogen have seven protons in the nucleus. Changing the number of protons changes the element. Neutrons are neutrally charged particles and may vary between atoms of the same element. As an example, hydrogen atoms each have one proton but may have zero, one or two neutrons depending on the isotope. Chemically and physically all the isotopes of an atom behave similarly. Collectively, protons and neutrons are referred to as nucleons.
Atomic Binding Energy
The mass of an atom is less than the sum of the individual nucleons within the atom's nucleus. This anomaly results from the binding energy that holds the atom together. Remember that energy and mass are related as stated by Einstein's famous equation. Thus, the difference in mass between the atom and the sum of its nucleons is the atomic binding energy. The atomic binding energy of an alpha particle, essentially a helium nucleus of two protons and two neutrons, is more than a million times greater than the energy between the nucleus and the electron.
Atomic Binding Energy Curve
The atomic binding energy can be divided by the number of nucleons in the nucleus for each element to produce a graph. This graph reveals that two isotopes of iron, Fe-56 and Fe-58, and the nickel isotope Ni-62 have the most tightly bound nuclei. Elements with less mass than these atoms can yield energy from nuclear fusion, and heavier elements can yield energy from nuclear fission. However, fission and fusion typically involve elements at the far end in each direction.
Nuclear Fission
Heavier elements may split into smaller atoms, releasing an astounding amount of energy in the process. Fission of one gram of U-238 releases more than one million times the energy released by burning one gram of natural gas. Unfortunately, U-238 undergoes spontaneous fission at a very slow rate. However, if enough material is collected, known as the critical mass, fission may be induced by targeting the nucleus with a neutron. As the U-238 atom splits, additional neutrons are released which can split additional atoms. Other elements can be used for similar reactions, such as Pu-239. While these reactions are often identified with nuclear reactors and the devastation in World War II of Hiroshima and Nagasaki, ore deposits in Africa suggest that in earth's distant past this chain reaction was occurring naturally.
Nuclear Fusion
Fusion involves the combining of lighter elements to form heavier elements. The most obvious place for nuclear fusion is in our own sun. Within the sun, hydrogen nuclei are fused together into helium nuclei, releasing a tremendous amount of energy, only a small part of which reaches the earth. As stars exhaust their hydrogen fuel, other fusion processes begin, such as fusion of helium into carbon. Fusion reactions have been duplicated on earth in hydrogen bombs. Unlike fission research, which produced controlled reactions before weaponization, fusion reactions have yet to be controlled in such a way as to allow energy production. Among the challenges related to fusion research is containment, as the high temperatures of fusion reactions vaporize any substance into a plasma.