If a one kilowatt generator runs for an hour, it produces a kilowatt-hour of electric energy. The amount of energy handled by humanity is still small compared to the amount that strikes the earth. It's about one part in 50,000. Energy is conserved . In each transformation, some of the energy becomes unusable, usually in the form of heat.
All matter is composed of elements.
The important elements for our discussion of nuclear energy are uranium, plutonium, carbon and hydrogen. Uranium and plutonium are involved in nuclear energy production, and carbon and hydrogen are the main elements in conventional fuels like coal, oil and natural gas. An atom of an element consists of a nucleus surrounded by electrons. What element it is is determined by the number of protons in the nucleus, but the elements come in various isotopes , and the isotope is defined by the number of neutrons in the nucleus. Matter takes part in two kinds of reactions involving atoms of different kinds - chemical reactions and nuclear reactions.