Hydrogen is the lightest of all elements, having an atomic weight of 1.00794 amu. It is the most common element in the universe, and is the element from which, ultimately, all other elements are derived. This transformation takes place by way of atomic fusion in the center core of stars. The element was discovered in 1766 by Henry Cavendish. Hydrogen is highly combustible, forming an explosive mixture with oxygen. Much research is currently being carried out into the possible future use of the element as a fuel for automobiles.
Oxygen is element number 8 on the periodic table and has an atomic weight of 15.9994 amu. Oxygen is a colorless gas, vital to life as it is known and comprising some 20 percent of the Earth's atmosphere. It is the third most profuse element in the universe after hydrogen and helium, and the most plentiful element in the Earth's crust. Free oxygen was not present in the atmosphere until about 2 billion years ago, when it was first photosynthesized by plants.
Sodium is element number 11 on the periodic table and has an atomic weight of 22.9897 amu. It is a soft alkaline metal that is highly reactive with water, where it becomes very hot and gives off hydrogen, forming sodium hydroxide in the process. Small pieces of sodium will float and bounce around the surface of water, whereas larger pieces will explode. Sodium was first isolated in 1807 by Humphrey Davy. Molten sodium is widely used as a coolant in some types of nuclear reactors.
Chlorine is element number 17 on the periodic table and has an atomic weight of 35.4530 amu. Chlorine is a halogen alongside bromine, fluorine and iodine. In its natural state, it is a yellowish-green gas and is a disinfectant used widely in swimming pools. Chlorine was first isolated and studied by a Swedish chemist named Carl Scheele in 1774. Chlorine is poisonous when breathed in and was used as a poison gas during the Great War.