Hydrogen is the lightest of all the elements, and is the most abundant in
the universe, where it accounts for 93% of the total number or atoms and 76%
of the total mass. Hydrogen is contained in most stars, including the sun,
where it is continually being converted into helium by the process of
nuclear fusion, producing heat and light. Under high pressures, it can
become a solid with metallic properties.
On Earth, hydrogen occurs mainly with oxygen as water. Hydrogen is used in
the production of margarines, in welding, and in rocket fuel.
Hydrogen is a reducing agent, burns in air with a light blue flame and, when
heated, reactswith many other substances.


Deuterium - An isotope of hydrogen containing a neutron as well as a proton
in the nucleus. It accounts for 0.016% of natural hydrogen. Water
consisting of deuterium and oxygen is called deuterium oxide or heavy water,
and is used in nuclear reactors.

Tritium - An isotope of hydrogen with one proton and two neutrons. It rarely
occurs naturally and is radioactive, emitting beta particles.
