Hydrogen is an element that forms a diatomic molecule. Diatomic molecules are composed of two atoms of the same element and generally exist because the element is so reactive that it needs to bond to another atom. The reactivity of hydrogen contributes to many of its unique properties.

The physical properties of hydrogen are the things that can be observed or measured, such as its density of 0.0000899 g/cm. The melting point of hydrogen is -259.2 C and the boiling point is -252.8 C. Hydrogen is a colorless gas that is so much lighter than air that it can actually escape the gravitational pull of the earth and shoot off into space. Hydrogen is also the first element on the periodic table and has only on proton and one electron. Hydrogen does not have any neutrons.

Hydrogen is extremely combustible when in contact with oxygen. It is a non-metallic element, but behaves similarly to metals when in some bonding situations. Hydrogen is unique in that it can act like a metal in an ionic compound, donating electrons to the non-metal it bonds with or like a non-metal in a molecular compound, sharing electrons with another atom. Hydrogen has a relatively high electronegativity, which contributes to its affinity for bonding and to its diatomic nature.

Hydrogen partakes in a unique set of circumstances known as hydrogen bonding. A hydrogen bond is an attraction between two molecules in which the proton in a hydrogen atom of one molecule is attracted to unbonded pairs of electrons in another atom. For example, water undergoes strong hydrogen bonding where the hydrogen atoms of one molecule are attracted to the oxygen atom of another. It is this intermolecular force that holds water molecules together and accounts for special properties such as the high surface tension of water.