56Water (H 2 O) is the oxide of hydrogen and the most familiar oxygen compound. Its bulk properties
partly result from the interaction of its component atoms, oxygen and hydrogen, with atoms of
nearby water molecules. Hydrogen atoms are covalently bonded to oxygen in a water molecule but
also have an additional attraction (about 23.3 kJꞏmol−1 per hydrogen atom) to an adjacent oxygen
atom in a separate molecule. These hydrogen bonds between water molecules hold them
approximately 15% closer than what would be expected in a simple liquid with just Van der Waals
forces.
Oxides, such as iron oxide or rust, Fe 2 O 3 , form when oxygen combines with other elements
Due to its electronegativity, oxygen forms chemical bonds with almost all other free elements at
elevated temperatures to give corresponding oxides. However, some elements, such as iron which
oxidizes to iron oxide, or rust, Fe 2 O 3 , readily oxidize at standard conditions for temperature and
pressure (STP).
The surface of metals like aluminum and titanium are oxidized in the presence of air and become
coated with a thin film of oxide that passivates the metal and slows further corrosion. So-called
noble metals, such as gold and platinum, resist direct chemical combination with oxygen, and
substances like gold(III) oxide (Au 2 O 3 ) must be formed by an indirect route.
The alkali metals and alkali earth metals all react spontaneously with oxygen when exposed to dry
air to form oxides, and form hydroxides in the presence of oxygen and water. As a result, none of
these elements is found in nature as a free metal.
Caesium is so reactive with oxygen that it is used as a getter in vacuum tubes. Although solid
magnesium reacts slowly with oxygen at STP, it is capable of burning in air, generating very high
temperatures, and its metal powder may form explosive mixtures with air.
Oxygen is present as compounds in the atmosphere in trace quantities in the form of carbon dioxide
(CO 2 ) and oxides of nitrogen (NOx).