Oxygen is a colorless, odorless and tasteless gas slightly soluble in water. It is soluble in alkaline pyrogallol. It is slightly heavier than air. It can be liquefied by cooling under pressure to a pale blue liquid. It can also be solidified into a bluish white solid at -291'C. It is not inflammable but a supporter of combustion. It is paramagnetic. It exists in three isotopic forms with mass numbers 16, 17 and 18. Normally, the three isotopes of atomic weight 16, 17 and 18 are present in the ratio of 10000 : 1 : 8.



In accordance with the usual trend of p-block elements, oxygen the first member of group 16 elements differs from the other elements of the group. The difference in behaviour can be attributed to its high electronegativity, its small atomic size and non-availability of d-orbitals in its valence shell. Oxygen is second most electronegative element ( first is fluorine) of the periodic table. Its compounds are more ionic in nature while the other elements of the group give covalent compounds. Due to its small size and high electronegativity, oxygen is capable of forming hydrogen bonds in its compounds, e.g. water (H2O) and hence such compounds exist as associate molecules. This explains why water is liquid while hydrogen sulphide is a gas at ordinary temperature. It exhibits an oxidation state of -2 only except in OF2 where it exhibits +2 oxidation state. Other members show both negative and more frequently the positive oxidative states.

Oxygen, being smaller in size, is capable of forming pp ? pp multiple bonds with itself or with other elements. On the other hand, other elements of the group being large do not form multiple linkages. The ability of oxygen of forming multiple bonds in turn leads to its abnormal characteristics. Its diatomic and gaseous nature; other elements exists as 8-membered ring or zig-zag chain structures and are solid in nature. It has low melting point, boiling point and density. Since multiple bonds are shorter, the oxygen-oxygen bond distance in oxygen is less which leads to high bond energy and hence more energy must be supplied in order to break the oxygen-oxygen bond. Thus oxygen is inert although it is the second most electronegative element. Oxide ion is not easily polarized, so it is called a hard ion while the remainder ions of this group are called as soft ions.

Since oxygen has no d-orbital it shows a maximum valency of 2, while other elements having empty orbitals exhibit a maximum covalency of six. Of the 16 group elements, only oxygen shows paramagnetism indicating the presence of unpaired electrons. Oxygen is found in free state in air as O2 gas to the extent of 21% by volume or 23% by weight. In the combined state, it is present in water (89% by weight), earth's crust (about 50%) and in plants and animal tissues (50-70%). In fact combined oxygen is much more plentiful than free oxygen.