Oxygen is the most abundant element in erath's crust (46 % by weight). Oxygen is present in Group 16 in the periodic table. The other elements in this period are: Sulphur (S), Selenium (Se), Tellurium (Te). The general electronic configuration of the elements present in Group 16 is: ns² np⁴. Oxygen is smallest in size among other elements in the same group. It exists as O₂ molecule where O=O bond is found. These O₂ molecules are associated through van der Waal's force of attraction which are very weak, because of that O₂ exists as gas. Except oxygen, other elements do not form divalent molecule and they have d-orbitals, so they can show catenation property by which large number of atoms are connected via covalent bonds. Hence those elements are not gaseous. Sulphur, Selenium and Tellurium are solid,
