Oxygen (O), Sulphur (S), Selenium (Se), Tellurium(Te) and Polonium(Po) are all called group 16 elements. They are also called as chalcogens.

Oxygen is the most abundant of all elements on our earth. Dry air contains 20.946% oxygen by volume. However, the abundance of Sulphur in the earth’s crust is only 0.03-0.1%. Combined Sulphur exists primarily as sulphates such as gypsum CaSO4 .2H2O, Epsom salt MgSO4 .7H2O, baryte BaSO4 and sulphides such as galena PbS, zinc blende ZnS, copper pyrites CuFeS2 . Traces of Sulphur occur as hydrogen sulphide in volcanoes. Organic materials such as eggs, proteins, garlic, onion, mustard, hair and wool contain Sulphur.

Selenium and Tellurium are found in their sulphide ores whereas Polonium is a radioactive element formed as a decay product of uranium and thorium minerals.

General electronic configuration of Group 16 elements: ns2 np4

Order of atomic radii: Po>Te>Se>S>O but size of oxygen is very small

Order of Ionization enthalpy: Ionization enthalpy decreases down the group. O>S>Se>Te>Po

Order of Electron Gain enthalpy: OSe>Te>Po

Order of Electronegativity: O>S>Se>Te>Po this implies that the metallic character increases from Oxygen to Tellurium. And oxygen has the second highest electronegativity after fluorine.

Oxygen and Sulphur are non-metals, Selenium and Tellurium are metalloids, Polonium is a metal. Oxygen exists as diatomic molecule (O2 ) whereas Sulphur exists as polyatomic molecule(S8). Oxygen exhibits -2,-1,1,2, Sulphur exhibits -2,2,4,6, Selenium exhibits -2,2,4,6, Tellurium exhibits -2,2,4,6 and Polonium exhibits 2,4 oxidation states. Oxygen behaves anomalously because of its small size and high electronegativity. The absence of d-orbitals limits its covalency to 4.

Hydrides of Group 16 elements: Acidic character: H2Te> H2Se> H2S> H2O this is because the bond dissociation enthalpy order is as follows: H2O> H2S>H2Se> H2Te. Thermal stability order: H2O>H2S>H2Se>H2Te>H2Po All hydrides except water have reducing properties in the order H2Te>H2Se>H2S

Oxides of Group 16 elements: All the elements form EO2 and EO3( only formed by Sulphur, Selenium and Tellurium) type oxides where both are acidic oxides. O3 is a gas, SO2 is a gas, SeO2 is a solid. Reducing property of dioxides: SO2>SeO2>TeO2

Halides of Group 16 elements: They form EX6 , EX4 and EX2. Stability of the halides: F>Cl>Br>I. Among hexahalides only hexafluorides are stable. All hexafluorides are gaseous in nature and have octahedral structure. SF6 is exceptionally stable for steric reasons. Among tetrafluorides SF4 is a gas, SeF4 is a liquid and TeF4 is a solid. They have sp3d hybridization and trigonal bipyramidal structure. The well known mono halides are dimeric in nature.

Few compounds of Group 16 elements:

Dioxygen (O2): Dioxygen can be obtained in the laboratory by heating oxygen containing salts such as chlorates, nitrates and permanganates or by the thermal decomposition of the oxides of metals low in the electrochemical series and higher oxides of some metals or by decomposition of hydrogen peroxide. On large scale it can be prepared from water or air. Industrially, dioxygen is obtained from air by first removing carbon dioxide and water vapor and then, the remaining gases are liquefied and fractionally distilled to give dinitrogen and dioxygen. Dioxygen is a colorless and odorless gas. Dioxygen reacts with all the metals and non-metals except few noble metals. It’s the major component in normal respiration and combustion processes, and also in oxyacetylene welding. Non-metal oxides are acidic in nature but some of the higher oxides of metals are also acidic in nature. In general metal oxides are basic in nature. There are few other oxides which are amphoteric and neutral.

Ozone (O3): Ozone is an allotropic form of oxygen. It is formed from atmospheric oxygen in the presence of sunlight. This ozone layer protects the earth’s surface from UV radiations. When a slow dry stream of oxygen is passed through a silent electrical discharge, conversion of oxygen to ozone (10%) occurs. The product is known as ozonized oxygen. Ozone is unstable and releases high amount of heat on converting into oxygen so high concentrations of ozone can be extremely dangerous. Ozone is a powerful oxidizing agent and is harmless only in small concentrations. It is used as a oxidizing agent, bleaching agent, germicide, disinfectant and for sterilizing water.

Sulphur: Sulphur exists in many allotropic forms. The two important forms are yellow rhombic (α-Sulphur) and monoclinic (β -Sulphur). Rhombic Sulphur crystals are formed on evaporating the solution of roll Sulphur in CS2. It is insoluble in water and is in yellow color. Monoclinic Sulphur is prepared by melting rhombic Sulphur in a dish and cooling. It is soluble in CS2. Above 369K β -Sulphur is stable and below 369K α-Sulphur is stable where 369K is called transition temperature. These two forms have S8 molecules.

Sulphur Dioxide(SO2): Sulphur dioxide is formed together with a little Sulphur trioxide when Sulphur is burnt in air or oxygen. In the laboratory it is readily generated by treating a sulphite with dilute sulphuric acid. Industrially, it is produced as a by-product of the roasting of sulphide ores. Sulphur dioxide is a colorless gas with pungent smell and is highly soluble in water. When moist SO2 acts as a reducing agent. It is angular in shape. SO2 is an anti-chlor and also used in refining of petroleum and sugar.

Sulphuric Acid (H2SO4): Sulphuric acid is one of the most important industrial chemicals worldwide. It is manufactured by the Contact Process which involves three steps. It is a colorless, dense, oily liquid. It has low volatility, strong acidic character, strong affinity for water and acts as an oxidizing agent. A nation’s industrial strength can be judged by the quantity of sulphuric acid it produces and consumes. So sulphuric acid is a very important compound of Sulphur.

Group 16 elements and their study is a very important part of inorganic chemistry.