Group 16 (Oxygen group) elements

The second remaining p-block group is Group 16, the oxygen family or chalcogens ("ore-formers"), so called because most naturally occurring ores are oxides or sulphides of other elements. This group comprises oxygen, sulphur, selenium, tellurium and polonium. Oxygen, the most abundant element on earth, occurs both as free dioxygen gas (making up above 20% of the atmosphere by weight and by volume) and, more abundantly still, in combined form as oxide minerals; oxygen and sulphur together make up about 46.6% and 0.034% respectively of the earth's crust by weight. Sulphur occurs both as sulphates (such as gypsum and Epsom salt) and as sulphides (such as galena and zinc blende), and is also found in volcanic emissions. The heavier chalcogens, selenium and tellurium, are comparatively scarce and are typically found as selenide and telluride impurities associated with sulphide ores. Down the group, physical state changes from the diatomic gas oxygen to solids; atomic radius, density, melting point and boiling point all rise steadily from oxygen to polonium as heavier, more metallic elements are reached, while every member retains the common outer electronic configuration ns2 np4, which underlies the group's characteristic -2 oxidation state (though the heavier members increasingly also show positive oxidation states, e.g. +4 and +6).

Oxygen: the atmosphere is about 23% oxygen and water is about 83% oxygen by mass, and most of the world's rocks contain combined oxygen; industrially, oxygen is separated by the fractional distillation of liquefied air. In the laboratory it can be prepared by decomposing hydrogen peroxide, either catalytically with manganese dioxide (2H2O2 --MnO2--> 2H2O + O2) or by oxidation with acidified potassium permanganate (5H2O2 + 2MnO4- + 6H+ -> 5O2 + 8H2O + 2Mn2+), or by the thermal decomposition of certain metal oxides or oxoanions: 2HgO -> 2Hg + O2; 2BaO2 -> 2BaO + O2; 2KClO3 --MnO2 catalyst, heat--> 2KCl + 3O2; 2KNO3 --heat--> 2KNO2 + O2. Under ordinary conditions oxygen exists as the diatomic, paramagnetic gas O2 (its paramagnetism, arising from two unpaired electrons in antibonding molecular orbitals, is a classic test case for molecular orbital theory over simple Lewis structures). Like nitrogen and fluorine, oxygen forms strong hydrogen bonds in its compounds (e.g. water). Oxygen has two allotropes: ordinary dioxygen (O2) and ozone or trioxygen (O3); although only negligible amounts of ozone form at sea level, it is generated continuously in the upper atmosphere by the action of ultraviolet light on O2 (O2 <=> 2O, then O + O2 <=> O3). In the laboratory ozone is generated by passing a silent electric discharge through oxygen; at about 20,000 V roughly 10% of the oxygen is converted, giving a mixture called ozonised oxygen, from which pure, pale-blue ozone can be isolated by the fractional distillation of liquefied ozonised oxygen. The ozone molecule is bent and symmetrical, with delocalised (resonance) bonding shared equally between the two O-O linkages. Chemically, oxygen and ozone behave very differently: oxygen combines directly with many metals and non-metals to form oxides, reacting with s-block metals even at room temperature, and some finely divided (pyrophoric) metals ignite spontaneously in air even though a solid lump of the same metal is unaffected. Ozone, by contrast, is a much more powerful oxidising agent and reacts with many substances that dioxy …