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Group 6 elements GROUP 6 ELEMENTS The most important elements in this group are oxygen and sulphur, both of which are non-metals. The other elements in this group, selenium, tellurium and polonium (radioactive) are metalloids. Each element is characterised by having six electrons in its outer shell. Oxygen, sulphur, selenium and tellurium are also known as the CHALCOGENS, a word that comes from the Greek meaning ‘brass maker’, because all these elements are found in copper ores, and copper is the most important metal in making brass. The most common member of this group is oxygen, itself the most abundant element in the Earth’s crust and which also makes a quarter of the mass and just over a fifth of the volume of the Earth’s atmosphere. Oxygen is essential for most living things, and its most important compound is water (H2O, see page 13). Oxygen and sulphur combine to form sulphur dioxide SO2 – an acidic gas, illustrating the non-metal chemistry of sulphur. Relative size of a hydrogen atom Group 6 elements have six electrons in the outer shell 6 2 Oxygen, from the Greek for acid forming, ‘oxys genes’; chemical symbol O; Group 6, period 2; non-metal; colourless as oxygen gas and dark blue as ozone gas; atomic number 8; approximate relative atomic mass 16. 6 8 2 Sulphur, from the Sanskrit ‘sulvere’; chemical symbol S; Group 6, period 3; non-metal; yellow solid; atomic number 16; approximate relative atomic mass 32. Selenium, from the Greek for the moon, ‘selene’; chemical symbol Se; Group 6, period 4; metalloid; grey solid; atomic number 34; approximate relative atomic mass 79. 6 18 18 8 2 6 18 8 2 Tellurium, from the Latin for earth, ‘tellus’; chemical symbol Te; Group 6, period 5; metalloid; silvery-white solid; atomic number 52; approximate relative atomic mass 128. 6 18 32 18 8 2 (Right) Oxygen makes up 21% of the atmosphere by volume and 23% by mass. It is produced by plants during photosynthesis and used by living things during respiration. Polonium, named after Poland; chemical symbol Po; Group 6, period 6; metalloid, radioactive; grey solid; atomic number 84; approximate relative atomic mass (209). 54 Demonstration 1: oxygen dissolves in water and so can oxidise some dissolved substances Concentrated ammonia solution is added from a pipette to a test tube containing white copper(I) chloride (햲). This produces a colourless solution of copper(I) ammine. However, the copper(I) ammine is not stable in air and rapidly oxidises to copper(II) ammine which is a deep indigo-blue colour (햳). 햳 햲 The way to see that this effect is produced by oxidation is to leave the tube to stand. Within a few moments, a very deep blue layer begins to form on the surface of the solution that can be reached by the air. The first signs occur in the shape of the meniscus where the liquid meets the air. Over some minutes, air dissolves in the solution to about one and a half centimetres, and thus the blue colour gradually extends into the solution. If the blue part of the liquid is taken off by using a 햴 pipette, the surface part of the solution rapidly returns to being blue as oxidation occurs at the meniscus again. Remarks Deep blue copper(II) ammine Ammonia solution The penetration of oxygen into water, and the very reactive nature of oxygen, are also the reasons that nails rust when left to stand in water (햴). Colourless copper(I) ammine EQUATION: The rusting of iron Copper(I) chloride Iron + water + oxygen ➪ iron(III) hydroxide ➪ iron(III) oxide + water 4Fe(s) + 6H2O(l) + 3O2(g) ➪ 4Fe(OH)3(s) ➪ 2Fe2O3(s) + 6H2O(l) Rust 55