Oxygen Group (Group VIA) Trends
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Summary of Oxygen Group (Group VIA) Trends:
1. The elements become progressively more metallic down the column. Polonium has chemical and physical properties of a metal and tellurium is borderline.
2. Chemically the metallic character of the heavier elements is reflected in their increased tendency to form cationic species, the ionic character and basicities of their oxides, and their increased tendency to form complexes.
3. The non-metal character of the earlier members of the Group is the molecular nature of the stable elemental allotrophs, the ability to form anions, e.g. O2- and S2-, which result from completing the octet. Compounds resulting from these anions, e.g. Na2E, CaE (E = O, S, Se ..) become progressively more covalent down the Group as a result of the decreasing electronegativiety of the chalcogens (group VIA).
4. The thermal stabilities of the hydrides EH2 decrease down the column primarily because of the decreasing EH mean bond enthalpies.
5. The abilities of these hydrides to form hydrogen bonds decreases rapidly down the Group after oxygen and this has a dramatic effect on the boiling and melting points of the hydrides.
6. The formation of molecular compounds with strong multiple bonds is particularly important for oxygen which forms strong pπ-pπ bonds, e.g. CO, dπ-pπ bonds with transition metals, e.g. OsO4, “allow” main group compounds to achieve hypervalency, e.g. R3PO, and even fπ-pπ bonding, e.g. UO22+. Multiple bonding is less significant for the heavier elements, although sulfur and selenium provide some examples, e.g. selenoketones, R3PS, and R3PSe.
7. The increasing size of the atoms leads to compounds with progressively larger maximum coordination numbers (coordinative unsaturation). Oxygen usually has a coordination number of 2 or 3 with a few examples of 4 coordination, sulfur exhibits a maximum coordination number of 6, and higher coordination numbers of 8 have been observed for Te.
- These maximum coordination numbers have an impact on the case of hydrolysis of the halides, e.g. the rate of hydrolysis is: TeF6 > SeF6 > SF6
- Also, the octahedral anionic complexes [MX6]2- (X = halide) are more commonly observed for Se, Te, and Po.
8. Oxygen has a strong preference for the -2 formal oxidation state, whereas the heavier elements exhibit oxidation states of 2, 4, and 6. The alternation in oxidation state stability effect is observed far more in Group VIa than Group Va.
9. The tendency towards catenation reaches a maximum at sulfur, which forms a wide range of ring and chain compounds, e.g. Sn, XSnX, [O3S(Sn)SO3]2-,and S82+.