Group 7, numbered by IUPAC nomenclature, is a group of elements in the periodic table. It contains manganese (Mn), technetium (Tc), rhenium (Re) and bohrium (Bh). This group lies in the d-block of the periodic table, and are hence transition metals. This group is sometimes called the manganese group or manganese family after its lightest member; however, the group itself has not acquired a trivial name because it belongs to the broader grouping of the transition metals. The group 7 elements tend to have a major group oxidation state (+7), although this trend is markedly less coherent than the previous groups. Like other groups, the members of this family show patterns in their electron configurations, especially the outermost shells resulting in trends in chemical behavior. In nature, manganese is a fairly common element, whereas rhenium is rare, technetium only occurs in trace quantities, and bohrium is entirely synthetic.
Physical properties The trends in group 7 follow, although less noticeably, those of the other early d-block groups and reflect the addition of a filled f-shell into the core in passing from the fifth to the sixth period. All group 7 elements crystallize in the hexagonal close packed (hcp) structure except manganese, which crystallizes in the body centered cubic (bcc) structure. Bohrium is also expected to crystallize in the hcp structure. The table below is a summary of the key physical properties of the group 7 elements. The properties of bohrium are either unknown or predicted, as they have not been measured.
Chemical properties Like other groups, the members of this family show patterns in its electron configuration, especially the outermost shells:
All the members of the group readily portray their group oxidation state of +7 and the state becomes more stable as the group is descended. Technetium also shows a stable +4 state whilst rhenium exhibits stable +4 and +3 states. Bohrium may therefore also show these lower states as well. The higher +7 oxidation state is more likely to exist in oxyanions, such as perbohrate, BhO4−, analogous to the lighter permanganate, pertechnetate, and perrhenate. Nevertheless, bohrium(VII) is likely to be unstable in aqueous solution, and would probably be easily reduced to the more stable bohrium(IV).
Compounds
Oxides
Manganese
Manganese forms a variety of oxides: MnO, Mn3O4, Mn2O3, MnO2, MnO3 and Mn2O7. Manganese(II) oxide is an inorganic compound that forms green crystals. Like many monoxides, MnO adopts the rock salt structure, where cations and anions are both octahedrally coordinated. Also like many oxides, manganese(II) oxide is often nonstoichiometric: its composition can vary from MnO to MnO1.045. Manganese(II,III) oxide is formed when any manganese oxide is heated in air above 1000 °C. Considerable research has centred on producing nanocrystalline Mn3O4 and various syntheses that involve oxidation of MnII or reduction of MnVI. Manganese(III) oxide is unlike many other transition metal oxides in that it does not adopt the corundum (Al2O3) structure. Two forms are generally recognized, α-Mn2O3 and γ-Mn2O3, although a high pressure form with the CaIrO3 structure has been reported too. Manganese(IV) oxide is a blackish or brown solid occurs naturally as the mineral pyrolusite, which is the main ore of manganese and a component of manganese nodules. The principal use for MnO2 is for dry-cell batteries, such as the alkaline battery and the zinc–carbon battery. Manganese(VII) oxide is dark green in its crystalline form. The liquid is green by reflected light and red by transmitted light. It is soluble in carbon tetrachloride, and decomposes when in contact with water.
Technetium
Technetium's main oxides are technetium(IV) oxide and technetium(VII) oxide. Technetium(IV) oxide was first produced in 1949 by electrolyzing a solution of ammonium pertechnetate under ammonium hydroxide. It has often been used to separate technetium from molybdenum and rhenium. More efficient ways are the reduction of ammonium pertechnetate by zinc metal and hydrochloric acid, stannous chloride, hydrazine, hydroxylamine, ascorbic acid, by the hydrolysis of potassium hexachlorotechnetate or by the decomposition of ammonium pertechnetate at 700 °C under an inert atmosphere. It reacts with oxygen to produce technetium(VII) oxide at 450 °C. Technetium(VII) oxide can be prepared directly by the oxidation of technetium at 450-500 °C. It is a rare example of a molecular binary metal oxide. Other examples are ruthenium(VIII) oxide and osmium(VIII) oxide. It adopts a centrosymmetric corner-shared bi-tetrahedral structure in which the terminal and bridging Tc−O bonds are 167pm and 184 pm respectively and the Tc−O−Tc angle is 180°.
Rhenium Rhenium's main oxides are rhenium(IV) oxide and rhenium(VII) oxide. Rhenium(IV) oxide is a gray to black crystalline solid that can be formed by comproportionation. At high temperatures it undergoes disproportionation. It is a laboratory reagent that can be used as a catalyst. It adopts the rutile structure. It forms perrhenates with alkaline hydrogen peroxide and oxidizing acids. In molten sodium hydroxide it forms sodium rhenate:
2 NaOH + ReO2 → Na2ReO3 + H2O Rhenium(VII) oxide can be formed when rhenium or its oxides or sulfides are oxidized a 500-700 °C in air. It dissolves in water to give perrhenic acid. Heating Re2O7 gives rhenium(IV) oxide, signalled by the appearance of the dark blue coloration. In its solid form, Re2O7 consists of alternating octahedral and tetrahedral Re centres. It is the raw material for all rhenium compounds, being the volatile fraction obtained upon roasting the host ore. Rhenium, in addition to the +4 and +7 oxidation states, also forms a trioxide. It can be formed by reducing rhenium(VII) oxide with carbon monoxide at 200 C or elemental rhenium at 4000 C. It can also be reduced with dioxane. It is a red solid with a metallic lustre that resembles copper in appearance, and is the only stable trioxide of the group 7 elements.
Halides
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