Oxidation States Oxidation States In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by integers, which can be positive, negative, or zero. In some cases, the average oxidation state of an element is a fraction, such as 8/3 for iron in magnetite (Fe3O4). The highest known oxidation state is +8 in the tetroxides of ruthenium, xenon, osmium, iridium, and hassium, and some complexes involving plutonium, while the lowest known oxidation state is −4 for some elements in the carbon group. The increase in oxidation state of an atom through a chemical reaction is known as an oxidation; a decrease in oxidation state is known as a reduction. Such reactions involve the formal transfer of electrons, a net gain in electrons being a reduction and a net loss of electrons being an oxidation. For pure elements, the oxidation state is zero.
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The definition of the oxidation state listed by IUPAC is as follows: Oxidation state :- A measure of the degree of oxidation of an atom in a substance. It is defined as the charge an atom might be imagined to have when electrons are counted according to an agreed-upon set of rules: (1) the oxidation state of a free element (uncombined element) is zero (2) for a simple (monoatomic) ion, the oxidation state is equal to the net charge on the ion (For example, Cl- has an oxidation state of -1) (3) hydrogen has an oxidation state of 1 and oxygen has an oxidation state of −2 when they are present in most compounds. (Exceptions to this are that hydrogen has an oxidation state of −1 in hydrides of active metals, e.g. LiH, and oxygen has an oxidation state of −1 in peroxides, e.g. H2O2 or −1/2 in superoxides, e.g. KO2) (4) the algebraic sum of oxidation states of all atoms in a neutral molecule must be zero, while in ions the algebraic sum of the oxidation states of the constituent atoms must be equal to the charge on the ion. For example, the oxidation states of sulfur in H2S, S8(elementary sulfur), SO2, SO3, and H2SO4 are, respectively: −2, 0, +4, +6 and +6. The higher the oxidation state of a given atom the greater its degree of oxidation; the lower the oxidation state the greater its degree of reduction. Oxidation State of Chromium Chromium has atomic number 24 and electronic configuration is [Ar] 3d5 4s1 due to more stable half filled orbitals. They show the oxidation state + 2, + 3, + 4, + 5, + 6. Generally Chromium forms compounds with oxidation state +2, +3 and +6.
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The +2 state of chromium is easily oxidized in aqueous solution to the +3 state. The +3 and +6 states are more stable in chromium. Chromium (III) – They can be obtained by dissolving the element chromium in acids like HCl or H2SO4. Generally they formed the octahedral complexes. For example-the dark green complex chromium(III) chloride hydrate [CrCl2(H2O)4]Cl, Chromium(III) hydroxide (Cr(OH)3), chromium(III) oxide (Cr2O3), Anhydrous chromium(III) chloride (CrCl3) etc. Chromium (VI)- These compounds are powerful oxidants at low or neutral pH. For examplechromate anion (CrO42-) and dichromate (Cr2O72-) anions. Their halides are also known like hexafluoride CrF6, chromyl chloride (CrO2Cl2). Other examples are Chromic acid (H2CrO4), chromium (VI) oxide CrO3 which is the acid anhydride of chromic acid. Chromium (V) – Only few compounds are known. For examples-peroxochromate (V), chromium (V) fluoride (CrF5) etc. Chromium (IV) - They are more common than Cr(V). For example- tetra halides like CrF4, CrCl4 and CrBr4 etc. Chromium (II) – For example- CrCl2, chromos acetate (Cr2(O2CCH3)4) etc.
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