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Example 4.9 What is meant by ‘disproportionation’ of an oxidation state? Give an example. Solution When a particular oxidation state becomes less stable relative to other oxidation states, one lower, one higher, it is said to undergo disproportionation. For example, manganese (VI) becomes unstable relative to manganese(VII) and manganese (IV) in acidic solution. VI 2– + VII – IV 3 Mn O 4 + 4 H 2 Mn O 4 + Mn O + 2H 2 2 Intext Question 4.9 Explain why Cu ion is not stable in aqueous solutions? 4.4 Some 4.4.1 Oxides and Oxoanions of Metals Important These oxides are generally formed by the reaction of metals with oxygen at high temperatures. All the metals except scandium form Compounds of MO oxides which are ionic. The highest oxidation number in the Transition oxides, coincides with the group number and is attained in Sc O to 2 3 Elements Mn O2. B7yond group 7, no higher oxides of iron above Fe O are 2 3 V + IV know2+ Besides thIVoxides, the2+xocations stabilise V as VO , V as 2 VO and Ti as TiO . As the oxidation number of a metal increases, ionic character decreases. In the case of Mn, Mn O is a covalent green oil. Even CrO 2 7 3 and V O 2ave5low melting points. In these higher oxides, the acidic character is predominant. Thus, Mn O gives HMnO and CrO gives H CrO and H Cr O . 2 7 4 3 2 4 2 3– 7 V 2 i5, however, amphoteric though mainly acidic and it gives VO 4 as well as VO + salts. In vanadium there is gradual change from the basic 2 V 2 t3 less basic V O and t2 a4photeric V O . V O dissolves i2 a5ids 2 4 2+ to give VO salts. Similarly, V O reacts wi2h a5kalies as well as acids to give VO 3 and VO  respectively. The well characterised CrO is basic 4 4 but Cr O 2s 3mphoteric. Potassium dichromate K Cr O 2 2 7 Potassium dichromate is a very important chemical used in leather industry and as an oxidant for preparation of many azo compounds. Dichromates are generally prepared from chromate, which in turn are obtained by the fusion of chromite ore (FeCr O ) with sodium or 2 4 potassium carbonate in free access of air. The reaction with sodium carbonate occurs as follows: 4 FeCr O +28 N4 CO + 7 O 2 3 2 8 Na Cr2 + 2 4e O + 8 CO 2 3 2 The yellow solution of sodium chromate is filtered and acidified with sulphuric acid to give a solution from which orange sodium dichromate, Na Cr O .22H O2ca7 be cr2stallised. + + 2Na Cr2 + 2 H4 Na C2 O +22 7a + H O 2 105 The d- and f- Block Elements Reprint 2026-27