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The formulae for the oxides and oxy-acids of chlorine based on the modern electronic theory of valency, are somewhat different from the above. In some cases it is assumed that a pair of electrons necessary for the formation of a valency link and usually shared by two atoms, are provided by the same atom, when the formulae are as follows, the donation of a pair of electrons being shown by ->:
  
The oxygen electrons are denoted by dots, those of chlorine by crosses, that of hydrogen by a circle; an ordinary valency bond is formed by two atoms sharing a pair of electrons, a double bond by four shared electrons.
Chlorine dioxide contains an odd electron (unpaired), and is a representative of the somewhat rare class of odd molecules, which are generally coloured, magnetic, and unstable. In the new formulae for HClO2, HClO3 and HClO4 the oxygens other than that in the hydroxyl group (the hydrogen of which is ionisable) are attached by coordination bonds, or semi-polar double bonds, rather than by ordinary double bonds. The corresponding bromine and iodine compounds are similarly formulated. Iodine, in addition, forms a very stable H5IO6, which can be formulated as (HO)5I->O.
The element manganese, which occurs in the same group of the Periodic System as chlorine, is often assumed to be hepta-valent in the compound potassium permanganate, KMnO4, which resembles the perchlorate in crystalline form:
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