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Valencies are sometimes classified as.positive and negative, the former being the valencies of atoms or radicals attracted to the negative electrode (cathode) in electrolysis, and the latter those attracted to the positive electrode (anode). In this way, we first consider the electrochemical character of the element, apart from its valency. This electrochemical character is epitomised in the table on p. in; metals and hydrogen are electropositive and oxygen and halogens electronegative; other elements are sometimes electropositive and sometimes electronegative.
In its compounds with hydrogen or metals an element is assumed to be electronegative (except metals in hydrides, such as Li+H-), whilst in its compounds with oxygen, halogens or sulphur it is electropositive. This polar character of elements was particularly emphasised in the electrochemical or dualistic theory of Berzelius (1811). Owing to the fact that carbon is an element without electrochemical individuality, the theory was of little service in the early study of carbon compounds, and since this branch of chemistry monopolised the attention of many chemists in the last century, Berzelius's theory was neglected or considered to be erroneous.
Elements of low valency (halogens; alkali metals) are outstandingly negative or positive; as the hydrogen valency increases, the sharp definition of properties falls off, and in elements of the maximum hydrogen valency of four, especially carbon, it has practically disappeared. For the present, we may call the valency of alkali metals +1, that of halogens in simple compounds -1, that of sulphur in H2S -2, in H2SO4 +6, that of chlorine in Cl2O7 +7, and so on, whilst that of carbon we call simply 4. The notation is extended to radicals; the valency of NH4 is +1, that of SO4 is -2. The full meaning of this classification of valencies is explained later by the electrical theory of the atom.
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