Most of the trends in chemical properties of elements, such as diagonal relationships, inert pair effect, effects of lanthanoid contraction etc. will be dealt with along the discussion of each group in later units. In this section we shall study the periodicity of the valence state shown by elements and the anomalous properties of the second period elements (from lithium to fluorine).
(a)Periodicity of Valence or Oxidation States
The valence is the most characteristic property of the elements and can be understood in terms of their electronic configurations.
The valence of representative elements is usually (though not necessarily) equal to the number of electrons in the outermost orbitals and / or equal to eight minus the number of outermost electrons as shown below. Nowadays the term oxidation state is frequently used for valence.
Consider the two oxygen containing compounds: OF2 and Na 2 O. The order of electronegativity of the three elements involved in these compounds is F > O > Na. Each of the atoms of fluorine, with outerelectronic configuration 2s 22p 5 , shares one electron with oxygen in the OF2 molecule.
Being highest electronegative element, fluorine is given oxidation state –1. Since there are two fluorine atoms in this molecule, oxygen with outer electronic configuration 2s 2 2p 4 shares two electrons with fluorine atoms and thereby exhibits oxidation state +2. In Na2O, oxygen being more electronegative accepts two electrons, one from each of the two sodium atoms and, thus, shows oxidation state –2.
On the other hand sodium with electronic configuration 3s 1 loses one electron to oxygen and is given oxidation state +1. Thus, the oxidation state of an element in a particular compound can be defined as the charge acquired by its atom on the basis of electronegative consideration from other atoms in the molecule.
(b) Anomalous Properties of Second Period Elements
The first element of each of the groups 1 (lithium) and 2 (beryllium) and groups 13-17 (boron to fluorine) differs in many respects from the other members of their respective group.
For example, lithium unlike other alkali metals, and beryllium unlike other alkaline earth metals, form compounds with pronounced covalent character; the other members of these groups predominantly form ionic compounds.
In fact the behaviour of lithium and beryllium is more similar with the second element of the following group i.e., magnesium and aluminium, respectively. This sort of similarity is commonly referred to as diagonal relationship in the periodic properties.
What are the reasons for the different chemical behaviour of the first member of a group of elements in the s- and p-blocks compared to that of the subsequent members in the same group? The anomalous behaviour is attributed to their small size, large charge/ radius ratio and high electronegativity of the elements.
In addition, the first member of group has only four valence orbitals (2s and 2p) available for bonding, whereas the second member of the groups have nine valence orbitals (3s, 3p, 3d). As a consequence of this, the maximum covalency of the first member of each group is 4 (e.g., boron can only form BF4 [ ]− , whereas the other members of the groups can expand their valence shell to accommodate more than four pairs of electrons e.g., aluminium AlF6 3 [ ] − forms).
Furthermore, the first member of p-block elements displays greater ability to form pp – pp multiple bonds to itself (e.g., C = C, C º C, N = N, N º N) and to other second period elements (e.g., C = O, C = N, C º N, N = O) compared to subsequent members of the same group.