Question

The valences of p block elements are equal to group number or 8 minus the group number. Therefore, they show one lower valency and one higher valency. However, if we go to the bottom elements of any group, the compounds with lower valency are predominant whereas higher valency becomes rather uncommon. Explain.

Answer 1

The p-block is on the right side of the standard periodic table and encompasses elements in groups 13 to 18. The electrons present in the outermost shell are called valence electrons. It has been found that in the case of p-block elements, the valency of an element is either equal to the number of valence electrons or eight minus the number of valence electrons, e.g. groups 13, 14, 15, 16.

Complete step by step answer:
The valency of an element is the number of electrons which it loses, joins or shares with other atoms to attain the nearest inert gas configuration. Thus, the valency of the elements of groups 1, 2 and 13 is equal to the number of their valence electrons. The elements of group 14 have four electrons in their respective valence shells and thus attain noble gas configuration only by sharing with four more electrons of other atoms. Therefore, the valency of elements of group 14 is also equal to the number of their valence electrons. It is due to the Inert pair effect. It is increasing the stability of oxidation states that are two less than the group valency for the heavier elements of groups 13,14 ,15 and16.

For example, thallium (Tl) in group 13. The +1-oxidation state of Tl is the most stable, while $T{l^{+III}}$ compounds are comparatively rare. The stability of the +1-oxidation state increases in the following sequence: $A{l^{+I}}\; < G{a^{+I}}\; < I{n^{+I}}\; < T{l^{+I}}$.

Note: The elements of group 15, 16 and 17 have 5, 6 and 7 electrons in their respective valence shells and hence they acquire $3\left( {8 - 5} \right),2\left( {8 - 6} \right)$ and $1\left( {8 - 7} \right)$ electrons either by gaining or sharing electrons from other atoms to attain the nearest inert gas configuration. Thus, the valency of the elements of groups 15, 16 and 17 is equal to eight minus the number of electrons in their respective valence shells.