The valency of an element may be defined as the combining capacity of element.
The valency of an element may be expressed in terms of the number of electrons that an atom of the element makes available for bonding. The valency of an element is determined by the number of electrons in the outermost shell. These electrons are known as valence electrons.
In case of representative elements, the valency is generally equal to either the number of valence electrons or eight minus the number of valence electrons. However,· the transition elements, exhibit variable valency.
ARIATION OF-VALENCY IN THE PERIODIC ABLE
variation in a Period
The number of valency electrons increases from 1 to 8 on moving across a period, the valency of the elements with respect to hydrogen and chlorine increases from 1 to 4 and then decreases from 4 to zero. Table 6.14 shows the valencies of the elements of 2nd and 3rd periods. The valencies of the elements have been shown in brackets.
Variation in a Group
On moving down a group, the number of valence electrons remains .same and, therefore, all the elements in a group exhibit same valency. For example, all the elements of group 1 have valency equal to 1 and those of group 2 have valency equal to 2.
Table 6.14. Variation of Valencies and Type of Bonding Among Elements of Second and Third Periods
Example 6.15 Predict the formulae of the stable binary compounds that would be formed by the following pairs of elements:
(i) silicon and oxygen (ii) aluminium and bromine
(iii) calcium and iodine
Solution. (i) Silicon belongs to group 14. Its valency is 4. The valency of oxygen is 2. Thus, the binary compound between silicon and oxygen would have formula Si02
(ii) Aluminium belongs to group 13. Its valency is 3. The valency of bromine is 1. Thus, the binary compound between aluminium and bromine would have formula AlBr 3
(iii) Calcium belongs to group 2. Its valency is 2. The valency of iodine is 1. Thus, the compound between calcium and iodine would have formula CaI 2