Valence or valency is one of the most fundamental properties of the elements and it can be studied with the help of electronic configuration or by the number of electrons constituting the valence shell (outermost shell) of an atom. Electrons which are found in the outermost shell are generally known as valence electrons and the number of valence electrons determines the valency or valence of an atom. The valency of atoms of s-block and p-block elements are generally given as the number of valence electron or eight minus the number of valence electrons. Whereas, in d-block and f-block elements valency is determined not only on the basis of valence electrons but also on d and f orbital electrons. However, the general valencies of these d and f block elements are 2 and 3.
Variation along a period:
While moving left to right across a period, the number of valence electrons of elements increases and varies between 1 to 8. But the valency of elements, when combined with H or O first, increases from 1 to 4 and then it reduces to zero. Nowadays, valency is also termed as oxidation state. Consider two compounds containing oxygen Na2O and F2O. In F2O, the electronegativity of F is more than oxygen. Hence, each of F atoms will attract one electron from oxygen i.e. F will show -1 oxidation state and O will show +2 oxidation state. Whereas, in the case of Na2O, oxygen is highly electronegative than sodium atom. So oxygen will attract two electrons from each sodium atom showing -2 oxidation state and Na will have +1 oxidation state. The oxidation state of the element represents the charge possessed by an atom due to loss or gain of electrons (due to the electronegativity difference between the combining atoms) in the molecule.
Variation within a group:
As we move down in a group the number of the valence electron does not change. Hence, all the elements of one group have the same valency.
This was general about the periodicity of valence or oxidation states. To know more about the properties of elements and its trend in periodic table subscribe Byju’s on Youtube.