CBSE Class 12-science Answered

Lower oxidation states are favoured by the heavier members of the p-block. The elements of group 13-18 have 3 to 8 electrons in their valence shell (ns²np^l-6) and, therefore, exhibit, oxidation state of + 3, +2, so on. However, it bas been observed that in addition to + 3 oxidation state, they also exhibit oxidation state of +1. The +1 oxidation state becomes more and more stable as one goes down the group for example in case of group 13 from B, Al, Ga, In to Tl. The +1 oxidation state of Tl is more stable than +3 oxidation state. The reason for this is attributed to the inert pair effect. In the case of last element, after the removal of one electron from p-orbital, the remaining ns² (e.g. 6s²) electrons behave like stable noble gas and do not take part formation. This reluctance of the s-electron pair to take part in chemical combination is called inert pair effect. Hence, there is high stability of lower oxidation states for the heavier elements of p-block. The variability of oxidation states, a characteristic of transition elements, arises out of incomplete filling of d orbitals in such a way that their oxidation states differ from each other by unity, e.g.,+2,+3,+4,+5 etc.