Oxidation State of Group 15 Elements
Group 15 elements consist of nitrogen, phosphorus, arsenic, antimony and bismuth. As we move down the group, there is a transition from non-metallic to metallic through metalloid character. The elements nitrogen and phosphorus are non-metals, arsenic and antimony are metalloids and lastly, we have bismuth which is a typical metal. The general valence shell electronic configuration of these elements is ns2np3. The s orbital is completely filled and p orbital is half-filled which makes them stable in nature. The chemical properties of these elements are determined by the oxidation states exhibited by them.
Chemical Properties and Oxidation State of group 15 elements
The elements of group 15 generally exhibit -3, +3 and +5 oxidation states. The tendency to exhibit -3 oxidation state decreases as we move down the group due to an increase in the size of the atom and the metallic character. Bismuth hardly forms any compound in oxidation state -3. In fact, the stability of the +5 state also decreases as we move down the group. BiF5 is the only well-characterized Bi(V) compound.
Due to the inert pair effect, the stability of +5 state decreases and +3 state increases as we move down the group in the periodic table. Nitrogen reacts with oxygen and also exhibits +1, +2, +4 oxidation states. On the other hand, phosphorus shows +1 and +4 states in some oxoacids.
In nitrogen, the oxidation states from +1 to +4 tend to disproportionate in acidic solution. In the case of phosphorus, the intermediate oxidation states disproportionate into +5 and -3 in both acids and alkalis. Whereas considering the case of arsenic, antimony, and bismuth, the +3 state is stable with respect to disproportionation.
Nitrogen has only 4 electrons in its outermost shell (one in s orbital and 3 in p) which is available for bonding, hence it exhibits a maximum covalency of 4. The heavier elements have a vacant d orbital in the valence shell which is used for bonding.