In group 5 elements, the valence shell electronic configuration is ns2np3. These elements are extra stable due to completely filled s-orbitals and half-filled p-configuration. In these elements, the s orbitals are completely filled, and p orbitals are half-filled. These qualities make their electronic configuration extra stable compared to other p-block elements. Because of the same reason, they have higher ionization enthalpies than gr-14 elements (Carbon family). There is a very small in size from As to Bi due to completely filled d- and f- orbitals in heavier members as they have poor shielding effect. The size of the nucleus progressively increases as the atomic number increase. Therefore, as you move down the nitrogen family: atomic radius increases, ionization energy decreases, ionic radius increases, and electronegativity decreases. Most p-block elements are non-metals but in group 15, the elements go from non-metal to the metalloid and metallic down the group.
Fig 1: Group 5 electronic configuration
Group 15 elements assume the inert pair effect. This happens because the tendency of ns2 electron pair to participate in a bond formation reduces with increasing atomic size. Within group 15, the with the increasing atomic number the higher oxidation state becomes less stable with respect to the lower oxidation state. This trend is called ‘inert pair effect’. Likewise, the energy needed to unpair the electrons is more than the energy that is released in the formation of two additional bonds. For instance, nitrogen compounds disproportionate in acid solutions.
3HNO2 HNO3 + H2O + 2NO
Similarly, phosphorus has nearly all intermediate oxidation states that disproportionate into +5 and –3 which can form both in alkali and acid. But the +3 oxidation state in case of arsenic, antimony and bismuth becomes increasingly stable and do not undergo disproportionation.
Nitrogen can complete its valence shell by:
1.) Electron gain: N3- ion. This is found in saltlike nitrides.
2.) formation of electron pair bonds: single bonds NH3,multiple bonds: N≡N: -N=N-, or NO2
3.) formation of electron pair bonds with electron gain, NH2– or NH2-
4.) formation of electron pair bonds with electron loss as does it happen in ammonium ions
Nitrogen does not form pentahalide due to non-availability of the d orbitals in its valence shell. Due to the small size and high electronegativity, nitrogen forms p-p pi bonds while other elements can also form d-p pi bonds. Therefore, nitrogen occurs with a triple bond that is highly stable. The bond enthalpy of a triple bond is extremely high which is at around 941.4 KJ/mol. The thermodynamic stability of the compound makes dinitrogen is stable under conventional conditions.NF3, NCl5, R3N=O type compounds cannot exist while PCl5, R3P=O type compounds occur in nature. Other elements do not form diatomic compounds. Pentahalides are more covalent than trihalides. In the case of nitrogen, only NF3 is known to be stable due to high polarizing nature of F-atom Trihalides except BiF3 are predominantly covalent in nature. BiF3 is ionic due to the high metallic character of Bismuth.
