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Group 15: Anomalous Properties

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Group 15: Anomalous Properties - Lesson Summary

Generally the first element of any group differs from the rest of the elements with respect to their properties. Nitrogen differs from the rest of the elements in its properties. The unique properties of nitrogen are attributed to its small atomic size, high electro-negativity or high ionisation enthalpy, the non–availability of d-orbitals and the propensity to form multiple bonds. Only nitrogen can form nitride ions by gaining electrons due to its small size and high electro-negativity.

Nitrogen, being smaller in size, can effectively form p pi – p pi bonds with other atoms of itself and atoms of other elements with a small size and high electro-negativity such as oxygen and carbon. The other elements do not form p   pi - p pi bonds because of their relatively larger size. In other words the p orbitals of these elements are larger in size.
Dinitrogen is a diatomic molecule with a triple bond between the two atoms.
The bond enthalpy of a triple bond is very high about 941.4 KJ/mole. Dinitrogen is inert at ordinary conditions. For the same reason, it forms several compounds.
They are   Compound Formulae Sodium nitrate NaNO 3 Potassium nitrite KNO 2 Sodium azide (azo) NaN 3 Nitrous oxide N 2O Nitric acid (oxo acid) HNO 3 Potassium cyanide KCN  
The other elements in the group, such as phosphorus, arsenic and antimony, exist as tetra atomic molecules. In all these molecules, only single bonds are present between the atoms.

But bismuth forms metallic bonds in its elemental state. The catenation tendency is less for nitrogen when compared to the other elements of the group. This is because the higher inter-electronic repulsions   between the lone pair of electrons present on the nitrogen atoms.

The high inter-electronic repulsions in dinitrogen are attributed to its smaller bond length or small size of nitrogen atoms. Nitrogen doesn’t form d pi – p pi bonds due to the absence of d orbitals.
Phosphorus can form d pi – p pi bonds. EX: Phosphorus Oxo chloride & triethyl phosphate.
Due to the absence of d orbitals in nitrogen, it cannot form   d pi - d pi bonds.
Due to the availability of vacant d orbitals in the remaining elements of group 15 forms compounds such as triethyl phosphine and triphenyl arsine, form d pi – d pi bonds with transition elements. 


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