76.CHEMICAL BONDING (23)- Covalent Bonding(22) – Drago Rule (1).

In this post, we will study yet another rule ,which is important while studying hybridization in molecules.

This rule was stated to explain certain observations , which could not be explained by hybridization theory alone.

Why is Drago rule needed?

1] To explain anomalous observations in bond angles of Group 15 and 16 hydrides-

Group 15 elements → Nitrogen (N) , Phosphorous (P) , Arsenic (As) and Antimony (Sb).

Hydrides of these elements → Ammonia (NH3) , Phosphine (PH3) , Arsine (ArH3) , Stibine (SbH3) .

The bond angles for the above four compounds are –



As seen in the above figure, the bond angle drops suddenly from ammonia to phosphine by 13º. There is just a 1º difference between bond angles of phosphine ,arsine and stibine.Why does the bond angle drop so much going from ammonia to phosphine?

Similar observation can be made for group 16 elements too.


In the above figure , oxygen , sulphur ,selenium and tellurium are all Group 16 elements.Their hydrides show similar behaviour as Group 15 elements too. Why does the bond angle drop so much going from water to H2?


2] To explain  solubility differences of certain compounds –

Considering the above set of compounds again, it is very surprising to see that ammonia is very easily soluble in water. It forms ammonium hydroxide (NH4OH) , which we all have extensively used in our labs .However, phosphine ,arsine and stibine ,  donot dissolve /partially dissolve in water. If there outer electronic configurations are the same and the hybridisation is same, then all these compounds should exhibit similar solubility. However, practically the observations tell a different story ! What is the reason for this solubility difference?


3] To explain reaction of ammonia and phosphine  with acids-

Ammonia readily forms ammonium chloride with dilute hydrochloric acid (HCl).However, phosphine only reacts with concentrated HCl . Why can’t phosphine react with diute acid?

NH3+ dil HCl → NH4Cl .

PH3+ dil HCl → NO REACTION .

PH3+ conc HCl → PH4Cl .

4] Water and ammonia act as ligands in forming many coordination compounds.Other compounds like PH,ArH3H2Se never form such coordination compounds, Why?
(We shall learn coordination chemistry in much detail in the chapters to follow. For now just remember that coordination chemistry deals with compounds having a central metal ion bonded to negative ions or neutral species , which are called ligands). 

All the above discrepancies can be explained by Drago’s rule. In the next post, we shall study the rule and how it explains the odd behaviour of molecules. Till then,

Be a perpetual student of life and keep learning….

Good day !



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