The Polarizability of Single Bonds

Electron distribution as a negative ion [Y] approaches the back side of C. 

The ease with which the valence electrons are displaced from their resting position depends to a very large extent on the attraction the atomic kernel of X has for electrons. The greater affinity for electrons it has, the closer it holds them to itself, and the more difficult it is for an approaching ion to displace them. Consequently, polarizability decreases in moving across a row in the periodic table, and the following series of decreasing bond polarizabilities is observed  

Similarly as the atomic kernel of X becomes larger, the electrostatic control which the nucleus is able to exert upon its valence electrons becomes smaller, and bond deformation becomes easier as we descend within a group of the periodic table  

This is the order of reactivity of alkyl halides in displacement reaction which is familiar to all organic chemists. Although it might not have been predicted, the polarizability factor almost always appears to outweigh the permanent polarization factor in reactions of saturated molecules. 

It is interesting that as a carbon atom becomes more highly branched, it behaves more and more as if it were repelling electrons towards the next atom. Thus the reactions of saturated molecules suggest that the electron density at Z decreases in the order  

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