Hardness in elimination vs substitution

We can rationalize selectivity for elimination and substitution, or attack of H vs attack on C in terms of hard and soft electrophiles. In a SN2, the carbon centre is a soft electrophile—it is essentially uncharged, and with leaving groups such as halides the C–X sigma* is a relatively low-energy LUMO. 

Substitution is therefore favoured by nucleophiles whose HOMOs are best able to interact with this LUMO—>soft nucleophiles. 

In contrast, the C–H sigma* is higher in energy because the atoms are less electronegative. This, coupled with the hydrogen’s small size, makes the C–H bond a hard electrophilic site, and as a result hard nucleophiles favour elimination 

Source: organic chemistry- Clayden