Explain why inspite of nearly the same electronegativity, oxygen forms hydrogen bonding while chlorine does not.

Both chlorine and oxygen have almost the same electronegativity values, but chlorine rarely forms hydrogen bonding. This is because in comparison to chlorine, oxygen has a smaller size and as a result, a higher electron density per unit volume.

Oxygen atom can form hydrogen bond whereas chlorine does not. The tendency for hydrogen bonding depends upon

1. Small size and 

2. High electronegativity values

Although the electronegativities of O and Cl are nearly the same yet chlorine does not form hydrogen bond due to its larger size (99 pm) as compared to oxygen (66 pm).