Size of the hydrogen atom

The small size of the hydrogen atom allows the unshared pair of an F, O, or N atom of one molecule to approach the H atom in another very closely. It is significant that hydrogen bonding occurs only with these three nonmetals, all of which have small atomic radii. 

The calculated volume has been shown to suffer a significant contraction due to the protonation. This effect is mainly attributed to a reduction in the size of the hydrogen atoms of the methyl groups. 

The charge distribution in groups such as OH, NH and FH is very eccentric as far as the positive pole is concerned, that is to say, it can also be stated that the positive pole of the dipole lies externally, this in connection with the small size of the hydrogen atom bound as an ion (no electron cloud ). This gives rise to much stronger interaction than in other molecules with the same or even larger dipole moment. So much so that the complex of phenomena connected with this is discussed separately as the hydrogen bond ( 45). 

Note that the small size of the hydrogen atom allows for close interactions. 

Q Hydrogen bonding among water molecules. The small size of the hydrogen atoms allows the molecules to get very close and thus to produce strong interactions. 

Figure 10.3 (a) The polar water molecule, (b) Hydrogen bonding ( ) among water molecules. Note that the small size of the hydrogen atom allows for close interactions. 

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