Re: Why do the calculated bond lengths of H compounds have large deviations

The bond lengths quoted in most textbooks and chemistry references are 
average bond lengths. The actual length of any bond in a molecule is unique 
to that molecule. The others atoms connected to the two atoms of which you 
are measuring the bond length play an extremly important part in the bond.

For example the bond length of the C-H bonds in ethane CH3-CH3 is 1.10A 
whilst the C-H bond length in ethylene CH2=CH2 is 1.07A, the difference in 
the two molecule's C-H bond lengths is directly related to the difference 
in the bond between the two carbon atoms. Comparing these values with the 
average bond length of a C-H bond, which is given as 1.08A, then neither of 
them are quite correct. This is because given average bond lengths are only 
meant to be used a guide and give a reasonable approximation of the length.

Anyway hope that helps,

David Barker