Bond energies and the Periodic Table

It is the almost constancy of XY bond energies, for each bond order, from molecule to molecule that makes the definition of average values for bond energies a particularly useful one. Table 10.1 shows such average values in relation to the Periodic Table, as taken from ref. 127. 

A similar effect is found for example for the single bonds N-O, P-O, and As-O, with energies 201, 335 and 301kJmol respectively and for N-F, P-F, and As-F whose bond energies are, respectively, 283, 490 and 440kJmol  

The small size of the first row atoms shows up in high ionization potentials too and in higher electronegativities than the elements below in the same group. 

With the notable exception of N-N, 0-0, and F-F, the X-X bond energy in general increases in each period, thus following the increase of effective nuclear charge, for example, 105, 293 and 346kJmol for Li-Li, B-B and C-C, respectively. 

Since tt bonding is in general less effective than a bonding, the X=X bond energies are less than twice the corresponding X-X values, with the exception of N=N and 0=0 due to the anomalous low values for N-N and 0-0. For example, for C-C we have 346kJmol and for C=C we have 602kJmor  

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