Data from Chemistry LibreTexts — Strength of Covalent Bonds, Table 8.8.1: Average Bond Energies (kJ/mol) for Commonly Encountered Bonds at 273 K:

$$ \begin{array}{cr} \hline \text{Bond} & D/\pu{kJ mol-1} \\ \hline \ce{C-C} & 346 \\ \ce{N-N} & \approx 167 \\ \ce{O-O} & \approx 142 \\ \ce{F-F} & 155 \\ \hline \end{array} $$

I couldn't understand the trend of bond dissociation energy of various single bonds. Why

$$D(\ce{C-C}) > D(\ce{N-N}) > D(\ce{F-F}) > D(\ce{O-O})?$$

I had mugged up the trend the now, and according to me BDE should continuously increase while going from left to right in a period due to decrease in size and increase in ENC. Why this irregular trend?

  • 3
    $\begingroup$ A simple explanation would be using the idea of lone pair-lone pair repulsions. As we go from C-C to F-F, there is an increasing number of lone pairs on the bonding atoms. Thus, this increasing destabilising interaction results in the bond energies following that particular trend. $\endgroup$ Aug 1 '18 at 5:11

In chemistry there is nothing like always. You have to see any element characteristics than decide its property. The case you are asking about:

  1. Noteworthy exceptions are single bonds between the period 2 atoms of groups 15, 16, and 17 (i.e., $\ce{N, O, F}$), which are unusually weak compared with single bonds between their larger congeners. It is likely that the $\ce{N–N, O–O,}$ and $\ce{F–F}$ single bonds are weaker than might be expected due to strong repulsive interactions between lone pairs of electrons on adjacent atoms.
  2. $\ce{N-N}gt \ce{F-F}$, in $\ce{F}$ there are greater number of lone pairs than in $\ce{N}$ so it experiences strong repulsion between its electrons that make it vulnerable to be broken.
  3. $\ce{F-F} \gt \ce{O-O}$ it's so because $\ce{F}$ atom is very small that enables strong electrostatic attraction between nucleus and electrons For further information on 3) you can follow this link.

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