Question

Write the bond energy order of:
${F_2},C{l_2},B{r_2},{I_2}$

Answer 1

The bond energy is the average of all bond dissociation energies of a single type of bond in a given molecule. Stability of the molecule increases with the increase in the bond energy, per electron pair bond, of a molecule.

Complete Step by step answer: In the periodic table, if you observe. You will find that Fluorine, Chlorine, Bromine and Iodine are group 17 elements arranged in increasing order of period or row. i.e. fluorine is in the first row. Chlorine is in the second row. Bromine is in the third row and iodine is in the fourth row.
Now we know that, when we form down in the periodic table, a new orbital is added with each period. That means, the size of the atom keeps on increasing. That means, the distance between the last electron and nucleus keeps on increasing. That means, the energy required to break their bond keeps on decreasing.
Therefore, the bond energies of Fluorine, Chlorine, Bromine and Iodine should decrease down the group as the size of the atom increases. But there is one exception to this group. That is, because of the small size of fluorine, there is an inter-electronic repulsion in fluorine atoms. Because of which, the energy required to dissociate the last electron of fluorine is less than that of chlorine.
$\therefore$ Bond energy order is
$C{l_2} > {F_2} > B{r_2} > {I_2}$
Lower the bond energy more reactively will be the halogen. The bond dissociation energy of fluorine is lower than that of Chlorine bond.

Note: Here, it is important to understand that, though, we associate bond dissociation energy with the size of the atom. It is not the size of the atom that gives us the bond dissociation energy. It is the energy required to break the last electron of an atom that tells us about the bond dissociation energy of the atom. So, sometimes, even though a molecule is small. Like in this case fluorine. We might need less energy to break the last electron because the electrons themselves repel each other as they are too close due to the small size.