Covalent bond nature in electronegativity gap

Question

Does a large difference in electronegativity mean the covalent bond is weaker?

In a covalent bond between two atoms of different electronegativities, the bonding electrons are pulled towards the more electronegative electron. This makes the bond polar (eg. $HCl$).

In a diatomic molecule, the electronegativity is the same so the electrons are equally attracted to either atom. Does this make diatomic covalent bonds stronger than the former? And if so, as the difference in electronegativity increases, does the strength of the covalent bond decrease?

Answer 1

Strength of covalent bond depends on many factors, one of the key factors is "extent of overlapping of orbitals". For e.g., if you consider $\ce{H2}$ molecule, there is a head on overlapping of $\mathrm{1s-1s}$ orbitals involving two hydrogen atoms, but in the case of, say, $\ce{HCl}$, there is a head on overlapping of $\mathrm{1s-3p_z}$ orbital ($\mathrm{1s}$ of hydrogen, and $\mathrm{3p_z}$ of chlorine), and we all know that $\mathrm{p-s>s-s}$, that is, overlapping between a $\mathrm{p}$-orbital and a $\mathrm{s}$-orbital is stronger than $\mathrm{s}$ and $\mathrm{s}$ orbital, and stronger the overlapping more is the covalent bond energy.

So, it largely depends on the type of orbitals involved and the nature of overlapping.

Check this to learn more about overlapping.

Answer 2

Well, I would say that the strength of the covalent bond is related to the level of polarisation abilities of the molecule in addition to the point stated by peaceHoper . In an ionic compound , polarisation is defined by the distortion of the anion by the cation . The more the polarisation , the more the covalent characteristics of the molecule . And thus , it results in higher strength of the covalent bond . It also depends on the type of orbitals involved but it's already been covered.

According to Fajan's rule , the smaller the anion and the larger the cation ,the greater the polarisation of the molecule . In H-H , both the atoms are the same . But in H-Cl , the cation H+ is really small compared to the relatively a lot bigger Chloride anion (let's hypothesise the molecule to be ionic ; not a bad one as well since there are some ionic characteristics present in it) . Hence, the polarisation of HCl is much more than H-H and therefore it has a stronger covalent bond.