Question

why is orbital overlap a guide to assessing the strengths of chemical bonds? why sometime it is?

Answer 1

According to Valence Orbital Theory, Two atoms having unpaired electron in their valence orbital can form a chemical bond by overlapping with each other.

Now the strength of chemical bond depends on the extent of overlapping of valence atomic orbitals. Better the overlapping, stronger will be the chemical bond formed.

As we know that s-orbitals are spherical and p-orbitals are along the axis.

Therefore extent of overlapping will be in order : p-p > s-p > s-s ; And same will be the strength of bond formed by overlapping of atomic orbitals.

Atomic orbitals can overlap either along the molecular axis or perpendicular to it. The axial overlapping results in formation of Sigma bond while the lateral overlapping results in the formation of Pi-bond.

An axial overlapping is always better than lateral overlapping. Therefore, Sigma bonds are always stronger then Pi-bonds. For example -

In a NH₃ molecule,

N-atom has 5 electrons in valence shell (2s and 2p) and each H-atom has 1 electron in 1s orbital

Now at the time of bonding, each 2p orbital of N-atom overlaps with the 1s orbital of H-atom and as a results 3 Sigma bonds are formed.

But Valence Bond Theory (VBT) is unable to explain some compounds like O₂ molecule. According to VBT O₂ should be a diamagnetic (No unpaired electrons) whereas O₂ molecule is paramagnetic with 2 unpaired electrons.

Also VBT is unable to explain tetravalency of C-atom. According to VBT carbon should be a divalent atom.

We can explain only covalent bonding by VBT.