Chemical Bonding and Molecular Structure — Chemistry STD 11 Science — Question

1. $\sigma$-bond

Explanation:

The reason that $\sigma$ bonds ( and lone pairs) determine the geometry is that they form the basic skeleton of the molecule, $\sigma$ bonds are formed by head-on overlap of atomic orbitals, meaning that they are oriented a long the imaginery axis connecting two atomic nuclei, and hence concentrate electron density in the region directly between the nuclei, $\pi$ bonds, on the other hand, are essentially orthogonal to the $\sigma$ bond skeleton, and are substantially weaker.

Moreover, $\pi$ bonds do not exist in isolation, meaning any $\pi$ bond between two given atoms is always formed secondarily to the $\sigma$ bond between said atoms.

As such, $\pi$ bonds do not alter the basic idealized geometry of a molecule as dictated by $\sigma$ bonding.