Write the limitations of the Lewis concept and the main postulates of $VSEPR$ theory.

Limitations of Lewis concept: The Lewis concept is unable to explain the shapes of molecules.
Valence Shell Electron Pair Repulsion $(VSEPR)$ theory: This theory provides a simple procedure to predict the shape of covalent molecules. Sidgwick and Powell in $1940$ proposed a simple theory based on the repulsive interactions of the electron pairs in the valence shell of the atoms.
Postulates of $VSEPR$ Theory:
- The shape of a molecule depends upon the number of valence shell electron pairs (bonded or non-bonded) around the central atom.
- Pairs of electrons in the valence shell repel one another since their electron clouds are negatively charged.
- These pairs of electrons tend to occupy such positions in space that minimize repulsion and thus maximize the distance between them.
- The valence shell is taken as a sphere with the electron pairs localizing on the spherical surface at maximum distance from one another.
- $A$ multiple bond is treated as if it is a single electron pair,and the two or three electron pairs of a multiple bond are treated as a single super pair.
- Where two or more resonance structures can represent a molecule,the $VSEPR$ model is applicable to any such structure.
- The repulsive interaction of electron pairs decreases in the order: $[Lone \ pair \ (lp) - Lone \ pair \ (lp)] > [Lone \ pair \ (lp) - Bond \ pair \ (bp)] > [Bond \ pair \ (bp) - Bond \ pair \ (bp)]$