Question

Write the limitation of Lewis concept and main postulates of VSEPR theory?

Answer 1

Hint: Lewis dot structure is entirely based on octet rule and it failed to explain certain molecules such as $H_2$,$C{l}_2$, $I{F}_3$, $PC{l}_5$, $S{F}_6$, $B{F}_3$,$BC{l}_3$ etc. VSEPR theory based on the number electron pair around the central atom, repulsion between the electron bond around the central atom and succeeded to explain all the molecules above mentioned that Lewis theory failed to explain.

Complete step by step answer:
Lewis concept is representation of the valence shell electrons in a molecule which shows how the electrons are arranged around each individual atom of a molecule.

Limitation of Lewis concept:
-This fails to explain the cause of covalent bond formation and the nature of attractive force between the adjacent atoms in molecules like$H_2$,$C{l}_2$.
-Fails to explain the geometry of a molecule.
-It could not explain the release of energy during formation of covalent bonds.
-It violates octet rule the evidence is the formation of molecules where the central atom has more eight electrons in its valence shell correctly. Eg:$I{F}_3$,$PC{l}_5$, $S{F}_6$ etc. and formation of electron deficient central atoms where there are less than eight electrons on the central atom. Eg:$B{F}_3$, $BC{l}_3$.
Valence shell electron pair repulsion (VSEPR) theory is a model used to predict the geometry of the individual molecules from the number of electron pairs surrounding their central atoms.

Postulates of VSEPR theory are:
-The spatial arrangement of the molecules depends upon the number of valence shell electron pairs around the central atom.
-The electron pair surrounding the central atom repels one another and moves apart from each other so that there is no further repulsion between them. As a result, the molecules have minimum energy and maximum stability.
-There are two types of electron pairs, one bond pairs and other lone pairs. The bond pairs of electrons are those shared between two atoms while the lone pair are the valence electron pairs that are not involved in bonding.
-Order of the repulsion between the electron pairs are: lp-lp > lp-bp > bp-bp where, lp is lone pair and bp is bond pair.

Additional Information:: The first postulate was derived with reference to Lewis dot structure theory. Even though it failed to explain many things but it was a stepping stone for VSEPR theory. Let's see some limitations of VSEPR theory. This theory fails to explain isoelectronic species. Since VSEPR theory does not take in size of substituent atoms into consideration it fails to explain about compounds of transition metal. According to VSEPR theory the halides of group 2 elements have linear geometry but in actual they are bent

Note: Lewis fails to explain correct geometry, bond length, bond angle, nature of bond etc. VSEPR theory explains all the properties mentioned in above such as geometry, bond length, bond angle, nature of bond etc.