Explain why the valence-shell electron-pair repulsion model, or theory, is used

Question

Explain why the valence-shell electron-pair repulsion model, or theory, is used in the study of molecular geometry. Tell why it is more difficult to determine the geometry of a molecule if the central atom has both lone pairs and bonding pairs. How is it determined that a carbon dioxide molecule is linear in shape, rather than being a bent molecule? Name the two quantum mechanical theories used to describe covalent bond formation and the electric structure of molecules. Discuss three different examples of valence bond treatment of tending in polyatomic molecules. (1) sp^3hybridization, (2) sp hybridization, and (3) sp^2Hiybridization. What type of bond formation gives acetylene its planar geometry. Define sigma bond and pi bond. Give one instance where the valence bond theory cannot satisfactorily account for observed properties of molecules. Define homonuclcar diatomic molecules. Why is the structure of a benzene ring pictured as a hexagon with a circle within it? Explain what is meant by the terms buckyball, and buckytube.

Explanation / Answer

1

Lewis concept is unable to explain the shape / geometry of the molecules.but valence shell electon pair repulsion theory ( VSEPR ) / model has been successfully used to explain molecular geometry , because of the following reasons,

........(i) the theory provides a simple procedure to predict the shape of covalent molecules .

........(ii) it is based on the repulsive interactions of the electron pairs in valence shell of the atoms.

........(iii) The main postulate /basis for this theory that

''the valence shell is taken as a sphere with electron pairs localizing on the spherical surface at maximum distance from one another "

.................is quite justifiable.

..........(iv) the theory is able to predict the geometry of a large number of compounds accurately.

.......... (v) it is also quite successful in determining geometry when the energy difference between possible structures is very small.

............(vi) the theory conveniently divides the molecules into two categories as

( a ) molecule in which central atom has no lone pair and

(b) molecule in which the central atom has one or more lone pairs of electron , thereby explaining the repulsive interactions between electrons in the order :

Lone pair (lp) - lone pair (lp) > lone pair (lp ) - bond pair (bp ) > Bond pair (bp) - bond pair (bp)

2.

It is more difficult to predict geometry of the molecule when central atom has both lone pair and bond pair of electrons because,

(i) Lone pair - lone pair repulsive interactions are greater than simply bond pair - bond pair repulsive interaction within electrons , with the result that -

.......(a) there is a deviation from idealised shape,

........( b) there is an alteration in bond angle,

........(c) Since as per postulates of VSEPR theory " the pair of electrons tend to occupy such positions in space so as to minimise repulsion and thus maximise distance from one another. " , the molecule attains a distorted geometry or bent shape .

3 Structure of CO2 molecule.

The experimentally determined carbon to oxygen bond length in CO2 is 115 pm. The length of normal carbon to oxygen double bond ( C=O ) and carbon to oxygen triple bond (C=_ O )are 121 & 110 pm respectively. The carbon to oxygen bond length in CO2 (115 pm ) lies between the values of C=O and C=_ O. Obviously , it points to consider CO2 molecule as a hybrid of the canonical or resonance forms of three possible structures,

.................O=C=O ,<---------> O - C =_ O <----------> O =_ C - O

........................I................................II........................................III

Thus, the hybrid structure of CO2 has been assigned as

.......................................................................O=C=O

The above structure has an energy lower than either of the two canonical forms , thereby stabilizing the molecule without any lone pair of electron imparting a linear shape to the molecule.

8. Homonucleur di atomic molecules are defined as a molecule in which the two atoms of the same element with atomicity two are bonded with a double bond. (ie. having a bond order = 2).

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Please post the other six questions separately. Glad to help .

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