Question

1) Explain the main concept behind the VSEPR theory of molecular shape. How do both number and type of electron pairs influence the shape of a molecule?

2) Explain how you were able to determine molecular polarity. What two factors did you need to consider?

3) Although PH3 and BH3 both contain bonds of similar polarity, one molecule is polar while the other is nonpolar

Answer 1

Following is the Answer to the first question (i.e. Question - 1 ) of the given ...Question Set.

Solution:

• Step - 1:

Valence Shell Electron Pair Repulsion Theory (i.e. VSEPR) is a set of rules, from which a chemist can/may predict the shape of an isolated molecule.

This theory is basically based on, the premise that groups of electrons surrounding a central atom repel each other, and that also to minimize the overall energy of the molecule, these groups of electrons ...try to get as far apart, as possible.

The above idea, that a pair of e ^- , in any valence orbital, always feels some electrical repulsion,  suggests that this causes the orbital (thus the geometry) of the molecule, to shift slightly, and thereby can influence shape of a molecule.

• Step - 2:

1) (i.e.. influence of number of electron pairs...)   

The number of electron pairs, influences shape, because the pairs try to space out evenly and at the same time, they repel each other. As the number of elctron pair increases, the more space will be taken up as the eleectrons try to maintain distance and push away from each other, and therefore changes shape of the molecule.

2) (i.e.. influence of Type of electron pairs...)   

The Type of electron pairs, can also influence shapes of molecules, due to the following reasons. a pair of electrons, which are not shared between two nuclei, repel each other more, because they only feel the pull of one positive nucleus.

A shared pair of electrons between two nuclei don't repel each other as much as the previous case, where the pair of electrons are not shared between two nuclei, and there is not as much distortion of the orbital, if compared, and thereby doesn't influence the shape of teh molecule, as much as the above type of pair of electrons.