VSEPR theory stands for valence shell electron pair repulsion theory. VSEPR theory is used to predict the geometry of an individual molecule based on the number of electron pairs that surround the central atom. The valence electron pairs that surround the central atom tend to repel each other and therefore VSEPR theory adopts a structure which minimizes this repulsion and determines the geometry of a molecule.
Question 1: What are the important topics in VSEPR theory that needs to be focused in order to crack JEE?
Answer: According to JEE syllabus the following topics should be taken care of:
- Postulates of VSEPR theory
- Finding the shape of any molecule
- Examples of molecules and their structures
- Limitations of VSEPR theory
Question 2: Explain the postulates of VSEPR theory.
VSEPR theory can be explained as follows:
- The shape of any molecule can be explained by the repulsion between all the electron pairs that are present in the valence shell.
- The repulsion between two lone pairs is greater than the repulsion between a lone pair and a bond pair. Presence of a lone pair on the central atom causes slight distortion of the bond angles from their ideal values.
- The electronegativity difference between the central atom and the other atoms determines the magnitude of repulsion between the bonding pair of electrons.
- Triple bonds cause more repulsion than the double bonds. Similarly double bond causes more repulsion than a single bond.
Illustration 1: On the basis of VSEPR theory explain the structure of NH3 molecule.
Answer: In ammonia, N is the central atom. Nitrogen is a group 15 element and therefore has 5 electrons in its outmost shell. Three electrons of N are bonded with hydrogen and the rest two which do not take part in bonding form the lone pair. The outer shell then has a share in eight electrons that is three pairs bonded and one lone pair. These four pairs of electrons give rise to a tetrahedral structure where three positions are occupied by H atoms and fourth position by the lone pair. This shape may either be described as tetrahedral or pyramidal. The presence of lone pair causes slight distortion from 109°28’ to 107°48’.
Question 3: Which books should be referred for VSEPR theory while preparing for JEE?
Answer: NCERT chemistry textbook for class 11th covers all the postulates of VSEPR theory. Going through NCERT will give you a brief idea that you need to cover according to the JEE syllabus and build the fundamental understanding of the topic. Practice questions by solving previous year’s JEE papers.
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