Chemical Bonding

Question 6.37: The molecules NF3 and BF3 all have molecular formulae of the XF3. But they have different structural formulae. Keeping in view VSEPR theory sketch the shape of each molecule and explain the origin of differing in shapes.
Answer: 
Shape of BF3 molecule: In boron triflouride, there are three bond pairs around the central boron atom. For maximum separation and minimum repulsion, three electron pairs have trigonal arrangement. Thus, BF3 has a trigonal planer geometry with each bond angle of 180o.

Shape of NH3 molecule: In ammonia, there are three bond pairs and one lone pair around the central nitrogen atom. For maximum separation and minimum repulsion, four electron pairs have tetrahedral arrangement. However, the molecular geometry of ammonia is trigonal pyramidal due to three bond pairs. Due to greater repulsion exerted by lone pair, the bond angle is reduced from 109.50 to 107. 50.

Question 6.38: The species NH2-, NH3, NH4+ have bond angles of 105o, 107.5o and 109.5o respectively. Justify these values by drawing their structure.
Answer: Shapes of these species can be sketched and explain according to VSEPR theory. In NH2, there are two bond pairs and two lone pairs. According to VSEPR theory, lone pairs of a atom occupy more space than bond pairs around the central atom. Thus, lone pairs have ability to compress the bond angle to 105.5 In NH3, there is only one lone pair of electrons. This lone pair has also tendency to compress the bond angles, but to lesser extent than two lone pairs in NH2-. So bond angle is 107. 50. In NH4+, there is no lone pair of electrons. So all bond angle perfectly 109.5O.

Question 3.39: What is advantage of VBT over VSEPRT?
Answer: VSEPR theory predicts and describes the shapes of the molecules but does not explain the bond formation. Whereas, VB theory explain both shape of the molecules and bond formation in the molecules.

Question 3.40: What is difference between sigma bond and pi bond?
Answer:
Sigma bond: A covalent bond which is formed by head-on overlape of two atomic orbitals is called sigma (𝛔) bond. In 𝛔 bond electron density is maximum between the nuclei and symmetrical about line joining the two nuclei.
Pi bond: A bond which is formed by sideways overlapping of atomic orbitals is called pi bond. In 𝛑 bond electron density lies above and below the line joining the nuclei.

Question 3.41: Why π bond is weaker than σ bond?
Answer: A π bond is weaker than a σ bond due to following reasons.

1. There is lesser overlapping of atomic orbital in π bonds than in σ bond. 
2. The electron density is more diffused (being above and below the bond axis) in in π bonds than in σ bond. 

Hence distance between electrons and nuclei are greater in π bonds, therefore, relatively a lesser energy is required to break the π bonds than σ bond.

Question 3.42: Why π bond is more reactive than σ bond?
Answer: The electron cloud in π- bond is more diffused (above and below the bond axis). Therefor,  π- electrons are relatively away from nuclei. As a result, the two nuclei have only weak control on these electrons. Hence, π – electrons are more susceptible to attack by an electrophile.