Question

Dipole moment is used to predict the shape of molecules. 

1. Justify the statement based on the shapes of CO₂ and H₂O. 

2. Which is having high dipole moment? NH₃ or NF₃ ? Why?

Answer 1

1. Carbon dioxide is a linear molecule in which the two C=0 bonds are oriented in the opposite directions at an angle of 180°. Hence the two C=0 bond dipoles cancel each other and the resultant dipole moment of CO₂ is zero. Thus CO₂ is nonpolar molecule

On the other hand, water molecule has a bent structure in which two O-H bonds are oriented at an angle of 104.5°. Therefore, the bond dipoles of two O-H bonds do not cancel each other and the molecule will have a net dipole moment (1.85D).

2. The dipole moment of NH₃ is higher than that of NF₃. In both cases, the central N atom has a lone pair whose orbital dipole points away from the N atom. In NH₃ the orbital dipole due to the lone pair is in the same direction as the resultant bond dipole of the three N-H bonds. On the other hand, in the case of NF₃ , the resultant dipole of the three N-F bonds is in the opposite direction to the orbital dipole due to the lone pair. Thus, the orbital dipole due to the lone pair decreases the effect of the resultant N-F bond moments, which results in the low dipole moment of NF₃​​​​​​.