Chemical Bonding and Molecular Structure

4.26. In BF₃, B atom is sp² hybridised. In NH₃, N is sp³ hybridised.
After the reaction, hybridisation of B changes from sp² to sp³, while the hybridisation of N remains the same.

4.25. Electronic configuration of ₁₃Al in ground state= 1s² 2s² 2p⁶ 3s² 3p¹

Electronic configuration of ₁₃Al in excited state = 1s² 2s² 2p⁶ 3s¹ 3p_x¹3p_y¹
Hence, hybridisation will be sp² and this makes the geometry to be Trigonal planar (in case of AlCl₃).
In AlCl^–₄, the empty 3p_z orbital is also involved. So, the hybridisation is sp³ and the shape is tetrahedral

4.23. The dipole moment of NH₃ (1.47D) is higher than the dipole moment of NF₃ (0.24D), even though the molecular geometry is pyramidal for both the molecules. In each molecule, N atom has one lone pair. F is more electronegative than H and N? F bond is more polar than N? H bond. Hence, NF₃? is expected to have much larger dipole moment than NH₃. However, reverse is true. In case of ammonia, the direction of the lone pair dipole moment and the bond pair dipole moment is same whereas in case of NF₃, it is opposite. Thus, in ammonia molecule, individual dipole moment vectors add whereas in NF₃, they cancel each other.

4.22. BeH₂is a linear molecular (H—Be—H), the bond angle = 180°. Be—H bonds are polar due to difference in their electronegativity but the bond polarities cancel each other. Thus, molecule has resultant dipole moment of zero

4.21. According to VSEPR theory, if CH₄ were square planar, the bond angle would be 90°. For tetrahedral structure, the bond angle is 109°28? Therefore, in square planar structure, repulsion between bond pairs would be more and thus the stability will be less.

Moreover, the orbitals of carbon in ground and excited states look as shown below. In the excited state, the electrons from 4 H atoms occupy one 2s orbital and three 2p orbitals. This results in an sp³ hybridisation, which is the hybridisation of a tetrahedral geometry

4.19. N₂ < SO₂ < ClF₃ < K₂O

The ionic character in a molecule is dependent upon the electronegativity difference between the constituting atoms. The greater the difference, the greater will be the ionic character of the molecule.

4.17. Electronegativity: Electronegativity is the tendency of an atom to attract shared pair of electrons towards itself. Electronegativity of any given element is never the same.

It depends on the element it is bonded with in a compound. Electronegativity is a relative quantity and cannot be measured.
Whereas, electron gain enthalpy is the change in enthalpy when an electron is added to a neutral gaseous atom to form an anionic specie.

It can have a positive or negative value. Every element has a specific electron gain enthalpy value.