What is hybridisation? Explain the following molecules using hybridisation

a) Be Cl₂

b) BF₃

Hybridisation is the concept of mixing atomic orbitals into new hybrid orbitals (with different energies, shapes, etc., than the component atomic orbitals) suitable for the pairing of electrons to form chemical bonds in valence bond theory.

(a) BeCl₂

The electronic configuration of 'Be' in ground state is 1s² 2s². Since there are no unpaired electrons, it undergoes excitation by promoting one of its 2s electron into empty 2p orbital.

Thus in the excited state, the electronic configuration of Be is 1s² 2s¹ 2p¹.

In the excited state, the beryllium atom undergoes 'sp' hybridization by mixing a 2s and one 2p orbitals. Thus, two half filled 'sp' hybrid orbitals are formed, which are arranged linearly.

These half filled sp-orbitals form two σ bonds with two 'Cl' atoms.

Thus BeCl₂ is linear in shape with the bond angle of 180^o.

(b) BF₃

The electronic configuration of 'B' in ground state is 1s² 2s² 2p¹.

In excited state one electron from s orbital gets excited to p orbital and it becomes

1s² 2s¹ 2p_x¹ 2p_y¹. This 1 half filled s orbital and 2 half filled p orbitals intermix to give 3 sp² hybridised orbitals which overlap with empty p orbitals of 3 fluorine atoms to give BF₃. The shape is trigonal planar.