Hybridisation in Beryllium and Boron compounds

Beryllium (Be) and Boron (B) form covalent compounds because of their high ionization energies and this can be explained on the basis of hybridisation.

Hybridisation in Beryllium

The ground state electronic configuration of Be is 1s² 2s². The promotion of one '2s' electron to '2p' orbital permits the possibility of sp hybridisation. The two sp hybrid orbitals are oriented in opposite directions at 180 to give a linear geometry.

Illustration: Beryllium difluoride - BeF₂

The electronic configuration of fluorine is 1s2 2s² 2p_x² 2p_y² 2p_z¹ which has one half filled 'p' orbital. Beryllium has two half filled sp filled orbitals at 180^o angle. The half filled p orbital of fluorine overlaps with one of the 'sp' hybrid orbitals of Be to form a single sigma bond between Be and F. The other F atom also forms a single sigma bond with Be. The two bonds are opposite to each other and the molecule is linear.

Fig: 6.12 - Formation of BeF 2 molecule

Hybridisation in Boron

The ground state electronic configuration of B is 1s² 2s² 2p¹. The promotion of one '2s' electron to one of the empty '2p' orbital permits the possibility of 'sp²' hybridisation. The three 'sp²' hybrid orbitals are oriented at 120^o in a plane giving a trigonal geometry around the boron atom.

Illustration: Boron fluoride - BF₃

Fluorine has one half filled p orbital and beryllium has three half filled 'sp²' filled orbitals at 120^oangle. The half filled p orbital of fluorine overlaps with each of the sp² hybrid orbitals of Be to form sigma bond between Be and F giving it a trigonal planar or triangular planar geometry.

Fig: 6.13 - Formation of BF₃molecule

Problems

14. Arrange the following carbon atoms numbered C-1, C-2 and C-3 in the decreasing order of 's' character:

Solution

Decreasing order of 's' character

15. Why is NF₃ pyramidal, while BF₃, is triangular planar, though both are tetra atomic molecules?

Solution

In NF₃ nitrogen involves sp³ hybridisation and one position is occupied by a lone pair. Therefore the molecule is pyramidal. But in BF₃, B involves sp² hybridisation having triangular planer geometry.