The ground state and excited state outer electronic configurations of phosphorus (Z = 15) are

Phosphorus atom is sp³ d hybridized in the excited state. These orbitals are filled by the electron pairs donated by five Cl atoms as:
PCI₅

The five Sp³ d hybrid orbitals are directed towards the five corners of the trigonal bipyramidal. Hence, the geometry of PCI₅ can be represented as:

There are five P-Cl sigma bonds in PCl₅ Three P-Cl bonds lie in one plane and make an angle of 120° with each other. These bonds are called equatorial bonds. The remaining two P-CI bonds lie above and below the equatorial plane and make an angle of 90° with the plane. These bonds are called axial bonds. As the axial bond pairs suffer more repulsion from the equatorial bond pairs, axial bonds are slightly longer than equatorial bonds.

(i) Formation of BH3 (atomic No. of B is 5.)Both the carbon atoms are sp hybridised. Both the carbon atoms have also
two unhybridised orbitals which overlap sidewise with the similar o.tbitals of
the other carbon atom to form two 1t bonds




Orbital picture of BH₃ molecule
B atom gets hybridised to form three equivalent hybrid orbitals directed towards three comers of equilateral triangle with B atoms in the centre. Bond angle = 1200
(ii) C₂H₂

Both the carbon atoms are sp hybridised. Both the carbon atoms have also two unhybridised orbitals which overlap sidewise with the similar orbitals of the other carbon atom to form two \(\pi\) bonds

When two atoms with different electronegativity are linked to each other by covalent bond, the shared electron pair will not in the centre because of the difference in electronegativity. For example, in hydrogen flouride molecule, flouride haspeater electronegativity than hydrogen. Thus, the shared electron pair is displaced more towards flourine atom, the later will acquire a partial negative charge (\(\delta ^{ - }\)).At the same time hydrogen atom will have a partial positive charge (\(\delta ^{ + }\)). Such a covalent bond is known as polar covalent bond or simply polar bond. It is represented as
\(\overset { { \delta }^{ + } }{ \underset { 2.1 }{ H } - } \overset { { \delta }^{ - } }{ \underset { 4.0 }{ F } } \)

\({ (CH }_{ 3 }{ ) }_{ 3 }\ddot { N } \) is trimethyl amine.It has pyramidal geometry as shown below

[(CH₃)₃Si]₃ N has triangular planar geometry as shown in diagram below.

Due to the presence of empty d-orbital on Si, there is an effective back bonding.
Thus, the two species are not isostructural.In (CH₃)₃N, the N-atom assumes sp³ hybrid state where as in [(CH₃)₃Si]₃N, the N-atom assumes sp² hybridtate.