CBSE Class 11-science Answered

Structure of C₂H₂:

• The ground state configuration of ‘C’ being 1s² 2s² 2p_x¹ 2p_y¹ has only two unassociated electrons. Carbon makes four bonds because its valency is four.
• For this, 4 unpaired electrons are required. Hence, it promotes its 2s electron to the empty 2p_z orbital in the excited state.
• So, the excited state electronic configuration of carbon is 1s² 2s¹ 2p_x¹ 2p_y¹ 2p_z¹. Each carbon atom undergoes ‘sp’ hybridisation by using 2s and 2p orbitals in the excited state to give two half-filled ‘sp’ orbitals, which are arranged linearly.

Formation of PCl₅ (sp³d Hybridisation)

• The ground state and the excited state outer electronic configurations of phosphorus (Z = 15) are given below:

• The formation of five half-filled sp³d hybrid orbitals involves the intermixing of one 3s, three 3p and one 3d orbitals during excitation.
• They are then organised in a trigonal bipyramidal symmetry. While three orbitals are organised in trigonal planar symmetry, the remaining two are kept perpendicularly above and below this plane.
• These half-filled sp³d orbitals help phosphorus in forming five σ_sp3d-p bonds with chlorine atoms. Each chlorine atom makes use of half-filled 3p_z orbital for the bond formation.
• The PCl₅ molecule has a trigonal bipyramidal shape with 120° and 90° of ∠Cl–P–Cl bond angles.
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