methane, CH4 less repulsion between the bonding pairs of electrons 109.5° 90° less repulsion between the bonding pairs of electrons

.. ammonia NH3 less repulsion between the bonding pairs of electrons

.. .. water, H2O 109.5° (109.5°) 109.5° (107°) 109.5° (104.5°)

Formaldhyde H2CO

Hydrogen Cyanide HCN

Are there atomic orbitals on C, Si, N and O to accommodate the bonding electrons of the shapes determined experimentally? 3 p orbitals s orbital x x y y z z

Are there atomic orbitals on C, Si, N and O to accommodate the bonding electrons of the shapes determined experimentally? x y z x y z

Are there atomic orbitals on C, Si, N and O to accommodate the bonding electrons of the shapes determined experimentally? 4 sp3 orbitals x y z

Are there atomic orbitals on C, Si, N and O to accommodate the bonding electrons of the shapes determined experimentally? H 4 sp3 orbitals x C H y H z H

s + px + py = 3 sp2 orbitals pz orbital

H O C H

H C O H

s + px = 2 sp orbitals Py orbital Pz orbital

N C

C N

C N

C N

H C N

bonding hybrid orbitals VSEPR shape sp3 tetrahedral trigonal planar sp2 sp linear

Questions?

Review: Ethane Csp3-Hs sp3 Csp3-Csp3

Review: Ethane Csp3-Hs sp3 H H H ● C C ● H H H Csp3-Csp3

. . . . Hybrid atomic orbitals s orbital + - p orbital x and s - p x p x s + Why are hybrid orbitals used in chemical bonding? . . . .

molecular orbitals. a C-C bond Csp3 Csp3 energy

molecular orbitals. a C-C bond Csp3 Csp3 energy

molecular orbitals. a C=C bond Csp2 Csp2 energy

A model of ethene using hybrid and atomic orbitals. π-bond · H · H σ-bond · C C H H · H H C C H H

molecular orbitals. a C=O bond H : C O : H

In contrast to the rotation about single bonds, the rotation about double bonds is very difficult. 12 kJ energy

2px 2s 2pz 2py

Important consequences of Quantum Mechanics The locations of the electrons are described by atomic orbitals. A given orbital can contain only 2 electrons. (Pauli exclusion principle.) Each orbital has an associated energy and electrons will occupy the lowest energy orbitals first. Atomic orbitals on different atoms will can combine to give molecular orbitals, but only if their symmetry matches. good s bond bad – wrong symmetry No bonding!

These atomic orbitals and their hybrids will be used extensively in our models of chemical bonding. p orbitals s orbital x y H C, N, O z

Common hybrid orbitals used in bonding of carbon, nitrogen and oxygen. sp x y z 2pz 2px 2py 2s

Common hybrid orbitals used in bonding of carbon, nitrogen and oxygen. sp hybrid orbital angle 180° pz sp sp py 2pz 2px 2py 2s

Common hybrid orbitals used in bonding of carbon, nitrogen and oxygen. sp2 hybrid orbital angle 120° pz 2pz 2px 2py 2s

Common hybrid orbitals used in bonding of carbon, nitrogen and oxygen. sp3 hybrid orbital angle 109.5° 2pz 2px 2py 2s

Methane sp3 Csp3-Hs H H H C H H

Ethane sp3 H C H

Ethane sp3 H C ● H C ●

Ethane Csp3-Hs sp3 H H H ● C C ● H H H Csp3-Csp3

Ethane Csp3-Hs sp3 Csp3-Csp3