Orbitals and Covalent Bond Chapter 9 Orbitals and Covalent Bond

Atomic Orbitals Don’t Work to explain molecular geometry. In methane, CH4 , the shape s tetrahedral. The valence electrons of carbon should be two in s, and two in p. the p orbitals would have to be at right angles. The atomic orbitals change when making a molecule

Hybridization We blend the s and p orbitals of the valence electrons and end up with the tetrahedral geometry. We combine one s orbital and 3 p orbitals. sp3 hybridization has tetrahedral geometry.

In terms of energy 2p Hybridization sp3 Energy 2s

How we get to hybridization We know the geometry from experiment. We know the orbitals of the atom hybridizing atomic orbitals can explain the geometry. So if the geometry requires a tetrahedral shape, it is sp3 hybridized This includes bent and trigonal pyramidal molecules because one of the sp3 lobes holds the lone pair.

sp2 hybridization C2H4 Double bond acts as one pair. trigonal planar Have to end up with three blended orbitals. Use one s and two p orbitals to make sp2 orbitals. Leaves one p orbital perpendicular.

In terms of energy 2p 2p Hybridization sp2 Energy 2s

Where is the P orbital? Perpendicular The overlap of orbitals makes a sigma bond (s bond)

Two types of Bonds Sigma bonds from overlap of orbitals. Between the atoms. Pi bond (p bond) above and below atoms Between adjacent p orbitals. The two bonds of a double bond.

H H C C H H

sp2 hybridization When three things come off atom. trigonal planar 120º on s one p bond

What about two When two things come off. One s and one p hybridize. linear

sp hybridization End up with two lobes 180º apart. p orbitals are at right angles Makes room for two p bonds and two sigma bonds. A triple bond or two double bonds.

In terms of energy 2p 2p sp Hybridization Energy 2s

CO2 C can make two s and two p O can make one s and one p O C O

Breaking the octet PCl5 The model predicts that we must use the d orbitals. dsp3 hybridization There is some controversy about how involved the d orbitals are.

dsp3 Trigonal bipyrimidal can only s bond. can’t p bond. basic shape for five things.

PCl5 Can’t tell the hybridization of Cl Assume sp3 to minimize repulsion of electron pairs.

d2sp3 gets us to six things around octahedral

Molecular Orbital Model Localized Model we have learned explains much about bonding. It doesn’t deal well with the ideal of resonance, unpaired electrons, and bond energy. The MO model is a parallel of the atomic orbital, using quantum mechanics. Each MO can hold two electrons with opposite spins Square of wave function tells probability

What do you get? Solve the equations for H2 HA HB get two orbitals MO1 = 1sA - 1sB MO2 = 1sA + 1sB

The Molecular Orbital Model The molecular orbitals are centered on a line through the nuclei MO1 the greatest probability is between the nuclei MO2 it is on either side of the nuclei this shape is called a sigma molecular orbital

The Molecular Orbital Model In the molecule only the molecular orbitals exist, the atomic orbitals are gone MO1 is lower in energy than the 1s orbitals they came from. This favors molecule formation Called an bonding orbital MO2 is higher in energy This goes against bonding antibonding orbital

The Molecular Orbital Model Energy 1s 1s MO1

The Molecular Orbital Model We use labels to indicate shapes, and whether the MO’s are bonding or antibonding. MO1 = s1s MO2 = s1s* (* indicates antibonding) Can write them the same way as atomic orbitals H2 = s1s2

The Molecular Orbital Model Each MO can hold two electrons, but they must have opposite spins Orbitals are conserved. The number of molecular orbitals must equal the number atomic orbitals that are used to make them.

H2- s1s* Energy 1s 1s s1s

Bond Order The difference between the number of bonding electrons and the number of antibonding electrons divided by two