Presentation on theme: "Orbitals and Covalent Bond"— Presentation transcript:
1Orbitals and Covalent Bond Chapter 9Orbitals and Covalent Bond
2Molecular OrbitalsThe overlap of atomic orbitals from separate atoms makes molecular orbitalsEach molecular orbital has room for two electronsTwo types of MOSigma ( ) between atomsPi ( ) above and below atoms
3Sigma bonding orbitals From s orbitals on separate atoms++ ++++Sigma bonding molecular orbitals orbitals orbital
4Sigma bonding orbitals From p orbitals on separate atomsp orbitalp orbitalSigma bonding molecular orbital
5Pi bonding molecular orbital Pi bonding orbitalsp orbitals on separate atomsPi bonding molecular orbital
6Sigma and pi bonds All single bonds are sigma bonds A double bond is one sigma and one pi bondA triple bond is one sigma and two pi bonds.
7Atomic Orbitals Don’t Work to explain molecular geometry.In methane, CH4 , the shape is 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
8HybridizationWe 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.
12How we get to hybridization We know the geometry from experiment.We know the orbitals of the atomhybridizing atomic orbitals can explain the geometry.So if the geometry requires a tetrahedral shape, it is sp3 hybridizedThis includes bent and trigonal pyramidal molecules because one of the sp3 lobes holds the lone pair.
13sp2 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.
27Breaking the octetPCl5The model predicts that we must use the d orbitals.dsp3 hybridizationThere is some controversy about how involved the d orbitals are.
28dsp3 Trigonal bipyrimidal can only s bond. can’t p bond. basic shape for five things.
29PCl5 Can’t tell the hybridization of Cl Assume sp3 to minimize repulsion of electron pairs.
30d2sp3gets us to six things aroundOctahedralOnly σ bond
31Molecular 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 spinsSquare of wave function tells probability
32What do you get? Solve the equations for H2 HA HB get two orbitals MO2 = 1sA - 1sBMO1 = 1sA + 1sB
33The Molecular Orbital Model The molecular orbitals are centered on a line through the nucleiMO1 the greatest probability is between the nucleiMO2 it is on either side of the nucleithis shape is called a sigma molecular orbital
34The Molecular Orbital Model In the molecule only the molecular orbitals exist, the atomic orbitals are goneMO1 is lower in energy than the 1s orbitals they came from.This favors molecule formationCalled an bonding orbitalMO2 is higher in energyThis goes against bondingantibonding orbital
36The Molecular Orbital Model We use labels to indicate shapes, and whether the MO’s are bonding or antibonding.MO1 = s1sMO2 = s1s* (* indicates antibonding)Can write them the same way as atomic orbitalsH2 = s1s2
37The Molecular Orbital Model Each MO can hold two electrons, but they must have opposite spinsOrbitals are conserved.The number of molecular orbitals must equal the number atomic orbitals that are used to make them.
39Bond OrderThe difference between the number of bonding electrons and the number of antibonding electrons divided by two
40Only outer orbitals bond The 1s orbital is much smaller than the 2s orbitalWhen only the 2s orbitals are involved in bondingDon’t use the s1s or s1s* for Li2Li2 = (s2s)2In order to participate in bonds the orbitals must overlap in space.
41Bonding in Homonuclear Diatomic Molecules Need to use Homonuclear so that we know the relative energies.Li2-(s2s)2 (s2s*)1Be2(s2s)2 (s2s*)2What about the p orbitals? How do they form orbitals?Remember that orbitals must be conserved.
46B2 (s2s)2(s2s*)2 (s2p)2 Bond order = (4-2) / 2 Should be stable. This assumes there is no interaction between the s and p orbitals.Hard to believe since they overlapproof comes from magnetism.
47Magnetism Magnetism has to do with electrons. Remember that spin is how an electron reacts to a magnetic fieldParamagnetism attracted by a magnet.associated with unpaired electrons.Diamagnetism repelled by a magnet.associated with paired electrons.B2 is paramagnetic.
48MagnetismThe energies of of the p2p and the s2p are reversed by p and s interactingThe s2s and the s2s* are no longer equally spaced.Here’s what it looks like.
49Correct energy diagram s2p*p2p*p2p*2ps2p2pp2pp2ps2s*2s2ss2s
51Patterns As bond order increases, bond energy increases. As bond order increases, bond length decreases.Supports basis of MO model.There is not a direct correlation of bond order to bond energy.O2 is known to be paramagnetic.Movie.
58Consequences Paramagnetic Since 2p is lower in energy, favored by electrons.Electrons spend time closer to fluorine.Compatible with polarity and electronegativity.
59Names sp orbitals are called the Localized electron model s and p Molecular orbital modelLocalized is good for geometry, doesn’t deal well with resonance.seeing s bonds as localized works wellIt is the p bonds in the resonance structures that can move.