VALENCE BOND THEORY
HYBRIDIZATION
-
- BONDING THEORY
- LEWIS - PREDICTS THE NUMBER OF BONDS
BUT NOT GEOMETRY (BOND ANGLES AND LENGTHS)
- VSEPR - PREDICTS BOND ANGLES
- NEITHER THEORY TELLS WHY BONDING OCCURS
- TWO ADDITIONAL THEORIES PROVIDE A BASIS FOR WHY
-
- VALENCE BOND THEORY
- CONSIDER TWO HYDROGEN ATOMS SUFFICIENTLY FAR APART THAT THERE ARE NO INTERACTIONS
BRING THEM TOGETHER AND MEASURE THE EXTENT OF
TWO PLUS-MINUS ATTRACTIONS
- AS THEY GET CLOSER THE ATTRACTIONS ARE LARGER THAN THE REPULSIONS
- WHEN THEY ARE CLOSE ENOUGH, THE REPULSIONS INCREASE MORE THAN THE ATTRACTIONS
-
-
- BONDS FORM AT A CERTAIN DISTANCE BECAUSE THE ENERGY IS LOWER THEN IF THE ATOMS WERE CLOSER OR FARTHER APART!
WE WILL SAY THE BONDS FORMED DUE TO AN OVERLAP OF ORBITALS
CALL THE BOND A SIGMA (s) BOND DUE TO ITS HIGH DEGREE OF SYMMETRY
CAN ALSO "OVERLAP" OTHER ORBITALS
-
-
- BUT, THIS SIMPLE OVERLAP OF ORBITALS AND THE PAIRING OF ELECTRONS DOES NOT PREDICT GEOMETRY.
IF IT DID, METHANE, CH4, USING AN EXCITED CARBON ATOM
(1s2 2s1 2px1 2py1
2pz1), WOULD HAVE
2 DIFFERENT BOND ANGLES, 90o AND SOMETHING LARGER
2 DIFFERENT BOND ENERGIES
EXPERIMENTALLY ALL BOND LENGTHS, ANGLES, AND ENERGIES ARE THE SAME
- NEED TO EXPAND THEORY USING HYBRIDIZATION OF ATOMIC ORBITALS
-
- HYBRIDIZATION
- BOND ANGLES IN MOLECULES ARE DIFFERENT THAN ORBITAL ANGLES IN ATOMS
USEFUL ORBITALS CAN BE OBTAINED BY COMBINING (HYBRIDIZING) ENOUGH ATOMIC ORBITALS
(Y) FOR THE SIGMA BONDS AND NONBONDING ELECTRONS.
- COMBINE FOUR ORBITALS (WAVE EQUATIONS):
Y2s,
Y2px,
Y2py,
Y2pz
- GET FOUR NEW ORBITALS (WAVE EQUATIONS): Y, Y,
Y, Y
EACH ORBITAL IS IDENTICAL IN:
- CALL THESE NEW ORBITALS, "HYBRID ORBITALS"
SPECIFICALLY sp3 HYBRID ORBITALS
WE NOW HAVE: Ysp3,
Ysp3,
Ysp3,
Ysp3
-
-
- NOTE THAT THE Ysp3 IS DIRECTIONAL, WITH MOST OF THE
ELECTRON DENSITY ON ONE SIDE
-
-
- TO FORM BONDS, OVERLAP Y1sH AND
Ysp3C
-
-
REPEAT THE PROCESS FOR OTHER THREE UNPAIRED ELECTRONS TO GET CH4
GET THE SAME GEOMETRY AS OBSERVED BY EXPERIMENT!
- AMMONIA, NH3, AND WATER, H2O, ARE BOTH sp3 HYBRIDIZED.
USE HYBRIDIZED ORBITALS FOR SIGMA BONDING AND NONBONDING ELECTRONS
-
- BH3 HAS ONLY 3 PAIRS OF SIGMA BONDING AND NONBONDING ELECTRONS
- ONLY NEEDS 3 HYBRIDIZED ORBITALS
COMBINE:
Y2s,
Y2px, AND
Y2py
HYBRIDIZE TO GIVE:
Ysp2,
Ysp2,
Ysp2
CALCULATIONS GIVE THREE IDENTICAL ORBITALS
120o BOND ANGLES (TRIGONAL PLANAR)
-
- BeCl2 HAS ONLY 2 PAIRS OF SIGMA BONDING AND NONBONDING ELECTRONS
- ONLY NEEDS 2 HYBRIDIZED ORBITALS
HYBRIDIZE TO GIVE:
Ysp,
Ysp
CALCULATIONS GIVE TWO IDENTICAL ORBITALS
180o BOND ANGLES (LINEAR)
-
- PCl5 HAS 5 PAIRS OF SIGMA BONDING AND NONBONDING ELECTRONS
- NEEDS 5 HYBRIDIZED ORBITALS
COMBINE:
Y3s,
Y3px,
Y3py,
Y3pz, AND
Y3dx2
HYBRIDIZE TO GIVE:
Ysp3d,
Ysp3d,
Ysp3d,
Ysp3d,
Ysp3d
CALCULATIONS GIVE FIVE ORBITALS
120o AND 90o BOND ANGLES (TRIGONAL BIPYRAMID)
-
- SF6 HAS 6 PAIRS OF SIGMA BONDING AND NONBONDING ELECTRONS
- NEEDS 6 HYBRIDIZED ORBITALS
COMBINE:
Y3s,
Y3px,
Y3py,
Y3pz,
Y3dx2, AND
Y3dx2
HYBRIDIZE TO GIVE:
Ysp3d2,
Ysp3d2,
Ysp3d2,
Ysp3d2,
Ysp3d2, AND
Ysp3d2
CALCULATIONS GIVE SIX ORBITALS
90o BOND ANGLES (OCTAHEDRAL)
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- CONSIDER ETHYLENE, CH2=CH2
-
-
- EACH CARBON IS sp2 HYBRIDIZED
THE UNPAIRED ELECTRON IS IN A p ORBITAL
TWO p ORBITALS CAN OVERLAP END TO END TO FORM A SIGMA (s) BOND, OR
TWO p ORBITALS CAN OVERLAP SIDE BY SIDE TO FORM A PI (p) BOND
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- TRIPLE BONDS ARE ONE SIGMA BOND AND TWO PI BONDS USING THE py AND pz ORBITALS
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HYBRIDIZATION AND GEOMETRY
| NUMBER OF SIGMA BONDING AND NONBONDING PAIRS |
HYBRIDIZATION |
BOND ANGLE |
GEOMETRY |
| 2 |
sp |
180o |
LINEAR |
| 3 |
sp2 |
120o |
TRIGONAL PLANAR |
| 4 |
sp3 |
109.5o |
TETRAHEDRAL |
| 5 |
sp3d |
120 AND 90o |
TRIGONAL BIPYRAMID |
| 6 |
sp3d2 |
90o |
OCTAHEDRAL |
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