Covalent Bonding Orbitals
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Chapter 9, AP Chemistry
Atomic Orbitals: Hybridization
In molecules, the orbitals occupied by electron pairs are seldom "pure" s or p orbitals. Instead, they are "hybrid" orbitals, formed by combining s, p, and d orbitals.
s orbital + p orbital two "sp" hybrid orbitals
Be in BeF 2
s orbital + two p orbitals three "sp 2" hybrid orbitals
B in BF 3
s orbital + three p orbitals four "sp 3" hybrid orbitals
C in CH 4
sp hybridization in BeF 2, sp 2 hybridization in BF 3, and sp 3 hybridization in CH 4
Unshared pairs can be hybridized. In both NH 3 and H 2O, hybridization is sp 3
Only one of the electron pairs in a multiple bond can be hybridized.
When a bond consists of an electron pair in a hybrid orbital, the electron density is concentrated along the bond axis and is symmetrical about it. Such a bond is called a sigma bond.
-The "extra" electron pairs in a multiple bond are located in unhybridized orbitals that are not concentrated along the bond axis. Such bonds are called
5 electron pairs:
6 electron pairs:
Note that expanded octets do not occur with atoms in the 2nd period (e.g. N, O, F), since there are no 2d orbitals available for hybridization.
Molecular Orbital Theory (MO Theory)
When atoms containing atomic orbitals (AO) combine to form molecules, the AO's merge into molecular orbitals (MO's).
-Quantum mechanics is used to define/describe the shape of the MO formed. (Schrodringer's Wave Equation)
Constructive interference produces MO's that places the bonding e - 's between the nuclei of the bonding atoms.
e - 's are simultaneously attracted by both nuclei which lowers their energies.
the MO's produced are called bonding orbitals.
sigma bonding MO's ( b) are formed along the internuclear axis between the nuclei of the atoms.
pi bonding MO's ( b) are formed over/under the internuclear axis, but still between the nuclei.
Destructive interference produces MO's that places the electrons away from the space between the nuclei.
the nuclei repel which makes the bond less stable (i.e. higher energy).
these MO's are called antibonding orbitals.
sigma antibonding MO's ( *)
pi antibonding MO's ( *)
Bonding MO's have a lower energy and are filled with e - 's before the antibonding orbitals.
Antibonding orbitals increase the energy level more than bonding MO's lower energy, so if there are as many e
- 's in antibonding MO's as in bonding MO's, the molecule will be unstable.
MO's are formed by combining atomic orbitals
two atomic orbitals
high energy, antibonding MO's (unstable)
low energy, bonding MO's (stable)
Each MO can hold two electrons; MO's fill in order of increasing energy.
two 2s orbitals
b 2s + * 2s
b 2p + x * 2p x
b 2p + y * 2p y
b 2p + z * 2p z
Hund's rule is obeyed:
2 (10 valence e - 's)
2 (12 valence e - 's)
2 (14 valence e - 's)
2 (16 valence e - 's)
Terms to Know:
Paramagnetic - unpaired electrons present in solids, attracted into a magnetic field.
Diamagnetic - solids that contain paired electrons, repelled by a magnetic field.
Examples: In the table above N
2 and F 2 have paired electrons in their outer MO, so they are diamagnetic. O 2 has unpaired electrons in its outer MO, so it is paramagnetic. Ne 2 does not truly exist because as stated above under "B" it has the same number of electrons in antibonding orbitals as it does in bonding orbitals, so it would be unstable and unlikely to form.
Number of bonds (Bond Order) =
Number of bonds
(Bond Order) Number of unpaired
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