1. 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.
   1. • s orbital + p orbital two "sp" hybrid orbitals
        Be in BeF₂
        

        2s	2px	2py	2pz

      • s orbital + two p orbitals three "sp²" hybrid orbitals
        B in BF₃
        

        2s	2px	2py	2pz

      • s orbital + three p orbitals four "sp³" hybrid orbitals
        C in CH₄
        

        2s	2px	2py	2pz

   2. sp hybridization in BeF₂, sp² hybridization in BF₃, and sp³ hybridization in CH₄

   3. Unshared pairs can be hybridized. In both NH₃ and H₂O, hybridization is sp³

   4. Only one of the electron pairs in a multiple bond can be hybridized.
      

      sp2	sp3

   5. 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 pi bonds.

   6. Hybridization
      

      5 electron pairs:	sp3d	3s 3p 3d
      6 electron pairs:	sp3d2	3s 3p 3d

      
      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.

2. 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)

   1. 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.

   2. 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.

   3. 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	b2s + *2s
      two 2px orbitals	b2px + *2px
      two 2py orbitals	b2py + *2py
      two 2pz orbitals	b2pz + *2pz

      
      Arrangement: ^b_2s, ^*_2s, ^b_2p, ^b_2p, ^b_2p, ^*_2p, ^*_2p, ^*_2p

      Hund's rule is obeyed:	b2s	*2s	b2p	b2p	b2p	*2p	*2p	*2p
      N2 (10 valence e- 's)
      O2 (12 valence e- 's)
      F2 (14 valence e- 's)
      Ne2 (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 N₂ and F₂ have paired electrons in their outer MO, so they are diamagnetic. O₂ has unpaired electrons in its outer MO, so it is paramagnetic. Ne₂ 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.

   Bond Order

   	Number of bonds (Bond Order)	Number of unpaired electrons
   N2	3	0
   O2	2	2
   F2	1	0
   Ne2	0	0

   Number of bonds (Bond Order) =

This page was made by Erik Epp.