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Covalent Bonding Orbitals

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Chapter 9, AP Chemistry

  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.

      • s orbital + p orbital ---> two "sp" hybrid orbitals
        Be in BeF2
        2s 2px 2py 2pz
        1 electron 1 electron empty orbital empty orbital

      • s orbital + two p orbitals ---> three "sp2" hybrid orbitals
        B in BF3
        2s 2px 2py 2pz
        1 electron 1 electron 1 electron empty orbital

      • s orbital + three p orbitals ---> four "sp3" hybrid orbitals
        C in CH4
        2s 2px 2py 2pz
        1 electron 1 electron 1 electron 1 electron

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

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

    3. Only one of the electron pairs in a multiple bond can be hybridized.
      sp2 sp3
      lewis structure of O2 lewis structure of N2

    4. 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.

    5. Hybridization
      5 electron pairs: sp3d
      3s 3p 3d
      1 electron 1 electron 1 electron 1 electron 1 electron empty orbital
      6 electron pairs: sp3d2
      3s 3p 3d
      1 electron 1 electron 1 electron 1 electron 1 electron 1 electron

      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 (sigmab) are formed along the internuclear axis between the nuclei of the atoms.
      • pi bonding MO's (pib) 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 (sigma*)
      • pi antibonding MO's (pi*)

      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 split to form 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 sigmab2s + sigma*2s
      two 2px orbitals pib2px + pi*2px
      two 2py orbitals pib2py + pi*2py
      two 2pz orbitals pib2pz + pi*2pz

      Arrangement: sigmab2s, sigma*2s, pib2p, pib2p, sigmab2p, pi*2p, pi*2p, sigma*2p

      Hund's rule is obeyed: sigmab2s sigma*2s pib2p pib2p sigmab2p pi*2p pi*2p sigma*2p
      N2 (10 valence e- 's) 2 electrons 2 electrons 2 electrons 2 electrons 2 electrons empty orbital empty orbital empty orbital
      O2 (12 valence e- 's) 2 electrons 2 electrons 2 electrons 2 electrons 2 electrons 1 electron 1 electron empty orbital
      F2 (14 valence e- 's) 2 electrons 2 electrons 2 electrons 2 electrons 2 electrons 2 electrons 2 electrons empty orbital
      Ne2 (16 valence e- 's) 2 electrons 2 electrons 2 electrons 2 electrons 2 electrons 2 electrons 2 electrons 2 electrons

    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) = bond order


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