Molecular Orbitals and Hybridizations

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intro to molecular orbital (MO) theory

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Key Questions

  • In Carbon, Carbon bonds there are three types of hybridizations possible; SP3, SP2, and SP. SP3 is when a Carbon has a tetrahedral shape four sigma bonds, or simply put a Carbon with four Hydrogens attached. An SP2 hybridization is when the Carbon has three sigma bonds and one pi bond or for better visualization a double bonded Carbon with three Hydrogens attached to each Carbon. And finally an SP hybridization is a Carbon with two sigma bonds and two pi bonds better know as two Carbons with a triple bond and one Hydrogen on each Carbon to complete the 4 bonds needed for Carbon.

  • Molecular Orbital (MO) Theory tells you that any linear combination of atomic orbitals (AOs) gives you the corresponding molecular orbital(s). (Linear combination literally means moving the atomic orbitals towards each other linearly through space until they overlap.)

    They can overlap either in-phase (#+# with #+#) or out-of-phase (#-# with #+#).

    The linear combination of two #s# orbitals overlaps to give you a #sigma# (in-phase overlap) bonding MO or #sigma^"*"# (out-of-phase overlap) antibonding MO.

    http://img.sparknotes.com/

    The linear combination of two #p# orbitals overlaps to give you either a #sigma# (in-phase overlap) bonding MO or #sigma^"*"# (out-of-phase overlap) antibonding MO for colinear/head-on overlap, or a #pi# (in-phase overlap) bonding MO or #pi^"*"# (out-of-phase overlap) antibonding orbital for parallel/sideways overlap.

    http://chemed.chem.purdue.edu/

    http://chemed.chem.purdue.edu/

    The results of the orbital overlaps can be depicted in a Molecular Orbital Diagram. An example for #F_2# is depicted below:

    http://www.grandinetti.org/

    Notice how the antibonding MOs are higher in energy than the bonding MOs. This is because the out-of-phase overlap creates nodes where electrons can never be, promoting nuclear repulsion, increasing the energy of the antibonding MO higher than the energy of the original AOs.

    In contrast, the bonding MOs are lower in energy because the in-phase combination creates a favorable overlap that increases electron density between the two AOs, minimizing nuclear repulsion, lowering the energy.

  • An atomic orbital extends over one atom. A molecular orbital extends over more than one atom.

    An orbital is a region in space where an electron is most likely to be found.

    The simplest atomic orbital is the spherical #1s# orbital of hydrogen.

    Isolated H atoms with energy from www.science.uwaterloo.cajpg)

    When two H atoms get close enough, their orbitals merge to include both nuclei. We then have a molecular orbital.

    www.science.uwaterloo.ca

    Actually, we can get two molecular orbitals, because the electrons behave as waves.

    They can either reinforce or cancel each other in the region between the two nuclei. So the molecular orbitals can be either bonding or antibonding.

    But the main point is, a molecular orbital extends over more than one atom.

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