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2.1: Lewis and Valence Bond Models

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    319603
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    There are several models of bonding in organic molecules that have, at one point in their history, achieved scientific consensus. Each of them has their own pitfalls, but they are all useful to organic chemists, even today.

    Lewis model – sharing of two electrons by more than one atom can permit each atom to have a stable closed-shell electron configuration, aka octet rule: in forming bond, atoms either gain, lose, or share electrons (depending on the electronegativity of the atoms that make up the bond) to achieve a stable electron configuration characterized by eight valence electrons:

    • gain/loss of electrons results in ionic bonding
    • sharing of electrons results in covalent bonding
    atom # valence electrons # electrons needed for octet # bonds made to other atoms # lone pairs
    H 1 1 (duet) 1 0
    C 4 4 4 0
    N 5 3 3 1
    O 6 2 2 2
    F 7 1 1 3
    Li 1 7 (loses an electron to generate octet) 0

     

    Lewis dot structures:

    CH4O               C + (4 x H) + O = 

                            4 + (4 x 1) + 6 = 14 electrons

    Screen Shot 2021-05-20 at 8.40.09 AM.png

    Other examples: CH5N, C2H6O, CH6N+ (quick question: can you draw the Lewis structure for these molecules?)

     

    So, what do Lewis structures look like in three dimensions? We use Valence Shell Electron Pair Repulsion (VSEPR) to predict geometries. In essence, an electron pair, existing either in a bond or as an unbounded lone pair, associated with an atom will lie as far away from the atom’s other electron pairs as possible. An example is CH4, or methane:

    CH4                 C + (4 x H) =

                            4 + (4 x 1) = 8 electrons

    Screen Shot 2021-05-20 at 8.40.14 AM.png

    What is the simplest way to arrange an atom containing four identical substituents? A tetrahedron, of course. We say the geometry of methane is tetrahedral with bond angles of 109.5°. Other geometries include H2O (bent, 105°), NH3 (trigonal pyramidal, 107°), BF3 (trigonal planar, 120°), CH2O (trigonal planar, 120°), and CO2 (linear, 180°). Can you draw the Lewis structures for these molecules?

     

    Valence Bond model – 2nd generation response to the Lewis model, which is convenient but not the whole picture. In summary, electrons have wave-particle duality, so we can describe bonding as an interaction between waves. When two waves overlap in-phase, they reinforce each other to create a new wave, or chemical bond. The best example is molecular hydrogen. When the electrons in two 1s orbitals overlap, a new orbital is created that is known as a \(σ\) orbital, or \(σ\) bond.

    Screen Shot 2021-05-20 at 8.40.21 AM.png

    The video below is a review of the key concepts in the Lewis and Valence Bond models of bonding.


    2.1: Lewis and Valence Bond Models is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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