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9.10: VSEPR Theory

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    53753
  • In every scientist's life comes a moment when they realize that they were wrong. Sometimes the scientist may rethink a problem, and recognize where the solution went astray. Other times they find out when they go into the lab and an experiment doesn't work (or gets way too exciting). What has to happen then is a change in direction. The good scientist sees the problem and comes up with a new answer. Then, that answer has to be tested to see how it plays out.

    Putting atoms together to form compounds can be done on paper or in the lab. However, when the shape of the molecule made in the lab is different from the shape of the molecule drawn on paper, then one needs to rethink their ideas and find better explanations.

    VSEPR Theory

    In 1956, British scientists R.J. Gillespie and R.S. Nyholm recognized that the current model for explaining bond angles did not work well. The theory at that time relied on hybrid orbitals to explain all aspects of bonding. The problem was that this theory gave an incorrect prediction of bond angles for many compounds. They developed a new approach based on earlier work by other scientists that incorporated a consideration of electron pairs in predicting three-dimensional structure.

    The valence shell is the outermost electron-occupied shell of an atom. The valence shell holds the electrons that are involved in bonding and are the electrons shown in a Lewis structure. The acronym VSEPR stands for the valence-shell electron pair repulsion model. The model states that electron pairs will repel each other such that the shape of the molecule will adjust, so that the valence electron-pairs stay as far apart from one another as possible. Molecules can be systematically classified according to the number of bonding pairs of electrons; as well as the number of nonbonding, or lone pairs, around the central atom. For the purposes of the VSEPR model, a double or triple bond is no different in terms of repulsion than a single bond.

    Summary

    • The valence shell is the outermost electron-occupied shell of an atom.
    • The VSEPR model states that electron pairs will repel each other such that the shape of the molecule will adjust, so that the valence electron-pairs stay as far apart from one another as possible.
    • VSEPR theory allows more accurate predictions of molecular shape.

    Contributors and Attributions

    • CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.

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