Shape of hydrogen selenide.

  1. Why does H2Se have a bent shape?

    It has 6 valence electrons. 2 are used to bond with each of the hydrogens. 2 more up and 2 more down. So the repulsion of the lone pairs cancel off each other. Isn't this arrangement going to make the bond angle 180°?
     
  2. jcsd
  3. trollcast

    trollcast 291
    Gold Member

    The repulsive forces between the lone pairs is greater than the forces between the 2 bonding pairs so the bonding pairs get pushed closer to increase the distance between the 2 lone pairs.

    Another way to look at it is that molecules with 4 pairs of electrons form a tetrahedral shape, think methane, except in molecules like H2O or H2Se, 2 of the corners do not have another atom but they do have an electron pair.
     
  4. Push closer to where? If the two lone pairs are arranged at 180° to each other and so does the bond pairs, then maximum distance will be achieved, no?
     
  5. trollcast

    trollcast 291
    Gold Member

    [​IMG]

    Try looking at that and see how the arrangement of the bonds and electron pairs maximises the distances between them (I think you are maybe only thinking about moving them round in the same plane)

    I know the diagram says water but H2Se has the same electron geometry just the angles and lengths are a bit different.

    Ps. The perspective of that diagram isn't great since it appears the lone pairs are closer together than the bonding pairs but in reality the bonding pairs should be closer but it should show the sort of 3d arrangement of the pairs around the central atom.
     
    Last edited: Mar 18, 2013
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