Depto. Química Orgánica

Molecular Geometry

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15/07/2017
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ELECTRON PAIRS DETERMINE MOLECULAR GEOMETRY


The bonds around an atom, formed through electronic sharing, are made up of pairs of electrons. These pairs of electrons in a molecule occupy defined places in space, just like electrons in atoms. This achieves the minimum repulsion between the electrons. Achieving this minimum repulsion conditions the shape that the molecules adopt.
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The electron pairs are distributed around the nucleus of the central atom like balloons held together by the inflation nozzle.
BH3CH4NH3H2O
Image There are three pairs of bonding electrons around boron. There is nothing else. They are arranged in a flat trigonal shape. 
Image Flat trigonal shape
Image There are four pairs of bonding electrons around carbon. There is nothing else. They are arranged in a tetrahedral shape.
Image Tetrahedral shape
Image There are four pairs of electrons around nitrogen, but only three are bonding. The arrangement of the electrons is tetrahedral, but the bonds have...
Image Pyramidal shape
Image There are four pairs of electrons around oxygen, but only two are forming bonds. The arrangement of the electrons is tetrahedral, but the bonds have... 
Image Angular shape
In molecular models (JSmol in the header), the non-shared electron pairs that nitrogen and oxygen have are "invisible" but they are there implicitly, conditioning the molecular geometry.