In the VSEPR model, electron regions around a central atom arrange to reduce repulsion. Each single, double or triple bond counts as one region, and each non-bonding pair counts as one. Two regions give a linear arrangement, three a trigonal-planar arrangement and four a tetrahedral arrangement. Molecular shape names describe atom positions: four regions with one lone pair give trigonal pyramidal, while four with two lone pairs give bent.
Molecular polarity depends on bond dipoles and their directions. Symmetrical identical bonds can cancel, as in linear CO2 or tetrahedral CH4. In bent H2O and pyramidal NH3 the dipoles do not cancel. A polar bond alone does not prove that a whole molecule is polar; first identify the spatial arrangement and whether the bond dipoles balance.
Example 1
Determine the shape and polarity of CO2,O=C=O.
Carbon has two electron regions and no lone pairs, so the shape is linear. Equal C=O dipoles point oppositely and cancel; CO2 is non-polar.
Example 2
Determine the shape of NH3.
Nitrogen has three bonding regions and one lone pair. The electron arrangement is tetrahedral but atom positions are trigonal pyramidal.
Watch out: Bond dipoles can cancel in a suitable symmetric molecular shape.