Molecular Shapes VSEPR Theory Valence Shell Electron Pair Repulsion

Identifying the shape of a molecule First a Lewis structure must be made of the molecule. Then the number of atoms surrounding the atom and the number of unshared electron pairs on the central atom must be expressed in a formula where A represents the central atom and B represents the surrounding atoms and E represents the unshared electron pairs on the central atom.

Linear Shape All molecules containing only two atoms have a linear shape with the formula AB CO2 has the formula AB2 because it has two atoms surrounding the central atom and no unshared electron pairs on the central atom.

Bent Shape H2O has an AB2E2 formula because it has 2 pairs of unshared electrons on the central atom. SnCl2 has an AB2E formula with only one pair of unshared electrons on the central atom. Both molecules have a bent shape because the unshared electron pairs on the central atom repel the shared electron pairs as well as the shared electron pairs repeling each other.

Tetrahedral CCl4 has an AB4 formula. With four atoms surrounding the central atom and no unshared electrons the electrons repel each other equally in all directions creating a tetrahedral shape.

Triangular Planar Shape BF3 has an AB3 formula with three atoms surrounding the central atom and no unshared electrons around the central atom so the atoms surrounding the central atom are equal angles from each other and on the same plane as the central atom.

Triangular Pyramidal Shape NH3 has an AB3E formula with three atoms surrounding the central atom and one pair of unshared electrons on the central atom. A triangular pyramidal shape is produced by the unshared electron pairs repelling the shared electrons away from them thus creating the shape.

Polarity of Molecules The polarity of a diatomic molecule is the same as the polarity of the bond in the molecule. The polarity of a molecule with more than two atoms depends on both the shape of the molecule and the polarity of the bonds. If the molecule is arranged in such a way that there is a positively charged and negatively charged end of the molecule then the molecule is polar. If the molecule is arranged in such a way that there is the same charge exposed all the way around then the molecule is nonpolar.

Polarity Continued A tetrahedral molecule with the same atoms around the central atom would be nonpolar. A tetrahedral molecule with different atoms around the central atom could be polar if some of the atoms have higher electronegativities than the central atom and some have lower electronegativities. A triangular pyramidal molecule would be polar. A triangular planar may be polar or nonpolar depending on the polarity of the bonds in the molecule

Polarity Continued A bent molecule is polar. A linear molecule with identical atoms on both ends like carbon dioxide is nonpolar because it has the same charge on both ends of the molecule.

Properties of molecules because of polarity Polar molecules tend to have a greater attraction for each other than nonpolar molecules. For this reason polar molecules like water have a greater attraction and are liquids under normal conditions where as carbon dioxide molecules that are nonpolar and have little attraction for each other are gases under normal conditions.