Two-dimensional Lewis dot formulas help us understand the bonding within a molecule or polyatomic ion, but they do not give us a sense of the 3-dimentional shape of the particle. Valence Shell Electron Repulsion Theory (VSEPR) is often used to predict particle shape from a Lewis dot formula.The VSEPR theory focuses on the idea that electron pairs and electrons repel one another and that these repulsions are smallest when the electron pairs or groups of electron pairs are as far apart as possible. This will then be the most stable form or shape of a molecule or ion.We know from a study of Lewis formulas that molecules and polyatomic ions may contain single bonds, double bonds, triple bonds, and "lone pairs" of electrons that are not used for bonding. We also know that a particle contains one or more "central atoms" around which the rest of the atoms are arranged; we know that the rest of the atoms are bonded either directly or through other atoms to this center atom.In the VSEPR theory approach to particle shapes, you focus on two things.the central atomthe number of different electron groups around the central atomThe arrangement in space (geometry ) of the electron groups around a center atom controls the overall shape of a particle because all bonds radiate out from the central atom of the particle.An electron group may be 1 pair of electrons (single bond or lone pair), 2 pairs (double bond) or 3 pairs (triple bond). The carbonate ion, for example, has one double bond and two single bonds attached to the center carbon atom. Thus, there are 3 groups of electrons around the C even though there are 4 pairs of electrons on carbon. Two pairs of electrons point in the same direction, the double bond to O. The other two pairs go in two other directions, one pair to each remaining O. One double bond and two single bonds on the center atom are considered to be 3 electron groups.
