Why is VSEPR theory thought to explain how molecules and ions behave?
VSEPR theory, which stands for Valence Shell Electron Pair Repulsion, explains the shapes of molecules which in turn explains their interactions with other molecules. This helps predict properties such as solubility and approximate melting and boiling points.
VSEPR theory says that the shapes of molecules are determined by the repulsion of both bonding and non-bonding outer electrons. For example, the molecule CH4 has a tetrahedral shape because that's the shape that allows for the maximum repulsion of the electron pairs being shared by each hydrogen with the central carbon atom. CH4 is a non-polar molecule because the carbon-hydrogen bonds aren't polar. CF4 is also tetrahedral, for the same reason, but the carbon-fluorine bonds are polar (have charge separation due to unequal sharing of electrons). Since the molecule is symmetrical, the polarities of the four bonds, all acting in the direction away from C and toward an F, cancel out. CF4 is therefore also non-polar according to VSEPR theory.
An example of a polar molecule is water. The oxygen shares a pair of electrons with each hydrogen and also has two lone (non-bonding) pairs. The four pairs of electrons, both bonding and non-bonding, on oxygen spread out into almost a tetrahedral formation, with the lone pairs repelling a little more than the bonding pairs. When looking at just the atoms the molecule has a bent shape caused by the two lone pairs repelling the O-H bonds. The polarity is caused by oxygen attracting the shared electrons more than hydrogen does. The electrons being pulled toward oxygen from two hydrogens not at 180 degree angles creates dipoles that don't cancel out. Water's extreme polarity is called hydrogen bonding. The positive (H) ends of water molecules are attracted to the negative (O) ends of other water molecules, causing water molecules to stick to themselves. This is the reason for water's relatively high boiling point. A similar but heavier compound, H2S, is a gas near room temperature. It also explains why ionic substances dissolve so well in water. Water's positive and negative ends attract the oppositely charged ions and pull these molecules apart.