Water molecules collide with one another to cause the self-ionization reaction represented by this equation: 2H2O H3O+ + OH-
It is a reversible reaction so the equation is usually written with the arrows going in both directions: 2H2O H3O+ + OH-
The reaction does not form very much H3O+ or OH-. In one liter of water there are about 55 moles of water molecules, but only 1.0 x 10-7 moles of H3O+ and OH- are formed (at room temperature). So the concentrations of H3O+ and OH- in pure water are 1.0 x 10-7 M. It is the 7 in the exponent or power of this number that gives neutral water a pH of 7.