The chemical equation formula for water is expressed as H2O, but also is acceptable when displayed as HOH. In this equation, two hydrogen atoms react with an oxygen atom to form a single molecule of H2O.
Every water molecule is made of two hydrogen atoms and an oxygen atom and both come with their own sets of electrons. These electrons circle the center of the atom, giving it a positive or negative charge. In water molecules, electrons from oxygen and hydrogen atoms are shared, which gives the H2O molecule a special bond called a "hydrogen bond." This special bond is the source of many familiar characteristics of water, including the tendency to group together as a liquid.