~Valence and the structure of compounds.~ Compounds will be met from time to time which are apparent exceptions to the general statements just made in regard to valence. Thus, from the formula for hydrogen dioxide (H_{2}O_{2}), it might be supposed that the oxygen is univalent; yet it is certainly divalent in water (H_{2}O). That it may also be divalent in H_{2}O_{2} may be made clear as follows: The unit valence of each element may be represented graphically by a line attached to its symbol. Univalent hydrogen and divalent oxygen will then have the symbols H- and -O-. When atoms combine, each unit valence of one atom combines with a unit valence of another atom. Thus the composition of water may be expressed by the formula H-O-H, which is meant to show that each of the unit valences of oxygen is satisfied with the unit valence of a single hydrogen atom. The chemical conduct of hydrogen dioxide leads to the conclusion that the two oxygen atoms of its molecule are in direct combination with each other, and in addition each is in combination with a hydrogen atom. This may be expressed by the formula H-O-O-H. The oxygen in the compound is therefore divalent, just as it is in water. It will thus be seen that the structure of a compound must be known before the valences of the atoms making up the compound can be definitely decided upon. Such formulas as H-O-H and H-O-O-H are known as _structural formulas_, because they are intended to show what is known in regard to the arrangement of the atoms in the molecules. ~Valence and the replacing power of atoms.~ Just as elements having the same valence combine with each other atom for atom, so if they replace each other in a chemical reaction they will do so in the same ratio. This is seen in the following equations, in which a univalent hydrogen atom is replaced by a univalent sodium atom: NaOH + HCl = NaCl + H_{2}O. 2NaOH + H_{2}SO_{4} = Na_{2}SO_{4} + 2H_{2}O. Na + H_{2}O = NaOH + H. Similarly, one atom of divalent calcium will replace two atoms of univalent hydrogen or one of divalent zinc: Ca(OH)_{2} + 2 HCl = CaCl_{2} + 2H_{2}O. CaCl_{2} + ZnSO_{4} = CaSO_{4} + ZnCl_{2}. In like manner, one atom of a trivalent element will replace three of a univalent element, or two atoms will replace three atoms of a divalent element.