FORMULAE AND EQUATIONS
We have seen that the various chemical elements combine with one another in certain proportions. In order to express these varied reactions, chemical formulæ have been devised, which can be read at a glance, showing the changes which have taken place in any given combination. When one atom of one element combines with one atom of another, the letters signifying these elements are simply written side by side, thus: HCl. When, however, two atoms of one element combine with one of another, a small figure is placed under and to the right of the element, thus: H₂O. Here we see at a glance that two atoms of hydrogen have combined with one of oxygen, formingwater. This is the simplest type of formula, and is often known as theempiricalformula. There is, however, another way of writing a formula, which is more expressive, thus:
BA=C=B
This is known as astructuralorconstitutionalformula, and from it we can see at a glance that A is bivalent, C is quadrivalent, and B univalent. This type of formula shows us more readily than the other the structure of the molecule in question. The complexity of such formulæ naturally increases with the complexity of the molecules, and in many cases may be extremely intricate. Ordinary chemical formulæ, however, are written empirically. Any chemical changes which take place as the result of some reaction are expressed in this manner.