The whole‐number ratios of combining masses in chemical reactions can be readily explained knowing that the basic unit of all elements is the atom, which originally meant an indivisible particle. Therefore, one atom of carbon can react with either one atom of oxygen,
or two atoms of oxygen,
but not with, say, oxygen atoms. The reactions of carbon with oxygen are shown using chemical equations with reactants on the left and products on the right of the arrow.
A substance containing atoms of more than one element in a definite ratio is called a compound. The composition of a compound is shown in its chemical formula. In the chemical reaction of carbon and oxygen to form carbon dioxide, the elements are in a definite 1:2 ratio—one atom of carbon with two atoms of oxygen forming the compound carbon dioxide. When several atoms are so tightly bonded together that they physically behave as a unit, the unit is called a molecule. (See Figure 1.) Elements and compounds can be molecular. In the carbon dioxide reaction, the O 2 molecule contains 2 atoms of oxygen, and the CO 2 molecule has 3 atoms—1 carbon and 2 oxygen.
Figure 1. Atoms and molecules.