A chemical formula is a combination of chemical symbols that represents the chemical composition of a compound. At a minimum, a formula tells which elements are present in the compound and the relative amount of each element. The chemical formula most familiar to people is probably H 2 O, the formula for water. This formula says that water consists of two elements, hydrogen (H) and oxygen (O). Further, it says that the ratio of the two elements is two parts hydrogen and one part oxygen. On a submicroscopic scale, the formula says that a molecule of water contains two atoms of hydrogen and one atom of oxygen.
A chemical formula can be determined in one of two ways: by experimentation or by prediction. For example, imagine that an entirely new compound has been discovered whose formula must be determined. That compound can be broken down in the laboratory and the elements present determined. Also, the ratio of the elements can be found. The formula obtained in this way shows the simplest possible ratio of the elements present and is known as the compound's empirical formula. The word empirical means "obtained by means of experimentation."
The empirical formula of a compound may not be its true or correct formula. Consider three different chemical compounds made of carbon and hydrogen only. The first compound contains one carbon atom and one hydrogen atom in each molecule. A molecule of the second compound consists of three carbon atoms and three hydrogen atoms joined to each other. And a molecule of the third compound contains six atoms of carbon and six atoms of hydrogen joined to each other.
The empirical formula for all three compounds is CH because the ratio of carbon to hydrogen is 1:1 in each. But the true formula is different for the three compounds. It is CH for the first compound, C 3 H 3 for the second, and C 6 H 6 for the third. The true, correct, or molecular formula for most chemical compounds also can be determined experimentally.
A second way of writing the chemical formula of a compound is by making intelligent guesses. When sodium reacts with chlorine to form sodium chloride, for example, each sodium atom loses one electron and each chlorine atom gains one electron. It makes sense to assume that the formula for sodium chloride is NaCl. To form the compound, every sodium atom needs one chlorine atom, so their final ratio should be 1:1.
Atom: The smallest particle of which an element can exist.
Chemical formula: A combination of chemical symbols that shows the composition of a compound.
Compound: A substance that contains two or more elements combined in a fixed proportion.
Empirical: Based on observation or experimentation.
Molecule: A particle formed by the combination of two or more atoms.
Valence: The tendency of an atom to gain or lose electrons in reacting with other atoms.
Chemists now know enough about the chemical elements to use this method with confidence. The tendency of any given element to lose or gain electrons in forming a compound is called its valence. The valence of sodium, for example, is +1, and the valence of chlorine, −1. Using valences, chemists can write the formulas for most chemical compounds with a high degree of accuracy.
Molecular formulas are the simplest kind of formulas to write because they tell only the minimum amount of information: the kind and number of atoms present in a compound. Structural formulas are a more complex type of formula because they also show how the atoms in a molecule are arranged in space.
Structural formulas. The structural formula for water is H—O—H. The dashed lines (—) in this formula are called bonds. They stand for the electrons that hold each hydrogen atom to the oxygen atom.
Another example of a structural formula is the expanded structural formula. It shows not only the elements present (for example, hydrogen, carbon, and oxygen) and the ratio of those elements in the compound (for example, CH 4 O), but also the arrangement of those atoms in comparison to each other. Thus, in an expanded structural formula you can see that three hydrogen atoms are attached to the carbon atom and one hydrogen atom is attached to the oxygen atom.
The only disadvantage of an expanded structural formula is the time and space required to write it out. Because of this disadvantage, chemists have developed an abbreviated kind of structural formula known as a condensed structural formula. The condensed structural formula for methanol can be written as:
CH 3 —OH or CH 3 OH
Both the condensed and structural formulas for methanol provide the same information.
When students are first beginning to study chemistry, they generally have to write expanded structural formulas. With practice, however, they soon develop the ability to write condensed formulas.
Spatial formulas. Other kinds of chemical formulas contain even more information about the structure of a molecule. For example, the structure of the water molecule shown above (H—O—H) is not quite correct. The hydrogen atoms in a water molecule do not really stick out in opposite directions from each other. Instead, the O—H bonds are bent slightly at an angle to each other.
More sophisticated formulas may be necessary for compounds whose three-dimensional shape is important. The compound known as 1,3-dichlorocyclobutane is an example. The compound consists of four carbon atoms connected to each other in a ring. The ring can be thought of as a square piece of cardboard with one carbon atom at each corner. Attached to two carbon atoms at opposite corners are two chlorine atoms. This molecule can be represented in two different ways, with both chlorine atoms on the same side of the carbon ring or on opposite sides of the ring. The two molecules look different from each other, and two different kinds of 1,3-dichlorocyclobutane can actually be found in the laboratory. Formulas that show special three-dimensional shapes are sometimes known as conformational formulas.