CHEMICAL FORMULAS

5.2. CHEMICAL FORMULAS

Writing a formula implies that the atoms in the formula are bonded together in some way. The relative numbers of atoms of the elements in a compound are shown in a chemical formula by writing the symbols of the elements followed by appropriate subscripts to denote how many atoms of each element there are in the formula unit. A subscript following the symbol gives the number of atoms of that element per formula unit. If there is no

subscript, one atom per formula unit is implied. For example, the formula H 3 PO 4 describes a molecule containing three atoms of hydrogen and four atoms of oxygen, along with one atom of phosphorus. Sometimes groups of atoms which are bonded together within a molecule or within an ionic compound are grouped in the formula within parentheses. The number of such groups is indicated by a subscript following the closing parenthesis. For

example, the 2 in (NH 4 ) 2 SO 4 states that there are two NH 4 groups present per formula unit. There is only one SO 4 group; therefore parentheses are not necessary around it.

68 CHEMICAL BONDING

[ CHAP. 5

EXAMPLE 5.1. How many H atoms and how many P atoms are there per formula unit of (NH 4 ) 3 PO 3 ? Ans.

There are three NH 4 groups, each containing four H atoms, for a total of 12 H atoms per formula unit. There is only one P atom; the final 3 defines the number of O atoms.

In summary, chemical formulas yield the following information:

1. Which elements are present

2. The ratio of the number of atoms of each element to the number of atoms of each other element

3. The number of atoms of each element per formula unit of compound

4. The fact that all the atoms represented are bonded together in some way You cannot tell from a formula how many atoms of each element are present in a given sample of substance,

because there might be a little or a lot of the substance present. The formula tells the ratio of atoms of each element to all the others, and the ratio of atoms of each element to formula units as a whole.

EXAMPLE 5.2. (a) Can you tell how many ears and how many noses were present at the last Super Bowl football game? Can you guess how many ears there were per nose? How many ears per person? (b) Can you tell how many hydrogen and oxygen atoms there are in a sample of pure water? Can you tell how many hydrogen atoms there are per oxygen atom? per water molecule?

Ans. (a) Since the problem does not give the number of people at the game, it is impossible to tell the number of ears or noses from the information given. The ratios of ears to noses and ears to people are both likely to be 2 : 1.

(b) Since the problem does not give the quantity of water, it is impossible to tell the number of hydrogen atoms or oxygen atoms from the information given. The ratios of hydrogen atoms to oxygen atoms and hydrogen atoms to water molecules are both 2 : 1.

The atoms of many nonmetals bond together into molecules when the elements are uncombined. For example,

a pair of hydrogen atoms bonded together is a hydrogen molecule. Seven elements, when uncombined with other elements, form diatomic molecules. These elements are hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine. They are easy to remember because the last six form a large “7” in the periodic table, starting at element 7, nitrogen:

N O F Cl Br