The Foundations of Chemistry

CHEMICAL FORMULAS

The chemical formulafor a substance shows its chemical composition. This represents
the elements present as well as the ratio in which the atoms of the elements occur. The
formula for a single atom is the same as the symbol for the element. Thus, Na can rep-
resent a single sodium atom. It is unusual to find such isolated atoms in nature, with the
exception of the noble gases (He, Ne, Ar, Kr, Xe, and Rn). A subscript following the sym-
bol of an element indicates the number of atoms in a molecule. For instance, F 2 indicates
a molecule containing two fluorine atoms, and P 4 a molecule containing four phospho-
rus atoms.
Some elements exist in more than one form. Familiar examples include (1) oxygen,
found as O 2 molecules, and ozone, found as O 3 molecules, and (2) two crystalline forms
of carbon—diamond and graphite (Figure 13-33). Different forms of the same element
in the same physical state are called allotropic modifications,or allotropes.
Compounds contain two or more elements in chemical combination in fixed propor-
tions. Many compounds exist as molecules (Table 2-2). Hence, each molecule of hydro-
gen chloride, HCl, contains one atom of hydrogen and one atom of chlorine; each mol-
ecule of carbon tetrachloride, CCl 4 , contains one carbon atom and four chlorine atoms.

2-2

2-2 Chemical Formulas 51

Figure 2-5 A computer reconstruction of the

surface of a sample of silicon, as observed with a

scanning tunnelling electron microscope (STM),

reveals the regular pattern of individual silicon

atoms. Many important reactions occur on the

surfaces of solids. Observations of the atomic

arrangements on surfaces help chemists understand

such reactions. New information available using the

STM will give many details about chemical bonding

in solids.

TABLE 2-2 Names and Formulas of Some Common Molecular Compounds

Name Formula Name Formula Name Formula

water H 2 O sulfur dioxide SO 2 butane C 4 H 10
hydrogen peroxide H 2 O 2 sulfur trioxide SO 3 pentane C 5 H 12

hydrogen chloride* HCl carbon monoxide CO benzene C 6 H 6
sulfuric acid H 2 SO 4 carbon dioxide CO 2 methanol (methyl alcohol) CH 3 OH

nitric acid HNO 3 methane CH 4 ethanol (ethyl alcohol) CH 3 CH 2 OH
acetic acid CH 3 COOH ethane C 2 H 6 acetone CH 3 COCH 3

ammonia NH 3 propane C 3 H 8 diethyl ether (ether) CH 3 CH 2 XOXCH 2 CH 3

*Called hydrochloric acid if dissolved in water.

An O 2 molecule.

O O

An O 3 molecule.

O

O

O

117 °