An aspirin molecule, C 9 H 8 O 4 , contains nine carbon atoms, eight hydrogen atoms, and
four oxygen atoms.
Many of the molecules found in nature are organic compounds. Organic compounds
contain CXC or CXH bonds or both. Eleven of the compounds listed in Table 2-2 are
organic compounds (acetic acid and the last ten entries). All of the other compounds in
the table are inorganic compounds.
Some groups of atoms behave chemically as single entities. For instance, an oxygen
atom that is bonded to a hydrogen atom and also to a carbon atom that is bonded to three
other atoms forms the reactive combination of atoms known as the alcohol group or mol-
ecule. In formulas of compounds containing two or more of the same group, the group
formula is enclosed in parentheses. Thus, ethylene glycol contains two alcohol groupsand
its formula is C 2 H 4 (OH) 2 (see structure in the margin). When you count the number of
atoms in this molecule from its formula, you must multiply the numbers of hydrogen and
oxygen atoms in the OH group by 2. There are twocarbon atoms, sixhydrogen atoms
and twooxygen atoms in a molecule of ethylene glycol.
Compounds were first recognized as distinct substances because of their different
physical properties and because they could be separated from one another by physical
methods. Once the concept of atoms and molecules was established, the reason for these
differences in properties could be understood: Two compounds differ from each other
because their molecules are different. Conversely, if two molecules contain the same
number of the same kinds of atoms, arranged the same way, then both are molecules of
the same compound. Thus, the atomic theory explains the Law of Definite Proportions
(see Section 1-5).
This law, also known as the Law of Constant Composition,can now be extended to
include its interpretation in terms of atoms. It is so important for performing the calcu-
lation in this chapter that we restate it here:Different pure samples of a compound always contain the same elements in the same
proportion by mass; this corresponds to atoms of these elements combined in fixed
numerical ratios.So we see that for a substance composed of molecules, the chemical formulagives
the number of atoms of each type in the molecule. But this formula does not express the
order in which the atoms in the molecules are bonded together. The structural formula
shows the order in which atoms are connected. The lines connecting atomic symbols rep-
resent chemical bonds between atoms. The bonds are actually forces that tend to hold
atoms at certain distances and angles from one another. For instance, the structural for-
mula of propane shows that the three C atoms are linked in a chain, with three H atoms
bonded to each of the end C atoms and two H atoms bonded to the center C. Ball-and-
stickmolecular models and space-fillingmolecular models help us to see the shapes and
relative sizes of molecules. These four representations are shown in Figure 2-6. The ball-
and-stick and space-filling models show (1) the bonding sequence,that is the order in which
the atoms are connected to each other, and (2) the geometrical arrangementsof the atoms
in the molecule. As we shall see later, both are extremely important because they deter-
mine the properties of compounds.52 CHAPTER 2: Chemical Formulas and Composition Stoichiometry
A model of ethylene glycol.