COMPOUNDS, AND CHEMICAL FORMULAS
Theory, introduced on a scientific basis by John Dalton in 1806,
revolutionized the way that humans thought about matter in the
universe. It created a view of matter that was elegant in its
simplicity - all of the changes that take place around us every day are
simply due to rearrangements of nature's fundamental building blocks,
which we call atoms.
a limited number of different types of atoms (92 that occur in nature),
from which all of the millions of substances around us are
"built." Each different type of atom (called an element) is represented by a
unique symbol. One interesting fact is that John Dalton himself
(who was a bit idiosyncratic) wished to use pictorial symbols for the
elements, using pictures that he thought represented some aspect of the
chemical or physical behavior of that particular element. He was
overruled in favor of using various combinations of letters as the
elemental symbols (less creative perhaps, but a lot faster to write and
more consistent from one person to the next). A list of the names
and symbols of all of the known elements is provided just inside the
front cover of the textbook.
It is important
to note that some of the elements are represented by 2-letter symbols
(e.g., He). In these cases, only the first letter of the symbol
is capitalized. This allows one to distinguish between things
such as Co (the element cobalt) versus CO (a compound consisting of the
two elements carbon and oxygen). Also, note that some elements
have symbols that don't seem to correspond to their names (such as Na
for sodium). This is because these elements have been known for
quite a long time, and the symbol usually corresponds to an earlier
name in another language, often Latin or Greek (for example, sodium is
still known in many countries by its earlier Latin name, natrium).
The term element can be used in two
different ways, which can at times be confusing. The term can be
used to describe the type of atom (e.g., water is a compound composed of the elements hydrogen and oxygen).
However, it can also be used to describe a substance that is composed
of only one type of atom (e.g., air
is a mixture of the elements nitrogen and oxygen). These
two statements imply very different things from an atomic perspective,
as illustrated below.
As we can see,
water is a single pure substance--it
consists of only one type of molecule. Each of these molecules
consists of 2 hydrogen atoms and 1 oxygen atom. Because water is
composed of more than one type of atom, it is called a compound. Air, on the other
hand, is a mixture--it
consists of two different types of molecules (i.e., two different
substances). Each substance (each molecule) is an element, because it consists of
only one type of atom. Note that nitrogen and oxygen both exist
as what we call diatomic
(meaning 2 atoms) molecules. Other diatomic elements are
hydrogen, fluorine, chlorine, bromine, and iodine.
One important implication of this is that if the compound water is
split into its two elements hydrogen (H2) and oxygen (O2),
it always results in twice as much hydrogen as oxygen. (Note, by
the way, that this is a chemical change.) If, on the other hand,
air is separated into its constituent substances nitrogen (N2)
and oxygen (O2), we would usually get about 21% oxygen, but
we could get less (for example, if the air is from someone's exhaled
breath) or more (for example, if the air is from an 'oxygen
bar'). There is no specific ratio of nitrogen to oxygen required
for a mixture of these two gases. The same is true for a mixture
of salt and water (or for any mixture).
Chemists use a system of symbolic notation that allows them to
communicate about the chemical identity and behavior of matter in a
very efficient manner. In addition to incorporating the elemental
symbols discussed earlier, the system also makes use of numbers in two
different ways, as indicated below.
Subscripts are used to indicate the number of each type of atom in a
molecule. For example, hydrogen (H2) molecules consist
of 2 hydrogen atoms. Water (H2O) molecules each
consist of 2 hydrogen atoms and 1 oxygen atom (note that if no
subscript is written, a 1 is implied).
Coefficients, which are written in front of the chemical formula of a
substance, are used to indicate the number of molecules. For
example, 3 H2O indicates the presence of 3 water
molecules. Note that to determine the number of each type of atom
in the collection of molecules, one need only multiply the coefficient
and the subscript. In other words, in 3 H2O there are 3 x 2 = 6
hydrogen atoms, and 3 x 1 = 3 oxygen atoms.