Chemistry Reference
In-Depth Information
element because it is only made up of one type of atom (atoms with 8 pro-
tons). Other common examples of elements include hydrogen (H
2
), carbon
(C), gold (Au), iron (Fe), and copper (Cu).
Have you ever noticed that when oxygen is written in a chemical equa-
tion, say the formula for photosynthesis that you studied in biology, it is
written with a small subscript “2” (O
2
)? The reason for this subscript is that
oxygen is a diatomic element. Most of the elements exist as monatomic forms,
which means that the smallest individual complete parts of these samples
of pure elements are single atoms. Seven of the known elements are called
diatomic elements, because they are found in nature in their elemental form
as two-atom molecules. Examination of a sample of pure chlorine, for ex-
ample, would reveal two-atom molecules of the element. For this reason,
when chlorine (Cl
2
) is represented in a chemical equation as a pure ele-
ment, it is also given the subscript “2.” The seven diatomic elements are
chlorine (Cl
2
), fluorine (F
2
), bromine (Br
2
), iodine (I
2
), hydrogen (H
2
), oxy-
gen (O
2
), and nitrogen(N
2
).
Comparing a Monatomic and a
Diatomic Element
One important characteristic of el-
ements is that they cannot be broken
down by ordinary chemical means.
Chemical reactions can break more
complex substances down into ele-
ments, but elements can only be bro-
ken down further by nuclear reactions.
As of this writing, there are presently
around 110+ known elements. I am
being somewhat vague on purpose
A molecule of
a diatomic
element, such
as O
2
.
An atom of a
monatomic
element, such
as Ca.
Figure 1-1a
here, because the number of known elements changes over time, and some
of the “man-made” elements are awaiting confirmation. A branch of nuclear
chemistry deals with trying to produce atoms of undiscovered elements,
and many of these heavier elements are made to “exist” for fractions of a
second, until they break apart into smaller parts. All of the known ele-
ments, whether natural or man-made, are represented on the Periodic Table
of Elements, where they are organized into similar groups.
Compounds are substances that are made up of two or more elements
chemically combined. The key to this definition is that the elements must
be chemically combined. If you physically mix together two elements, let's
say iron and copper, you get a mixture, which can be physically separated.
Compounds are formed by chemical reactions, where the individual ele-
ments lose their individual properties and take on the new properties of
the compound that is formed.
