DEVELOPING AND DESIGNING NEW CHEMICALS AND MATERIALS - Chemistry: Notable Research and Discoveries - page 3

pounds.) For example, two atoms of hydrogen (H) and one atom of
oxygen (O) react to form a molecule of the compound known as wa-
ter, written chemically as H 2 O. An element's chemical properties—the
chemical reactions it enters and the products formed from these reac-
tions—depend on the properties of the atom.
Mendeleyev noticed that when he listed the chemical elements known
at the time in order of atomic weight—the mass of the atom—elements
with similar properties appeared periodically. For instance, lithium,
sodium, potassium, rubidium, and cesium are all soft metals that tend
to engage in the same chemical reactions. Toss a pellet of pure sodium
into water and it will strongly react with water, producing hydrogen gas.
The same is true for the other elements of this group. When elements
are arranged in a table, columns form groups that have similar chemical
properties.
Mendeleyev observed some gaps in this early table. Rather than dis-
missing this observation as unimportant, Mendeleyev hypothesized that
the gaps represented as yet undiscovered elements. Using the groups of the
periodic table, he could predict some of the properties for these unknown
elements. For example, in 1871, Mendeleyev predicted the existence of an
element below silicon, which he called ekasilicon, and some of its proper-
ties, such as mass, density, and the nature of the compound it would pro-
duce when combined with oxygen. The German chemist Clemens Win-
kler (1838-1904) found this element in 1886. Germanium, as it was called
(for Germany), has many of the properties Mendeleyev predicted.
Later versions of the periodic table ordered the elements by atomic
number, a concept that had not been formulated in Mendeleyev's time.
The atomic number is the number of protons in the nucleus of an atom.
This number specifies each of the elements—for instance, oxygen has
an atomic number of eight, and all atoms of oxygen have eight protons
(though oxygen atoms may vary in the number of neutrons in the nucle-
us). As of May 2009, researchers have identified a total of 117 elements,
though many of the larger elements are highly unstable—chemists make
them during special experiments in the laboratory, but the atoms decay
quickly.
The periodic table of elements gives order to the bewildering variety
of substances found on Earth. People have been working with various ma-
terials for a long time, but the development of new substances was a slow
and fortuitous process. About 30,000 years ago, the first ceramic appeared,