DEVELOPING AND DESIGNING NEW CHEMICALS AND MATERIALS - Chemistry: Notable Research and Discoveries - page 6

arrangements of electrons. In a simplified description of an atom, elec-
trons orbit around the nucleus, as illustrated in the figure. (Neutral at-
oms have the same number of electrons as protons.) These orbits are
grouped in shells. As a general rule, many atoms are stable when they
have eight electrons in their outer shell. This “octet rule” is true for gases
such as neon, argon, and xenon, which rarely react with other elements.
Other atoms engage in chemical reactions that tend to satisfy this octet
rule; for example, oxygen, with six electrons in its outer shell, acquires
at least a share of two more.
Strong bonds form when atoms transfer one or more electrons, in
which case the bond is called an ionic bond, and when atoms share one or
more electrons, in which case the bond is known as a covalent bond. he
bond between each hydrogen atom and oxygen in H 2 O is covalent—the
oxygen atom shares an electron with each hydrogen atom. Oxygen has six
electrons in its outer shell and hydrogen has one, so the result is that the
oxygen atom obtains eight electrons in its outer shell (the usual six plus a
share in two more) and the hydrogen atoms have two apiece. (Hydrogen
is the smallest atom and is satisfied with only two electrons, as is helium,
which also has two.) A simple example of an ionic bond occurs when so-
dium reacts with chlorine to form sodium chloride, also known as table
salt. In this case, sodium donates the single electron in its outer shell to
chlorine, which needs only one electron to fill its outer shell.
Another bond, similar to covalent bonds, occurs in metals, in which
atoms share electrons with many of their neighbors. Other kinds of
bonds are weaker than covalent, ionic, and metallic bonds, and often in-
volve subtle interactions based on electric attraction or repulsion of the
charged components of molecules. For instance, proteins are large mol-
ecules that perform a variety of functions in biological tissues and are
formed by covalent bonding of a sequence of smaller molecules known
as amino acids. The sequence of amino acids folds into a certain shape,
which governs the protein's function. Keeping this shape intact is the
job of many weak interactions among the hundreds or even thousands
of atoms in the protein.
The structure formed by these bonded atoms is critical for many
compounds. Diamond and graphite are two substances with vastly dif-
ferent properties—diamond is one of the hardest substances known,
and graphite is soft and often used as a lubricant or as a pencil “lead”