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Molecules and compounds
The atoms of most elements interact with one another. If the interaction
is a sharing of electrons the interacting atoms are called a molecule.
Molecules are two or more atoms held together by shared electrons
and can be made up of atoms of the same element or atoms of different
elements. Molecules made up of atoms of different elements are called
compounds. (Compounds are also atoms of different elements held together
by electrostatic attraction [ionic bonds]).
The oxygen in the air consists of molecules made up of pairs of atoms
of the same element-oxygen. These pairs are represented by the chemical
formula O2. A chemical formula is a type of "shorthand"
used to describe a molecule. The atoms are represented by symbols,
such as O for oxygen.
A subscript shows the number of these atoms present in the molecule.
An example of a compound with which you are all familiar is water,
H2O. In this chemical formula, the symbol H stands for
the element hydrogen. The chemical formula H2O shows that
water is made up of two atoms of hydrogen and one atom of oxygen.
A space-filling model of one molecule of water is shown to the left.
Three factors influence whether an
atom will interact with other atoms. In addition, these factors influence
the type of interactions likely to take place. These factors are (1)
the tendency of electrons to occur in pairs, (2) the tendency of atoms
to balance positive and negative charges, and (3) the tendency of
the outer shell, or energy level, of electrons to be full. This third
factor is often called the octet rule.
The word octet means "eight objects"
and refers to the fact that eight electrons is a stable number for
an outer shell; the outer electron shell of many atoms contains a
maximum of eight electrons. (The first energy level is an exception
to this rule because it contains a maximum of two electrons.) The
octet rule states that an atom with an unfilled outer shell has a
tendency to interact with another atom or atoms in ways that will
complete this outer shell. Although the octet rule does not apply
to all atoms, it does apply to all biologically important ones-those
involved in the structure, energy needs, and information systems of
living things. These atoms and the molecules they make up are discussed
later in this chapter.
Atoms of elements that have equal numbers
of protons and electrons and have full outer-electron energy levels
are the only ones that exist as single atoms. Atoms with these characteristics
are called noble gases, or inert gases, because they
do not react readily with other elements. Many stories explain why
they are called noble, but they all center around the concept of nobility-those
who have everything (in this case a full outer shell), need nothing,
and interact little with others. Most of the noble gases-helium, neon,
argon, krypton, xenon, and radon-are rare. In addition, they are relatively
unreactive and unimportant in living systems. Helium and radon are
probably the best known and the least rare noble gases. Radon, in
fact, has gained more recognition in recent years. Formed in rock
or soil particles from the radioactive decay of radium, radon gas
can seep through cracks in basement walls and remain trapped in homes
that are not well ventilated. Prolonged exposure to radioactive radon
in levels greater than those normally found in the atmosphere is thought
to lead to lung cancer. Figure 2 shows the areas of the United States
in which radon gas is most prevalent.
Figure 2 Areas of the United States in which radon
gas is most prevalent. Note: A picocurie is a unit used to describe
the rate of disinitegration of the nucleus of a radioactive element.
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