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on the left side. The arrow means yields. The new chemical sub- explained by the making and breaking of chemical bonds.
stances are on the right side of the word equation and are called Chemical bonds can be explained in terms of changes in the
products. Reading the photosynthesis reaction as a sentence, electron structures of atoms. Thus, the place to start in seeking
you would say, “Carbon dioxide and water use energy to react, understanding about chemical reactions is the electron struc-
yielding plant materials and oxygen.” ture of the atoms themselves.
The plant materials produced by the reaction have more in-
ternal potential energy, also known as chemical energy, than the
reactants. You know this from the equation because the term 9.2 VALENCE ELECTRONS AND IONS
energy appears on the left side but not the right. This means that
the energy on the left went into internal potential energy on the As discussed in chapter 8, it is the number of electrons in the
right. You also know this because the reaction can be reversed outermost orbital that usually determines the chemical prop-
to release the stored energy (Figure 9.4). When plant materials erties of an atom. These outer electrons are called valence
(such as wood) are burned, the materials react with oxygen, and electrons, and it is the valence electrons that participate in
chemical energy is released in the form of radiant energy (light) chemical bonding. The inner electrons are in stable, fully oc-
and high kinetic energy of the newly formed gases and vapors. cupied orbitals and do not participate in chemical bonds. The
In words, representative elements (the A-group families) have valence
electrons in the outer most orbitals, which contain from one
plant oxygen carbon water
to eight valence electrons. Recall that you can easily find the
material + molecules → dioxide + molecules + energy
number of valence electrons by referring to a periodic table.
molecules molecules
The number at the top of each representative family is the
If you compare the two equations, you will see that burn- same as the number of outer orbital electrons (with the excep-
ing is the opposite of the process of photosynthesis! The en- tion of helium).
ergy released in burning is exactly the same amount of solar The noble gases have filled outer orbitals and do not
energy that was stored as internal potential energy by the plant. normally form compounds. Apparently, half-filled and filled
Such chemical changes, in which chemical energy is stored in orbitals are particularly stable arrangements. Atoms have a
one reaction and released by another reaction, are the result tendency to seek such a stable, filled outer orbital arrangement
of the making, then the breaking, of chemical bonds. Chemi- such as the one found in the noble gases. For the representa-
cal bonds were formed by utilizing energy to produce new tive elements, this tendency is called the octet rule. The octet
chemical substances. Energy was released when these bonds rule states that atoms attempt to acquire an outer orbital with
were broken, then reformed to produce the original substances. eight electrons through chemical reactions. This rule is a gen-
In this example, chemical reactions and energy flow can be eralization, and a few elements do not meet the requirement of
eight electrons but do seek the same general trend of stability.
There are a few other exceptions, and the octet rule should be
considered a generalization that helps keep track of the valence
electrons in most representative elements.
The family number of the representative element in the
+ + +
periodic table tells you the number of valence electrons and
what the atom must do to reach the stability suggested by the
Water Carbon Solar Green Oxygen
(H 2 O) dioxide energy plant (O 2 ) octet rule. For example, consider sodium (Na). Sodium is in
(CO 2 ) family IA, so it has one valence electron. If the sodium atom
can get rid of this outer valence electron through a chemical
A
reaction, it will have the same outer electron configuration
as an atom of the noble gas neon (Ne) (compare Figures 9.5B
and 9.5C).
When a sodium atom (Na) loses an electron to form a
+
+ sodium ion (Na ), it has the same, stable outer electron con-
+
figuration as a neon atom (Ne). The sodium ion (Na ) is still a
Plant Oxygen Water + Energy + Carbon
materials (O 2 ) (H 2 O) dioxide form of sodium since it still has 11 protons. But it is now a so-
(CO 2 ) dium ion, not a sodium atom, since it has 11 protons (11 posi-
B tive charges) and now has 10 electrons (10 negative charges) for
a total of
FIGURE 9.4 (A) New chemical bonds are formed as a green
plant makes new materials and stores solar energy through the 11 + (protons)
photosynthesis process. (B) The chemical bonds are later broken, 10 – (electrons)
and the same amount of energy and the same original materials 1 + (net charge on sodium ion)
are released. The same energy and the same materials are released +
rapidly when the plant materials burn, and they are released slowly This charge is shown on the chemical symbol of Na for the
when the plant decomposes. sodium ion. Note that the sodium nucleus and the inner orbitals

232 CHAPTER 9 Chemical Bonds 9-4