Chapter 2
Chemistry Comes Alive
31
2
eight electrons? Te number of electrons that can participate in
bonding is still limited to a total of eight. Te term
valence shell
(va
9
lens) specifically indicates an atom’s outermost energy level
or that portion of it
containing the electrons that are chemically
reactive. Hence, the key to chemical reactivity is the
octet rule
(ok-tet
9
), or
rule of eights
. Except for shell 1, which is full when
it has two electrons, atoms tend to interact in such a way that
they have eight electrons in their valence shell.
Types of Chemical Bonds
Tree major types of chemical bonds—
ionic
,
covalent
, and
hy-
drogen bonds
—result from attractive forces between atoms.
As noted earlier, when atoms combine with other atoms, they
are held together by
chemical bonds
. A chemical bond is not
a physical structure like a pair of handcuffs linking two people
together. Instead, it is an energy relationship between the elec-
trons of the reacting atoms, and it is made or broken in less than
a trillionth of a second.
The Role of Electrons in Chemical Bonding
Electrons forming the electron cloud around the nucleus of an
atom occupy regions of space called
electron shells
that consecu-
tively surround the atomic nucleus. Te atoms known so far can
have electrons in seven shells (numbered 1 to 7 from the nucleus
outward), but the actual number of electron shells occupied in a
given atom depends on the number of electrons that atom has.
Each electron shell contains one or more orbitals. (Recall from
our earlier discussion that
orbitals
are regions around the nucleus
in which a given electron is likely to be found most of the time.)
It is important to understand that each electron shell rep-
resents a different
energy level
, because this prompts you to
think of electrons as particles with a certain amount of potential
energy. In general, the terms
electron shell
and
energy level
are
used interchangeably.
How much potential energy does an electron have? Te an-
swer depends on the energy level that it occupies. Te attraction
between the positively charged nucleus and negatively charged
electrons is greatest when electrons are closest to the nucleus
and falls off with increasing distance. Tis statement explains
why electrons farthest from the nucleus (1) have the greatest
potential energy (it takes more energy for them to overcome
the nuclear attraction and reach the more distant energy levels)
and (2) are most likely to interact chemically with other atoms.
(Tey are the least tightly held by their own atomic nucleus and
the most easily influenced by other atoms and molecules.)
Each electron shell can hold a specific number of electrons.
Shell 1, the shell immediately surrounding the nucleus, accom-
modates only 2 electrons. Shell 2 holds a maximum of 8, and
shell 3 has room for 18. Subsequent shells hold larger and larger
numbers of electrons, and the shells tend to be filled with elec-
trons consecutively. For example, shell 1 fills completely before
any electrons appear in shell 2.
Which electrons are involved in chemical bonding? When
we consider bonding behavior, the only electrons that are im-
portant are those in the atom’s outermost energy level. Inner
electrons usually do not take part in bonding because they are
more tightly held by the atomic nucleus.
When the outermost energy level of an atom is filled to
capacity or contains eight electrons, the atom is stable. Such
atoms are
chemically inert
, that is, unreactive. A group of ele-
ments called the
noble gases
, which include helium and neon,
typify this condition
(Figure 2.5a)
. On the other hand, atoms
in which the outermost energy level contains fewer than eight
electrons tend to gain, lose, or share electrons with other atoms
to achieve stability (Figure 2.5b).
What about atoms that have more than 20 electrons, in
which the energy levels beyond shell 2 can contain
more
than
Helium (He)
(2p
+
; 2n
0
; 2e
)
Neon (Ne)
(10p
+
; 10n
0
; 10e
)
Ne
2e
2e
8e
2e
4e
2e
8e
1e
(b) Chemically reactive elements
Outermost energy level (valence shell) incomplete
Hydrogen (H)
(1p
+
; 0n
0
; 1e
)
Carbon (C)
(6p
+
; 6n
0
; 6e
)
C
O
H
1e
Oxygen (O)
(8p
+
; 8n
0
; 8e
)
Sodium (Na)
(11p
+
; 12n
0
; 11e
)
Na
2e
6e
(a)
Chemically inert elements
Outermost energy level (valence shell) complete
He
Figure 2.5
Chemically inert and reactive elements.
(
Note
: For
simplicity, each atomic nucleus is shown as a sphere with the atom's
symbol; individual protons and neutrons are not shown.)
previous page 65 Human Anatomy and Physiology (9th ed ) 2012 read online next page 67 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off