32
UNIT 1
Organization of the Body
2
Ionic Bonds
Recall that atoms are electrically neutral. However, electrons
can be transferred from one atom to another, and when this
happens, the precise balance of
1
and
2
charges is lost so
that charged particles called
ions
are formed. An
ionic bond
(i-on
9
ik) is a chemical bond between atoms formed by the
transfer of one or more electrons from one atom to the other.
Te atom that gains one or more electrons is the
electron ac-
ceptor
. It acquires a net negative charge and is called an
anion
(an
9
i-on). Te atom that loses electrons is the
electron donor
. It
acquires a net positive charge and is called a
cation
(kat
9
i-on).
(±o remember this term, think of the “t” in “cation” as a
1
sign.)
Both anions and cations are formed whenever electron transfer
between atoms occurs. Because opposite charges attract, these
ions tend to stay close together, resulting in an ionic bond.
One example of ionic bonding is the formation of table salt,
or sodium chloride (NaCl), by interaction of sodium and chlo-
rine atoms
(Figure 2.6)
. Sodium, with an atomic number of 11,
has only one electron in its valence shell. It would be very dif-
ficult to attempt to fill this shell by adding seven more. However,
if this single electron is lost, shell 2 with eight electrons becomes
the valence shell (outermost energy level containing electrons)
and is full. Tus, by losing the lone electron in its third energy
Sodium atom (Na)
(11p
+
; 12n
0
; 11e
)
Chlorine atom (Cl)
(17p
+
; 18n
0
; 17e
)
Sodium ion (Na
+
)
Chloride ion (Cl
)
Sodium chloride (NaCl)
(a) Sodium gains stability by losing one electron, and
chlorine becomes stable by gaining one electron.
CI
Na
+
(b) After electron transfer, the oppositely
charged ions formed attract each other.
+
Cl
Na
Cl
Na
(c) Large numbers of Na
+
and Cl
ions
associate to form salt (NaCl) crystals.
Figure 2.6
Formation of an ionic bond.
level, sodium achieves stability and becomes a cation (Na
1
). On
the other hand, chlorine, atomic number 17, needs only one
electron to fill its valence shell. By accepting an electron, chlo-
rine achieves stability and becomes an anion.
When sodium and chlorine atoms interact, this is exactly what
happens. Sodium donates an electron to chlorine (Figure 2.6a),
and the oppositely charged ions created in this exchange attract
each other, forming sodium chloride (Figure 2.6b). Ionic bonds
are commonly formed between atoms with one or two valence
shell electrons (the metallic elements, such as sodium, calcium,
and potassium) and atoms with seven valence shell electrons
(such as chlorine, fluorine, and iodine).
Most ionic compounds fall in the chemical category called
salts
. In the dry state, salts such as sodium chloride do not exist
as individual molecules. Instead, they form
crystals
, large arrays
of cations and anions held together by ionic bonds (Figure 2.6c).
Sodium chloride is an excellent example of the difference in
properties between a compound and its constituent atoms. So-
dium is a silvery white metal, and chlorine in its molecular state
is a poisonous green gas used to make bleach. However, sodium
chloride is a white crystalline solid that we sprinkle on our food.
Covalent Bonds
Electrons do not have to be completely transferred for atoms to
achieve stability. Instead, they may be
shared
so that each atom
is able to fill its outer electron shell at least part of the time. Elec-
tron sharing produces molecules in which the shared electrons
occupy a single orbital common to both atoms, which consti-
tutes a
covalent bond
(ko-va
9
lent).
Hydrogen with its single electron can fill its only shell (shell
1) by sharing a pair of electrons with another atom. When it
shares with another hydrogen atom, a molecule of hydrogen gas
is formed. Te shared electron pair orbits around the molecule
as a whole, satisfying the stability needs of each atom.
Hydrogen can also share an electron pair with different kinds
of atoms to form a compound
(Figure 2.7a)
. Carbon has four
electrons in its outermost shell, but needs eight to achieve stabil-
ity. Hydrogen has one electron, but needs two. When a methane
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