a.
Sod iu m :
1
valence electron
Ionic bonds • Figure 2.4
When sodium and chlorine atoms interact,
sodium gives up a valence electron to become
a stable sodium ion with 8 electrons in its outer
shell. Chlorine takes an electron to become a
chloride ion with 8 electrons in its outer shell.
Na
Na
donated
Atom
b. Ch lorin e:
7
valence electrons
O 0
0 0
o o
0 0
0 0
Electron
accepted
0 0
0 0
0 o
0 0
0 0
Atom
Ion
Both ions are electrically attracted by
an ionic bond and become a new ionic
compound called sodium chloride.
c.
Ionic bond in sodium chloride (NaCl)
0 0
O 0
O 0
O 0
0
0
0
0
Cl
0 0
O 0
0
0
0 0
d. Packing of ions in a
crystal of sodium chloride
Na+
Cl -
What aspect of the two ions makes
them stable after the ionic bond has formed?
Ions and ionic bonds
If an atom either
gives up
or
gains
electrons to conform to the octet rule, it becomes
an
io n
, an atom that has a positive or negative charge
due to having unequal numbers of protons and electrons.
An ion of an atom is symbolized by writing its chemical
symbol followed by the number of positive ( + ) or nega-
tive (-) charges. For example, Ca2+ stands for a calcium
ion that has two positive charges because it has given up
two electrons.
Let’s look at the ionic reaction between sodium, a
highly reactive metal, and chlorine, a poisonous gas (
F ig -
ure 2 .4 a
). When sodium gives up its valence electron to
chlorine, elemental sodium becomes a sodium ion (Na+)
that has eight electrons in its outer shell. When chlorine
accepts the electron from sodium, it becomes a chloride
ion (Cl-), also with eight electrons in its outer shell.
These two stable ions are electrically attracted (due to
the attraction between positively and negatively charged
ions). This electrical attraction holds the ions together
and is called an
io n ic b o n d
. The new ionic compound,
sodium chloride, is chemically different from either so-
dium or chlorine; it is what we know as common table
salt (
Figu re 2 .4 b
). Typically, ionic compounds dissolve
in water. Both positively and negatively charged ions re-
main as ions, but become attracted to water molecules
in solution.
Positively charged ions are called
cations
because they
are attracted to the negatively charged electrodes of elec-
trical sources called
cathodes
. In contrast, negatively
charged ions are called
anions
because they are attracted
to positively charged electrodes of electrical sources called
anodes
. So, Na+ is a cation, and Cl- is an anion.
The Building Blocks of Matter Fit Together to Make Ions and Compounds
27
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