Chapter 4 The Ionic Bond
4.1IONIC BONDING The valence electrons in metals are at relativ
ely high energy, so they have low ionization
energies and lose their valence electrons
readily. The unfilled valence orbitals in
nonmetals are at relatively low energy, so they have high electronegativities and gain electrons easily. When a metal atom comes in
to contact with a nonmetal atom, the valence
electrons of the metal are
transferred
to the low-energy orbitals of the nonmetal to produce
a cation and an anion. The oppositely charged ions are then held together by an electrostatic force called an
ionic bond
, and the compound that forms is an
ionic
compound
. Figure 4.1 considers the case of NaCl. When a sodium atom binds to a
chlorine atom, the high-energy electron on sodium transfers to the low-energy orbital on chlorine. The loss of an electron by Na produces the Na
1+ cation, and the gain of an
electron by Cl produces the Cl
1- anion. In order for a bond to be ionic, the valence orbital
of the cation must be very high in energy
, but only metals have high-energy valence
orbitals, so a good aid in identifying ionic compounds is that
ionic compounds almost
always contain a metal
*.
Some metals, such as Cu, Ag, Au, and Pt ar
e not reactive in their elemental form, but
most metals react with oxygen in the atmosphere and are found as their oxides. The metals of Group 1A are so reactive that they must be
stored in oil to keep them away from oxygen
and water. Sodium is a common element, but it is found in nature only as the Na
1+ ion as
in NaCl. Na metal must be prepared from its
ion. Thus, most reac
tions involving metals,
involve their ions, not the atoms, but ions and atoms are very different. For example, Na is a highly reactive solid, but Na
1+ is an inert ion that cannot be isolated without a
counterion.
† The properties of Na and Na
1+ are more different than are those of Na and K,
which have similar physical properties and almost identical chemical properties! Na and Na
1+ are totally different species, so
it is imperative that the charge on an ion be included
whenever the ion is written alone
. Thus, we write the dissociation of sodium chloride into
its ions as NaCl
→
Na
1+ + Cl
1-.
Example 4.1
Which of the following are ionic? CaCl, SF 2, KCl, CCl 2, Na 4O, F 2O, N 2O 2, FeCl 3(^3)
Ionic substances contain a metal and a nonmetal. Only CaCl
, KCl, Na 2
O, and FeCl 2
(^3)
contain metals, so only they are ionic. As discussed in the Chapter Introduction, bonds between nonmetals are covalent and their compounds are molecular. Thus, SF
, CCl 2
, 4
F^2
O, and N
O 2
are molecular because they contain only nonmetals. 3
Energy
Na
Na
1+
Cl
Cl
1-
Na + Cl
NaCl
Figure 4.1 Electron transfer and ionic bonding Na transfers an electron to Cl to produce Na
1+ and Cl
1- ions.
Coulombic attraction between
the oppositely charged ions
produces NaCl.
† Cations and anions are charged species, but only neutral materials
can be isolated. Therefore, cations must always be accompanied by anions and
vice versa
. That is, each ion must be accompanied by a
counterion. Cl1- is the counterion of Na1+ in NaCl.* We will see later in the chapter that there are also polyatomic cations
that form ionic compounds. The ammonium ion (NH1+ 4
) is the onlysuch ion treated in this text.© byNorthCarolinaStateUniversity