3.13 Correcting the Ionic Model: Polarizability
All ionic bonds have some covalent character. To see how covalent character can
arise, consider a monatomic anion (such as Cl) next to a cation (such as Na). As
the cation’s positive charge pulls on the anion’s electrons, the spherical electron
cloud of the anion becomes distorted in the direction of the cation. We can think
of this distortion as the tendency of the electron density of the bond to occupy
preferentially the region between the two nuclei, resulting in a bond that acts like
a covalent bond (FIG. 3.14). Ionic bonds acquire more covalent character as the
distortion of the electron cloud on the anion increases.
Atoms and ions with electron clouds that readily undergo a large distortion are
said to be highly polarizable. An anion can be expected to be highly polarizable if it
is large, such as an iodide ion, I. In such a large anion, the nucleus exerts only weak
control over its outermost electrons because they are so far away. As a result, the
electron cloud of the large anion is easily distorted and the ion is highly polarizable.
Cations, which have fewer electrons than their parent atoms, are not significantly
polarizable because the remaining electrons are held so tightly.
Atoms and ions that can cause large distortions are said to have high polarizing
power. A cation can be expected to have high polarizing power if it is small and highly
charged, such as an Al3 cation. A small radius means that the center of charge of a
highly charged cation can get very close to the anion, where it can exert a strong pull on the anion’s electrons. Compounds composed of a small, highly charged cation and
a large, polarizable anion tend to have bonds with considerable covalent character.
Cations become smaller, more highly charged, and hence more strongly polarizing,
from left to right across a period. Thus, Be2 is more strongly polarizing than Li,
and Mg2 is more strongly polarizing than Na. On the other hand, cations become
larger and hence less strongly polarizing down a group. Thus, Na is less strongly
polarizing than Li, and Mg2 is less strongly polarizing than Be2. Now we can see
that, because polarizing power increases from Li to Be2 but decreases from Be2 to
Mg2, it follows that the polarizing power of the diagonal neighbors Li and Mg2
should be similar. We can expect such similarities in the properties of other diagonally
related neighbors. These diagonal similarities underlie the diagonal relationships in
the periodic table introduced in Section 2.13 and illustrated in Fig. 2.32.