The ionic model, the description of bonding in terms of ions, is particularly appropriate
for describing binary compounds formed from a metallic element, especially
an s-block metal, and a nonmetallic element. An ionic solid is an assembly of cations
and anions stacked together in a regular array. In sodium chloride, sodium
ions alternate with chloride ions, and large numbers of oppositely charged ions are
lined up in all three dimensions (FIG. 3.1). Ionic solids are examples of crystalline
solids, or solids that consist of atoms, molecules, or ions stacked together in a
regular pattern. We explore these patterns in Chapter 6; here we concentrate on
the changes in the valence electrons that accompany the formation of ions and the
energetics of the formation of ionic solids.
3.1 The Ions That Elements Form
When an atom of a metallic element in the s-block forms a cation, it loses electrons
down to its noble-gas core (FIG. 3.2). In general, that core has an ns2np6 outer
electron configuration, which is called an octet of electrons. For example, a sodium
atom ([Ne]3s1) loses its 3s-electron to form Na_, which has the same electron
configuration as a neon atom, [Ne] or 1s22s22p6. The Na_ ions cannot lose more
electrons in a chemical reaction, because the ionization energies of core electrons
are too high. There are three main exceptions to octet formation. Hydrogen loses
its only electron to form a bare proton. Lithium ([He]2s1) and beryllium ([He]2s2)
atoms lose their 2s-electrons, leaving a heliumlike duplet, a pair of electrons with
the configuration 1s2, when they become Li_ and Be2_ ions. Some typical electron
configurations of atoms and the ions they form are shown in TABLE 3.1.
When the atoms of metals on the left of the p-block in Periods 2 and 3 lose
their valence electrons, they form ions with the electron configuration of the preceding
noble gas. Aluminum, [Ne]3s23p1, for instance, forms Al3_ with the same
configuration as neon. However, when the metallic p-block elements in Period 4
and later periods lose their s- and p-electrons, they leave a noble-gas core surrounded
by an additional, complete subshell of d-electrons. For instance, gallium
forms the ion Ga3_ with the configuration [Ar]3d10. The d-electrons of the p-block
atoms are gripped tightly by the nucleus and, in most cases, cannot be lost.
In the d-block, when occupied by electrons, the (n _ 1)d-orbitals lie below the
ns-orbitals in energy. Therefore, the ns-electrons are lost first, followed by a variable
number of (n _ 1)d-electrons. For example, to obtain the configuration of the
Fe3_ ion, we start from the configuration of the Fe atom, which is [Ar]3d64s2, and
remove three electrons from it. The first two electrons removed are 4s-electrons.
The third electron comes from the 3d-subshell, giving [Ar]3d5.
Many metallic elements, such as those in the p- and d-blocks, have atoms that
can lose a variable number of electrons. As we saw in Section 2.12, the inert-pair
ions, has a lower energy than a collection of widely separated sodium and chlorine
atoms. If the lowest energy can be achieved by sharing electrons, then the atoms
link through a covalent bond and discrete molecules are formed. Hydrogen and
nitrogen atoms bond together as molecules of ammonia, NH3, for example,
because a gas consisting of NH3 molecules has a lower energy than a gas consisting
of the same number of widely separated nitrogen and hydrogen atoms. A third type
of bond is the metallic bond, in which large numbers of cations are held together
by a sea of electrons. For example, a piece of copper consists of a stack of copper
ions held together by a sea of electrons, each of which comes from one of the
atoms in the sample (recall Fig. 2.27). We consider the metallic bond in more detail
in Chapters 6 and 7. In this chapter we examine ionic and covalent bonds.
The changes in energy responsible for the formation of bonds occur when the
valence electrons of atoms, the electrons in the outermost shells, move to new
locations. Therefore, bond formation depends on the electronic structures of
atoms discussed in Chapters 1 and 2.