Metals: Weak Covalent Bonding (Metallic Bonding)

Let us first consider the example of sodium (Na). Na has one valence electron, which is in the 3s orbital. When two Na atoms bond to form a gaseous Na₂ molecule, the two valence electrons (one from each Na atom) are found primarily between the two Na nuclei. To make a crystalline solid, many atoms are packed together in a regular pattern. In metal solids (Figure 2a), the atoms adopt a "closest packed" configuration, in which the atoms are equally spaced, and space between the atoms is minimized. Electrons in diffuse orbitals are not tightly constrained to a small space, and hence the interactions between one Na atom and an adjacent Na atom, or "nearest neighbor," are weak. At the same time, each atom in a metallic solid has many (up to 12) "nearest neighbors," i.e., each atom interacts with many other atoms. Thus, although the individual interactions between atoms are weak, there are many interactions, and the aggregate effect is a well-bonded metallic solid. (To illustrate this point, think of how a woven cloth is held together. Although the individual threads may be quite weak, when many threads are woven together, they form a strong cloth.)

The large number of nearest neighbors for metal atoms in a solid effectively causes the atoms to be surrounded in all directions by other atoms' valence-electron orbitals. Recall that when atoms are packed in a solid and interact, their valence orbitals overlap. Thus, in a metallic solid such as sodium, each atom's valence orbital can overlap with many other valence orbitals, in virtually all directions. Each Na atom is affected (perturbed) by its many neighbors and therefore, the valence atomic orbitals of all of the Na atoms "mix" to form an almost continuous band of orbitals that are very close in energy. Because these valence atomic orbitals have lost their individual identity in this aggregate of Na atoms, the band formed is called the valence band (Figure 3). (You may find it helpful to think about what happens to the individual identities of voters in an election. Going into the voting booth, each voter decides for himself or herself how to vote, just as a solitary atom has its own valence orbitals with a certain number of electrons. However, when the results are tallied, the individual identities of the voters are lost; the voters are simply divided into those who voted for one candidate and those who voted for the other candidate. Similarly, when many atoms bond together, the individual identities of the orbitals are lost; they form continuous bands that are divided into the filled and unfilled orbitals.)

The low-electronegative metal atoms "give up" their valence electrons, allowing them to be found throughout the "mixed" orbitals of the valence band. Hence, the band of orbitals is filled to a certain energy according to the number of valence electrons provided by all of the Na atoms in the solid. Because electrons are shared among atoms in these "mixed" orbitals, they form covalent bonds between the atoms in the solid. Each atom shares electrons with all its many neighbors in all directions, so these bonds are weak covalent bonds. Because the orbitals are large and diffuse, the bonds they form do not have significant directional preferences. Another term frequently used to describe this type of bonding is "metallic bonding," because solids with this type of bonding exhibit metallic properties and are therefore categorized as metals. As we shall see later in the tutorial, elements with metallic bonding are very good conductors of electricity, because of the nondirectionality of the orbitals (i.e., electrons can easily move in any direction). Metallic elements are not used in LEDs, although they are important components of the circuits used to power LEDs.

Figure 3 When metal atoms interact together to form a solid, their atomic orbitals mix to form a continuous band of orbitals. In the representation of 1 Na atom (left-hand side of diagram), the blue dot represents an electron in the 3s orbital; in the representation of many Na atoms (center of diagram), the blue dots again represent electrons in the "mixed" orbitals that are formed when many atoms interact. This "mixing" of orbitals is better represented by a "band" of orbitals (right-hand side of diagram), in which the blue area represents orbitals that are filled with electrons.