Chemical bonds do more than hold the atoms of a molecule together. They also hold the molecules together in liquids and solids. But before chemical bonds can be understood, it is necessary to have a clear understanding of the electron—the atom’s loosest, most mobile part.
The electron swarm
Electrons are negatively charged particles that swarm around the tight, compact nucleus of an atom like bees around a hive. Big, heavy atoms with a big, heavy nucleus are surrounded by a large swarm of electrons, while hydrogen, the lightest and simplest of elements with the smallest nucleus—a single proton—has just a single electron traveling around it. The negative charge of the electron balances the positive charge of the proton, leaving the hydrogen atom electrically neutral.
Amadeo Avogadro proposed the diatomic nature of hydrogen—that the element occurs as H2 molecules rather than as El atoms. But he didn’t know the reason for this. Neither Avogadro nor any other scientist of his time knew that the atom consists of a nucleus surrounded by a swarm of electrons. There is a kind of order in the way that the electron swarm is arranged around the nucleus.
The architecture of the electron swarm is both structural and quantitative—it depends on how the electrons are arranged, as well as on their numbers. In a simple model of the atom, the electron swarm is portrayed as being layered in shells, or concentric rings, like an onion. The inner ring has room for no more than two electrons. The other rings have room for a maximum of eight electrons. When the rings hold their maximum number—when the outer rings are full—the atoms of an element have the kind of stability that most elements try to attain. The so-called noble gases—helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn)—occur as single atoms with two electrons in the first ring, as in helium; those with a second ring have eight electrons in it, and those with a third ring also have eight in it. This makes these six elements inert, or chemically inactive—their atoms show little or no inclination to combine with others because they are stable by nature. But it’s a very different story with all the other elements.
Elydrogen, the lightest and simplest of all elements, has just a single electron in its first (and only) ring, it needs another electron to gain a measure of stability, so it teams up with a neighboring hydrogen atom. The two atoms share the two electrons, giving each atom the sense that it has its own two electrons. This sharing of two electrons by two atoms is what is called a covalent bond. A sequence of covalent bonds allow carbon atoms to link together to form carbon molecules.
Carbon has six electrons—four in its outer ring, two in its inner ring. To get the four it needs for a full outer ring, it hooks up with other needy carbon atoms and they share their outer electrons through covalent bonds to produce a total of eight around each carbon atom. The eight electrons give each bonded carbon atom the sense that its outer shell has a full complement. A whole network of carbon atoms can be linked covalently. In diamonds, the covalent bonds between carbon atoms are so strong and arranged so symmetrically in three dimensions that they create the hardest substance known. In graphite, the covalent bonds are arranged so that they are even stronger than diamond in two directions but much weaker in a third—an asymmetric arrangement—which is why graphite is so soft.
The electrons in the outer rings of all the elements are called valence electro ns— electrons whose interactions determine how the atoms of a substance bond together and what kind of chemical bond will form. Valence electrons have two characteristics that account for this: they are the electrons that come closest together when atoms collide (as they must in order to join and form a molecule); and they are farthest from the nucleus of their own atom, and so the least tightly held.
Valence electrons in a covalent bond are shared between atoms. In an ionic bond, an atom gives up valence electrons and another atom receives them. In this case, the electron donor, having lost a negatively charged particle, becomes a positively charged ion, or cation, and the receiver, having gained the negatively charged particle, becomes a negatively charged ion, or anion. The atom that gained the positive charge is said to have been oxidized, while the atom that gained the negative charge is said to have been reduced. These equal but opposite charges hold the ions together in an ionic bond. Salt is a good example of this.
Salt has the chemical name sodium chloride and the formula NaCI. The sodium atom, like hydrogen, has only a single electron in its outer ring, while the chlorine has seven electrons in its outer ring—one vacancy. Both elements—one a solid, the other a gas—are extremely reactive in this condition. Brought together, they readily combine, the solitary electron in the outer ring of the sodium atom being donated to the chlorine atom to fill its electron vacancy (the sodium atom having been oxidized to form a positive ion and the chlorine reduced to form a negative one).
Now, joined by the attraction of their charges, they achieve stability as common table salt.
The stability achieved by these two highly reactive elements does not prevent further change, however. Just drop salt into water and see what happens to the molecules: water easily breaks the bonds between them, as water molecules surround and stabilize each of the sodium and chlorine ions. The high solubility of these ions is one reason that the sea is so salty, though there are 5 elements in nature more abundant than sodium and 10 elements more abundant than chlorine.
The hydrogen bond
Another fascinating force of nature is the hydrogen bond— the major force that keeps all the HzO molecules together in liquids and solids. The water molecule is another example of a covalent compound. Each of the two hydrogen atoms in a water molecule gains a full complement of two electrons in its outer (and only) ring by sharing unpaired electrons of an oxygen atom’s outer ring. The three atoms of this molecule are not arranged in a line but form an angular “V” instead. This geometry allows the eight electrons in the valence shell of oxygen to interfere with each other the least The two electrons that make up the covalent bond between hydrogen and oxygen are attracted a bit more strongly to the oxygen atom, so that each hydrogen atom in a water molecule has a small positive charge and each oxygen atom a slight negative charge. Because of this built-in charge on a water molecule, the hydrogens of one water molecule exert a small pull on the oxygen of other water molecules. This small attraction between the hydrogen of one molecule and the oxygen of another is called a hydrogen bond. The hydrogen bonds among all the molecules make water liquid at room temperature.
Hydrogen bonds are also responsible for a peculiar and unusual characteristic that water displays as it passes from the liquid to the solid state. Instead of becoming denser, as most substances do when making this transition, it actually becomes less dense. This is why solid ice floats on liquid water, instead of sinking. As the water temperature approaches the freezing point—(32° F, 0° C)—the H,0 molecules string themselves out along the lines of their hydrogen bonds to form hollow rings. The result is a porous crystalline structure with many air spaces, making solid water less dense than the liquid form. For most other substances, the solid phase is denser than the liquid phase.
The metallic bond
One more chemical bond remains to be noted—the very important bond responsible for the flow of an electric current. In the type of covalent bond that holds the atoms of a metal like copper together—the metallic bond—valence electrons are free to wander from atom to atom, like a drifting cloud. When an electrical field is applied, the cloud of electrons moves in a definite direction and this movement is called an electric current. The electrons in a metal conduct heat as well as electricity. Apply heat to a metal in one place and the drifting electrons become energized, moving in random directions with increasing speed and generating collisions, and in this way transmitting heat. This unique, characteristic way in which atoms of a metallic element share their valence electrons gives these elements their uniquely “metallic” properties, such as electrical and thermal conductivity, luster; malleability, the capability of being hammered into thin sheets; and ductility, the capability of being drawn out into fine wire.