The Canon (23 page)

Atoms with a moderate degree of empty shell space often end up consorting together, and satisfying each other's closet cravings by swapping their outermost electrons back and forth, back and forth. In that way, they get the sensation of orbital satiety without becoming formally, electrically charged, as they would if they picked up one too many electrons full-time, or if they lost the members of their partially filled outer shell altogether. The shared electrons may sometimes be found closer to the outer shell of one atom, at other times nearer to the cloud around the other, but more often fluttering somewhere in between.

"On the one hand, the two atoms want to come together, because their shared electrons want to feel the effects of both positive nuclei,"
said Roald Hoffmann. "On the other hand, the nuclei don't want to get too close to each other. The compromise distance is the bond length, and it acts as a kind of spring linking the atoms together." Boing, boing, please do as I say, first you come closer, now hop away.

The participants in this covalent Slinky sling may be atoms of the same element. A couple of hydrogen atoms, for example, each with their lone electron in a shell built for two, may pool their particles covalently to form a molecule of H2, while the oxygen we breathe consists mostly of O
₂
, vaporous plumes of covalently twinned oxygen atoms that share not just one but two pairs of electrons per partnership.

Alternatively, a covalent vinculum may clasp together two entirely different elements into a so-called compound. Hydrogen and its only-child electron can be hitched to chlorine, which has seven of its seventeen electrons fluttering around an outer shell suited for eight, to form the familiar chemical, hydrogen chloride, a colorless, suffocating, corrosive gas used in making plastics and many other industrial operations. Nitrogen, with five electrons in its outer shell, and oxygen with six, both have outer orbital capacity for eight electrons and can join forces in a variety of permutations. One nitrogen covalently bonded to one oxygen gets you nitric oxide, NO, a clear, potent gas that is quite toxic in large quantities but that the body exploits judiciously for tasks like relaxing muscles, battling bacteria, sending signals in the brain, and engorging the genitals during sexual arousal. In another magic merging of nitrogen and oxygen, a covalently packaged pair of nitrogen atoms can be induced to fraternize covalently with a unit of oxygen, yielding nitrous oxide, N
₂
O, a sweet-smelling psychoactive gas that makes dentistry almost affable, if never truly laughable. The carbohydrates in our diet are covalently bonded armadas of carbon, hydrogen, and oxygen atoms—carbon and water—the exact proportions and positioning of each element in a given array determining whether the carbohydrate is complex and nutritious or sugary and suspicious.

As a rule, elements are more stable and less chemically reactive when they're in a bonded relationship than when they're out of one, for the same reason that married people are celebrated as society's source of levelheaded bourgeois dependability. When you are married, your coupling capacity is more or less filled, and you are considered "taken." Not for nothing is the emblem of marriage, the wedding ring, a closed circle. Similarly for chemical partners in bondage: their reactive parts are already busy and so are unavailable for other relations.

Molecular marriage does not, however, demand monogamy. Many elements have more than one reactive option, more than one electron
consigned to life in a half-full orbit and thus in a position to conjugate covalently with another atom. Many elements, then, are polygamous by nature, and each has its romantic limit, the maximum number of partners with which it can conjoin simultaneously. That figure is known as the element's valence number, from the Latin word
valentia,
for "power" or "capacity." The closer an element comes to filling all its gaps, the more stable, the less chemically predatory, it becomes. The reason why nitric oxide is such a prickly chemical is that, although its nitrogen and oxygen components are covalently linked, both still have room for more electrons and will readily engage in supplementary affairs or frank acts of larceny. Nitric oxide is particularly deft at stealing electrons from iron atoms at the core of hemoglobin molecules, disrupting hemoglobin's ability to convey oxygen throughout the body.

In the case of nitrous oxide, by comparison, all three of nitrogen's available outer electrons are fully engaged in covalent liaisons and are not open for further chemical dalliances, making laughing gas a reasonably benign compound when used in moderation. Nevertheless, the persistence of reactive prospects on the oxygen end of the coalition means that nitrous oxide also can disturb hemoglobin performance, and if you breathe the gas too long you will suffer gradual oxygen depletion and eventually laugh your last.

Nitrogen on its own is capable of extreme stability. In the absence of any pressure to bond cross-culturally with oxygen, hydrogen, or the like, two nitrogen atoms will readily fulfill each other's every need by sharing all three pairs of their available electrons. This triple-bonded nitrogen duet makes for an exceptionally doughty and unreactive molecule that lasts and lasts, which is why liquid nitrogen is the chemical of choice for long-term storage of such prized biomedical goods as blood, sperm, fertilized embryos, evidence from a crime scene. About 78 percent of our atmosphere consists of triple-bonded nitrogen gas, compared to the 21 percent assigned to oxygen; but while our lungs are designed to extract that oxygen from the air and put it to work in every cell of the body, and we are incapable of living without oxygen for more than a few minutes at a time, the nitrogen we inhale is of no use to us physiologically, and we either exhale it immediately, or excrete it later as waste. The nitrogen we do need, for our cells and our DNA, we obtain from food, where the nitrogen arrives in a form that has been conveniently "fixed" for us, that is, combined with oxygen and hydrogen, by compliant microorganisms in the soil; those microbes had "fixed" the nitrogen from the air and fed it to plants, which in turn fed it to us or to the animals we eat. From wherever on the food chain we can pinch it,
this molecularly domesticated form of nitrogen is essential to our perpetuation, and all of us can be said to have a nitrogen fixation.

Yet what feeds life can seed annihilation. Nitrogen may pair up with elements like oxygen or hydrogen, but when given a chance it will lock itself into nearly inert triple bondage with others of its kind. If you take a compound like nitroglycerin, then, and disturb its chemical structure, the newly liberated nitrogen atoms will quickly form their xenophobic alliances, releasing large amounts of energy in the process—which is why most explosives contain nitrogen.

Chemistry is about molecules, and the word "molecule," like so many scientific terms, has its precise and its casual definitions. Its meticulous meaning is a group of atoms linked together by covalent bonds, by a sharing of pairs of electrons. Yet even chemists sometimes dispense with the formalities and call any sort of chemically bonded substance a molecule, offhandedly referring to molecules of table salt, for instance, or the molecules of magnesium bromide in a bottle of milk of magnesia. In truth, sodium chloride, magnesium bromide, calcium chloride and the like are not molecules but ionic compounds, and though the hero here is still a bond, Sean Connery it is not. The ionic bond that brings us condiments, pebbles, eggshells, Alka-Seltzer, many household cleaning products, and a surprising selection of psychiatric drugs, is stiffer and more strait-laced than a covalent bond, less pliable, more predictable. A brick, a rock, the salt of the earth. An ionic bond is Roger Moore.

In contrast to a covalent bond, which can join together atoms of the same or different elements, an ionic bond can only assimilate the dissimilar. The reason for that is embedded in the term: an ionic bond is a bond between ions, or electrically charged atoms. It is the attraction that a negatively charged atom, laden with one or maybe more electrons than its proton content calls for, feels for a positively charged atom, one that has too few electrons to suit its nuclear desires. Some elements are quite prone to becoming negative ions, others to having an electron stripped away and leaving them positive, but no element is in jeopardy of both ionization fates. When ion-plus seeks ion-minus, you know there's no chance of incest.

The elements at greatest risk of electron loss are those with a single or maybe two electrons in an outer shell intended for throngs. Several inner layers of electron shells separate the outlier from the positive charges in the nucleus. A glancing blow, a brisk breeze, a winking neighbor, and, whoops, the electron's gone.

By contrast, elements likeliest to turn negative are those whose outer
shells are practically filled, but there's room for one more. Sure, the element can and often does enter into a covalent time-share, but, Oh, the temptation to go further: just one more electron, one little extra charge, and the entire house would be occupied in earnest, and how wholesome, how aesthetically gratifying that would feel. Just one last little after-dinner mint...

Consider, then, the lovely symmetry of salt. On one side we have sodium, a soft metal with the silvery sheen of herring scales. Sodium has eleven electrons, two in the innermost orbit, eight in the next, and, in orbit number three, a solitary sailor with a distinct propensity for jumping ship. Across the aisle, we see chlorine, a corrosive, greenish yellow gas. The outer shell of chlorine, as I mentioned earlier, is one electron shy of satiety, and so chlorine leans toward mean, toward stealing electrons where it can. You can't eat pure sodium, and you shouldn't breathe pure chlorine: they're both toxic. Put the two together, though, and enjoy the show. In a fiery reaction, the sodium atoms essentially wilt and shrug off their extra electrons into the palms of their chlorine counterparts. The sodium atoms in the sample are now electron-deprived and positively ionized, while the chlorines, in fully staffing their orbits, have turned negative (which grants them a name change to "chloride"). Now the two elemental tribes truly want each other. Now the sodium and chloride ions are drawn closer not by the middling desire to round out their shells, but through the much stronger draw of electromagnetic attraction.

At the same time, we have two competing pressures: the attractive tug opposites feel toward each other, and the repulsive sensation between the like-charged ions. As a result, the atoms quickly settle into a regular alternating pattern of chloride and sodium atoms. They stack up neatly in three dimensions like a balanced composition of oranges and grapefruits. These repetitive, geometrically elegant atomic arrays are crystals—salt crystals. What before were two substances that you wouldn't have fed to your old home economics teacher even after she gave you a C for sewing your apron pocket on upside down have condensed into a seasoning so precious that wars have been fought over it, and soldiers given money specifically to buy it—hence the world "salary," from the Latin
salarius,
a stipend for salt. If you were to look at some table salt under a microscope, you'd see just how crisply Pythagorean the grains are, like a sprinkling of art deco glass bricks. Bear in mind that each one of those crystals is an ensemble of a billion billion chlorine and sodium ions, more atoms per granule than there are stars in the Milky Way. Now, would you please pass the salt?

Yet another sort of atomic bond is the metallic bond, the almost socialist sharing of electrons among many atoms in, for example, a piece of copper wire, or the gold of a wedding ring, or the soft sodium sample before its encounter with chlorine. In a metallically bonded substance, the outermost electrons float about in what's often called an "electron sea," being tugged first toward one atom, then toward another, their fluidity accounting for a metal's capacity to conduct an electric current.

The bonds that bind atoms and ions together are all fairly strong glues, with the result, Roald Hoffmann has written, that under normal, nonsolar conditions, "the atoms cohere, move as a group." They cohere covalently as molecules, or ionically as salts, or ironically as metals. Beyond the coherent cliques are larger assemblages, gangling groups of molecules or ionic compounds that adhere together through a couple of bonds of their own. The two big-canvas bonds are weaker than those that marry atoms into molecules, yet they have proved indispensable to life, and ships, and sealing wax, and they give a pencil wings.

One of the critical cross-connectors is the hydrogen bond. The name is unfortunate, not only because it sounds uncomfortably close to "hydrogen bomb," but because it suggests a bond that links hydrogen to other atoms—to oxygen in H
₂
O, for example, or chloride in hydrogen chloride. The bonds in those cases are, however, covalent bonds, and they are far more serious than a hydrogen bond. In fact, the hydrogen bond is best exemplified by the stridently unserious image of Mickey Mouse: a big round head with two round ears on top. Mickey Mouse here is a molecule of water, with the head representing oxygen, the ears the two hydrogen atoms covalently linked to it. Fortunately, we can dispense with facial details and avoid the risk of copyright infringement.

It turns out that the electron pairs binding each hydrogen ear and the oxygen skull are not quite fairly, squarely, and roundly shared. They tend to spend a bit more time near the oxygen nucleus than near the proton core of either hydrogen atom. As a result, the ears of the Mickey molecule have a slight positive charge: their protons are not always fully counterbalanced by a constant cloud of negative charge. At the same time, because the oxygen atom is hogging a bit too much of the shared electrons' attentions, the bottom half of the mouse face has a five o'clock shadow of modest negative charge. The molecule is polarized; its distribution of charges gives it a directionality, an upside and a downside.

What happens when you put a whole lot of polarized Mickey Mouses together in one place—like, say, Lake Michigan? The chins of one molecule are drawn gently toward the ears of another, lending water an
overall shape and integrity that make Mickey quite mighty. Through the puzzle-piece fusing of tops and bottoms, hydrogen bonds account for water's exceptional clinginess, the tendency of droplets to stick together and trail one another loyally no matter where their scout leaders may venture. Hydrogen bonds are only about one-tenth as strong as covalent bonds, but what they lack in strength they make up for with elasticity. Because of hydrogen bonds, plants can drink water; even the crowns of towering redwoods can be quenched. Slender threads of water snake upward from the soil and through the plant's vasculature, to escape as water vapor via pores in the leaves. And as the leading edge of the water column evaporates into the air, hydrogen bonds pull up more fluid from below.