The Periodic table is to chemistry what the Bible and Koran are to religion and the Constitution to America. Its existence is a testament to the beauty of thought and inquiry, for in one document there is both tremendous information and inspiration to those who understand its message. For centuries, mankind has struggled to understand what surrounds us, the stuff we call “matter.” Rocks, trees, air and water can be seen, but what are they comprised of? Great philosophers struggled with the meaning of the world around us, including the concept of a smallest, indivisible particle, which we now call the atom. The Greeks postulated that everything was made up of earth, fire, air and water. And why not? It would be easy to imagine that these four “elements” could explain the things around us. But, as our forays into space demonstrate, mankind has a seemingly insatiable curiosity to better understand its world, and its place in the universe.
From earth, fire, air and water, thoughtful and creative explorers over the past two hundred years have led to the discovery of, for chemists, the fundamental building blocks of the universe. Unfortunately, those pesky physicists next door are breaking apart our precious atoms into even smaller particles, which they call quarks, bosons, leptons, gluons, and a myriad of other names that clearly illustrate their inability to invent dull names like their chemist cousins. But while those physicists build their alters of particle colliders, such as the LHC in Europe, and conduct experiments that they hope won’t create a black hole out of our planet, we chemists are still learning and applying the magic of the Periodic table.
Magic? The table would seem to simply list the elements in their increasing order of mass – until you look more closely. There are a few cases where that order doesn’t hold true. For example, cobalt precedes nickel in the periodic table. But cobalt’s mass is 58.93 AMU (atomic mass units) while nickel is lighter at 58.69 AMU. This seems an obvious mistake. Perhaps the international committee that guards the table from evil should be notified.
But the order of cobalt and nickel is correct. The table is arranged by the number of protons in the nucleus, not by atomic weight. It is the number of protons that gives an element its identity, not its mass. Nickel is placed after cobalt, even though it has smaller mass, because it has one more proton in its nucleus than cobalt. The inconvenient fact that nickel has less mass than cobalt is due to the number of neutrons in the nucleus. Although a neutral atom has equal numbers of protons and electrons, it can have differing numbers of neutrons, even within the same element. We call these wayward atoms isotopes.
Some elements, such as uranium, have so many neutrons that they are unstable and the atom can split apart all on its own, with a release of radiation energy. A quick scan of the periodic table allows one to identify elements that might exhibit this property. I search for elements having a much greater number of neutrons than protons. It doesn’t take long to spot several, such as radon (86), that with a mass of 222 has 86 protons but 136 neutrons. Radium (88) is similar with 88 protons and 138 neutrons. I wonder if that is why they call it radium. For elements beyond radium, the popularity of neutrons seems to grow rapidly. Ratios of neutrons to protons are all well over 2, including a whopping 2.6 for neptunium (93), which is located next to uranium (92). Many of these elements were discovered in colliders, not in nature. Their half-lives are too short. They fall apart, form different elements, and emit a lot of radiation in the process. The periodic table allows such predictions.
Protons and electrons define the behavior of an element. One is always equal to the other in a neutral atom. Elements are arranged by on the basis of their proton count and each is assigned an “atomic number” that reflects the number of protons. The difference between the atomic number and atomic mass is the number of neutrons. Electrons are too small to account for a meaningful difference at this level and their presence is ignored in mass calculations.
But what could explain the bizarre appearance of the table as drawn? A list of elements by atomic number should look like an Excel-ready table, not the random assortment of small boxes and symbols that we see. The periodic table appears to have been fabricated by someone with a bizarre sense of design. Hydrogen leads the parade with its perch atop the left hand column, but helium, the next element at 2, is placed to the far right of the table. This desolate first row of two, albeit popular, elements is followed by a second row containing 8 elements, with two in the first two columns and 6 in the last eight columns. The third row follows the pattern of the second, but the fourth row finds an insertion of ten elements stuck smack in the center. Confusion reigns in rows six and seven, which include a series of footnoted lanthanoid and actinoid elements that almost seem to be afterthoughts. In reality, both should be inserted into the table by splitting it between columns 2 and 3, just as was done before to neighbors hydrogen and helium. The split table would be connected by a long bridge of these elements strung along the bottom two rows. Why this isn’t actually done is simply a convenience for those who actually have to print the tables. Imagine having to place such an elongated table on a printed page. The ubiquitous periodic table wall charts would also take on an unwieldy appearance.
There is a reason for constructing the periodic table in this clearly odd fashion. Yes, the elements are arranged first in order of their atomic number, but due respect is also seriously given to their chemical properties. Early scientists aligned columns of elements to correspond with similarities between them in their chemical properties. Thus, while the two gases hydrogen and helium are next to each other in atomic number, they are spread apart by the full length of the table. Hydrogen was a reactive gas while helium was not. Hydrogen’s reactivity was more similar to the lithium below it; helium’s chemical properties were more similar to neon. Even though gaseous hydrogen looked quite different than solid lithium, they had similar chemical reactivities. Thus, the table also transmits some basic expectations as to how an element might fare in a chemical reaction, based on its location. One can correctly impute that since lithium is a reactive element, that cesium might also have similar properties and react in the same way, which it does. Likewise, since helium is very nonreactive, one might assume that the other elements in the same row are also nonreactive (e.g., neon, argon, etc.). In fact, they are so nonreactive that the whole column, or Group, is given the name of the “noble gasses” or “inert gasses.”
So arranged, the table thus becomes a powerful tool for prediction. It even allowed the prediction new elements when gaps appeared in early tables. These gaps have since been filled. New elements are added as they are discovered, usually as a result of experiments in colliders. Unfortunately, these newborns are recipients of obtuse names such as ununbium, ununquadium, and ununoctium, clearly a result of not allowing more playful physicists into the naming process.
What the early scientists didn’t know was that their construct of the table parallels the location of the elements’ electrons. Electrons are the negatively charged particles that orbit the nucleus. We always see them drawn as circles around the nucleus, similar to planets around a sun. But, in fact, the electrons take routes that look quite different from these pleasantly simple constructs. Most importantly, it is the electrons that control the reactions between elements. Electrons move in specific orbitals, oddly labeled s, p, d, and f, letters that are duplicated among differing energy shells and given names such as 1s, 2s, 3s, 2p, 3p, etc. As with people, electrons have a strong preference where they reside, with whom they get along with, and how many will fit into their particular group or orbital. They also tend towards isolation with the very strong exception that they all would love to emulate the electron configuration of the elements in the far right column, headed by helium and known as the noble gases.
We can illustrate chemical reactivity by analogy to people. The elements certainly have their own personalities, and we can understand their desires better by thinking as they do. So let’s take a trip to Elementland to look at their society. The fashion magazine Element Today has made the noble gases the envy of ElementLand, All the residents strive towards this ideal of social perfection. What distinguishes the noble gases is that they have what are known as “complete outer shells.” This means that their electrons have completely filled all their orbitals which results in a low energy state a situation eagerly sought by all elements. The resulting noble gas atom is quite happy to live on its own, in peace and quiet, and drawing the envy of everyone west of their location in the table. For their neighbors, it is a tantalizingly close foray to perfection.
Take elements 9 (Fluorine), 10 (neon), and 11 (sodium), for example. Fluorine and sodium know from Element Today that neon’s outer shell of electrons is the envy of the world. They are so close, yet sodium has one too many electrons and fluorine has one too few for either of them to match neon’s appareled beauty. In fact, sodium’s electron is in a new energy shell a fact that is confirmed by looking at the table and noting that sodium is the start of a whole new row.
The Wall Street brokers on sodium have offered the atom’s last electron on Element Bay (ebay), much to the shock of the nucleus, whose 11 positive protons strongly protest such a move. They enjoy the company of all 11 negative electrons and are not willing to yield their soul mates. Loss of an electron would make them a lowly ion. A deal is brokered wherein the purchasing entity must stick close to the remaining sodium ion as a condition of the sale. Fluorine spots the ebay ad and immediately agrees to the deal. Fluorine needs one more electron to match the configuration of its neighbor and in exchange for sticking around, it takes the outer electron from sodium and both atoms party, exultant in their both having become nouveau social elite. They form a strong relationship, known to the rest of the world as sodium fluoride. The sodium electron spends its time on the fluorine atom, but both atoms cling to each other as a result of the imbalance of charge. Protons in the nucleus are far less mobile and will only leave under extreme duress. So the happy compromise is to stick close together.
Sodium’s success with fluorine was noted on both ebay and Element Today. It generated intense interest among all sodium atoms, who collectively agreed it was sodium’s destiny to partner is such a manner. Most tellingly, sodium atoms seek out those elements in the same column as fluorine. This endeavor was a resounding success. Perhaps the most popular marriage was between sodium and chlorine. Chlorine had the same situation as fluorine, albeit on a different energy level. It only needs one more electron to make it attractive like Argon. Argon had been seen on Dancing with the Stars and her success was mesmerizing to chlorine, the next door neighbor. Their partnership began a long relationship that greatly impacted a new compound called humans. Humans called the partnership of sodium and chlorine “table salt,” which today seems demeaning, but salt had a profound impact on the history of the humans. It was a compound of great value when it was discovered that it could be used to cure foods for storage. It enabled ocean exploration and control of salt supplies was the genesis of both wars and revolts.
The vivacity of the elements immediately bordering the noble gases leads to many of the ionic compounds we see formed. The hydrogen, lithium, sodium, and potassium salts are well known, particularly the chlorides. Hydrogen chloride is a well-known acid known as hydrochloric acid. Hydrogen iodide is strong enough to etch glass.
What of the other elements? Their reaction pattern is again forecast by their attempts to fill their outermost electron shells. Carbon (element 6) is four doors down from neon. It would love to attract four electrons to be added to its outer shell. Hydrogen is again a popular regular on Element Bay and in this case carbon barters for four hydrogen atoms. All four contribute one electron each to the carbon atom and the combined entity becomes a molecule of methane, CH4.
The wonderful world of carbon includes its ability to bond with itself. Two and more carbons may join forces and share electrons in a joint venture to at least get closer to a noble gas a portion of their lives. An electron on one carbon will pair up with one from the second to form what is known as a covalent bond. That is, the electrons are shared between the two carbons in an elementary form of a time share condominium. It is the basis of a mutual benefit that can extend to unbelievable proportions. Hundreds and even thousands of carbons can bond together. The longer ones form the basis of many synthetic polymers. The complex ones form the backbone of proteins, carbohydrates and other life-critical substances.
Carbon also freely bonds with other elements, especially oxygen, phosphorous, nitrogen, and sulfur. These form organic acids, carbonates, phosphates, nitrates and sulfates. Carbons can also form more than one bond between themselves. These bonds are of high energy and readily react with other atoms. This property makes them useful for making other compounds.
A chemist can look at the periodic table and make judgments on an element’s reactivity based on its position on the table. The study of electrons in their shells and orbitals is given the quite cool name of quantum mechanics. It can be rolled off the tongue casually when trying to impress others. What it means is that you know the periodic table well enough to be able to assign an electron configuration label to each of an element’s electrons. The last electron on iron is 1s22s22p63s23p64s23d6. There. That’s that. Sounds cool, eh? Be impressed.
What is so inspirational about the periodic table is that so much of it was formed well before tools existed that allow us to understand atoms the way we do today. The table provides an elegant picture of what elements may or may not do based entirely on their placement in the table. The development of the table is a triumph of scientific method and incredible ingenuity by many. It provides both information and inspiration to those who understand its genesis.<?xml:namespace prefix = o ns = “urn:schemas-microsoft-com:office:office” />
