At first glance, nature appears to consist of countless numbers of very different materials. The world thus appears to be an extremely complex mixture of many different materials. But philosophers and scientists have long held another view of the world. They have found it difficult to believe that nature is really as complex as it appears to be. Instead, they have assumed that the many different materials we see result from the combination of a small number of fundamental substances called elements. Elements refer to a basic substance that cannot be broken down into anything simpler by ordinary chemical of physical means.
EARLY CONCEPTS OF THE ELEMENTS
The idea of an element probably originated with the ancient Greeks. At first, philosophers imagined that only one basic substance exists. They believed that everything was made of some variation of that substance. Thales taught that everything is made from water. By condensing, evaporating, and changing its outward form, water could take the appearance of all other materials, he said. Anaximenes held a similar view, but called air the one single element. For Heraclitus, fire was the elementary material from which everything else was formed.
Other philosophers claimed that two or more elements were necessary. Probably the most popular view was that of Aristotle, who taught that earth, air, fire, and water were the four elementary materials. Other scholars proposed other theories of the elements. Some described only two elements, mercury and sulfur while others added a third, salt.
MODERN CONCEPTS OF AN ELEMENT
As modern chemistry began to develop, questions about the nature of an element became more confused. Some scholars tried to keep the Greek concept of a handful of fundamental substances that might or might not be material substances. Others were aware of the increasing number of new materials being discovered that seemed to be fundamental materials. In 1661, the English philosopher Robert Boyle tried to resolve this issue. In his book, Sceptical Chymist, Boyle rejected the Greek idea of elements as being nonmaterial qualities. Instead, he suggested that the term element be reserved. Boyle s concept of the element differs from that of a modern chemist but it is important because it helped scholars to start seeing the elements in a different light, namely as concrete materials and not qualities .
Another philosopher who helped us understand the nature of elements was Antoine-Laurent Lavoisier. He is often called the Father of Modern Chemistry. In his 1789 textbook on chemistry, Trait El mentaire de Chimie, Lavoisier wrote, all substances which we have not yet been able by any means to decompose are elements to us. The definition that Lavoisier provided in 1789 is still considered valid today even though the specific list of elements that accompanied that definition has changed.
The search for substances that are true elements occupied scientists for more than 100 years after Lavoisier s time. The problem was that, until Moseley s discovery of the atomic number in 1913, scientists had no way of knowing how many elements could exist. Moseley was able to show, however, that scientists could never expect to discover more than about 100 elements.
THE ELEMENTS TODAY
Scientists now know that they have discovered all the elements-ninety-two of them- that exist in the natural world. In addition, they have found ways to produce another dozen or so synthetic elements. As chemists discovered more and more elements with a great variety of properties, a question began to emerge. Are the chemical elements really as different as they seem? Or is there some underlying principle that can be used to organize the apparent complexity that exists?
ORGANIZING THE ELEMENTS
By the mid-nineteenth century, chemists had discovered more than sixty elements. New elements were being announced every few years. Questions began to trouble scientists. First, how many true elements were there? Was it correct to think that only a handful of these basic materials really existed? Or would chemists continue to find an unlimited number with passing time? Second, was there some way that all these elements could be organized? Also was there families or groups into which they could be arranged?
The answer to the second question was provided in 1869 almost simultaneously by tow chemists, Dmitri Mendeleev in Russia and Lothar Meyer in Germany. Mendeleev and Meyer found a way to arrange the elements so that their properties were related to each other in a logical, orderly, and predictable way. This system of organization thus became known as the periodic law.
Both scientist listed the elements according to their atomic weights, beginning with the lightest element, hydrogen. When the elements were arranged in this way, Mendeleev and Meyer found that a pattern began to emerge. Sodium (Na) is chemically similar to lithium (Li), magnesium (Mg) has properties like those of beryllium (Be), aluminum (Al) has properties similar those of boron (B), and so on. The Mendeleev-Meyer discovery is summarized thus in the periodic law: When the elements are arranged in order according to their atomic weights, their properties are repeated in a periodic way . Credit for the discovery of the periodic law is usually given primarily to Mendeleev for two reasons. First, he used a more complete set of data in working out his version of the law. Second, and more important, he showed how the law could be used to predict the existence of new elements.
Mendeleev was able to predict the existence of new elements because of gaps that appeared in his original table of the elements. In his table the heavier element after calcium (Ca) in the chart is titanium (Ti) normally, one would expect to see Ti immediately after Ca, below aluminum. But Mendeleev knew that would be a mistake. In his table, like elements always occurred below each other, and titanium is like silicon (Si), not aluminum. So Mendeleev put titanium where it belonged on the basis of its properties. Then he predicted that a new element would be found to fill the missing space beneath aluminum. In 1879, the Swedish chemist L.F. Nilson discovered the missing element and named it scandium.
Most chemists quickly accepted Mendeleev s periodic law. However, it still contained a few problems. In Mendeleev s periodic table, he guessed that the chemists that had measured the weight of the tellurium and iodine had made an error in calculating it. For that reason his placement of the two elements was incorrect. But this time, Mendeleev was wrong. The correct explanation for this apparent confusion did not appear for nearly 50 years. Then the English physicist H.G. J. Moseley unraveled the puzzle. Moseley found that the elements could also be arranged according to their atomic number. Thus arrangement is almost the same as the atomic weight sequence but not exactly. However, when atomic numbers are used instead of atomic weights in building the periodic table, all problems remaining from Mendeleev s original work disappear. The result is the table known to us today.
ATOMIC STRUCTURE AND PERIODIC PROPERTIES
Meyer, Mendeleev, and Moseley had no way of knowing why the periodic law is true. The statements they wrote simply described patterns that they had observed among the elements. But they did not know why these properties depended on atomic weight or atomic number.
Today, chemists understand the connection between an element s atomic number and its properties. Chemists know that an element s properties depend largely on the number of electrons in its outer shell. The properties of an element are closely related to its atomic number. Thus, the periodic law. Atomic weight is not as useful in establishing the periodic law. The atomic weight of an atom includes neutrons, which carry no electric charge. The fact that two atoms have different numbers of neutrons has no effect on the number of electrons they contain and, thus, on their properties. The match between atomic weight and properties is, therefore, close, but not exact.
NEW ELEMENTS DISCOVERED
Scientists have reported the discovery of three new superheavy elements, atomic numbers 114, 116 and 118, since the beginning of 1999. Atoms of these elements are bigger and heavier than any previously known. Superheavy elements like these may help scientists discover secrets about the nucleus of an atom. These heavy, artificial atoms are not visible. If other scientists can reproduce the results, then the elements will be solid candidates to officially join the periodic table.
Since the 1960s scientists have theorized that an island of stability exists among the superheavy elements. These newly discovered stable superheavy atoms decay rapidly, but they last longer than smaller heavy elements. For example, although an atom of element 114 exists for only 30 seconds, it last 100,000 times longer than an atom of element 112.
The stable superheavy elements are believed to have a special combination of neutrons and protons in each nucleus. The combination allows the force that binds neutrons and protons to temporarily prevent the enormous electrical repulsion between protons from breaking up the nucleus.
Now that scientists have produced a few superheavy elements in the laboratory, some want to attempt to make element 126, which is believed to be in the island of stability . Other nuclear scientists are busy trying to recreate elements 114, 116 and 118 to confirm that scientists have found them.