simple table—the periodic table. Every element has its own box, with atomic number increasing as you read from left to right. Boxes are arranged so that elements in a vertical grouping have similar chemical behavior—that is, elements in the same column enter into similar reactions and combine to form similar compounds.
The periodic table of the elements, a fixture in chemistry lecture halls, was first written down by the Russian scientist Dmitri Mendeleyev (1834–1907) in 1869. He didn’t understand why a ranking of the elements in increasing order of weight had something to do with their chemical properties, but it seemed that it did. In addition, when Mendeleyev first wrote down the table there were two holes, corresponding to the elements we now call germanium and scandium. When these elements were discovered (in Germany and Sweden, as the names imply), it was considered an important piece of evidence that the organization of Mendeleyev’s table has a deep reason behind it.
Today we understand that elements can be grouped into rowsand columns because of the way electrons fit into shells of electrons, corresponding to different allowed Bohr energy levels. It turns out that electrons cannot be crowded too closely together. Like two cars in a parking lot, two electrons cannot share the same space. We call this the exclusion principle (because the presence of one electron excludes others).
Thus, there is space for only two electrons in the lowest Bohr electron shell. The atoms of hydrogen (one electron) and helium (two electrons) respectively fill that shell, so the inner electron shell becomes “closed,” to use physicists’ jargon. The next element, lithium, has three electrons, however, so the third electron must go into the next higher shell. Thus lithium, like hydrogen, has a single electron in its outermost shell, which explains why the two have similar chemical properties.
The second and third shells have space for eight electrons each, so we can squeeze in seven electrons after lithium before we start on another shell, and thus find another atom with one electron in its outermost shell. This element is sodium, with eleven electrons. Not surprisingly, you will discover hydrogen, lithium, and sodium in the same column of the periodic table, along with potassium, rubidium, cesium, and francium—all of which have one electron in their outer Bohr shells.
Using quantum mechanics, the subject of the next chapter, we can predict the number of spaces available for electrons in each atomic shell. It is this calculation that ultimately justifies and explains the periodic table.
FRONTIERS
One of the most striking areas of modern research is the rapid development of “microscopes” capable of producing photographs of individual atoms in a material. The best developed of these instruments, the scanning tunneling microscope (or STM), works by measuring the electric current that flows between a tiny, precisely positioned point and the atoms on the surface of a material. The closer the point of the atom, the greater the current. A typical STM photograph shows the presence of individual atoms on a surface.
A scanning tunneling microscope reveals the locations of individual iodine atoms coated on a surface of platinum. The distance between atoms is only about a billionth of an inch
. PHOTO COURTESY OF BRUCE SCHARDT, PURDUE UNIVERSITY.
As scientists improve the resolution of these microscopes, and as they learn more and more about the surfaces of materials allaround us—metals, plastics, paper, and skin, to name just a few—we can expect to see more spectacular pictures of atoms.
SUPERHEAVY ELEMENTS
Although natural elements occur only up to atomic number 94 (the radioactive element plutonium), artificial “superheavy” elements up to 114 (with the inelegant but temporary name “ununquadium”), and as high as 118, have been created in the laboratory by colliding less massive atoms together and hoping that some
The periodic table of the elements, a fixture in chemistry lecture halls, was first written down by the Russian scientist Dmitri Mendeleyev (1834–1907) in 1869. He didn’t understand why a ranking of the elements in increasing order of weight had something to do with their chemical properties, but it seemed that it did. In addition, when Mendeleyev first wrote down the table there were two holes, corresponding to the elements we now call germanium and scandium. When these elements were discovered (in Germany and Sweden, as the names imply), it was considered an important piece of evidence that the organization of Mendeleyev’s table has a deep reason behind it.
Today we understand that elements can be grouped into rowsand columns because of the way electrons fit into shells of electrons, corresponding to different allowed Bohr energy levels. It turns out that electrons cannot be crowded too closely together. Like two cars in a parking lot, two electrons cannot share the same space. We call this the exclusion principle (because the presence of one electron excludes others).
Thus, there is space for only two electrons in the lowest Bohr electron shell. The atoms of hydrogen (one electron) and helium (two electrons) respectively fill that shell, so the inner electron shell becomes “closed,” to use physicists’ jargon. The next element, lithium, has three electrons, however, so the third electron must go into the next higher shell. Thus lithium, like hydrogen, has a single electron in its outermost shell, which explains why the two have similar chemical properties.
The second and third shells have space for eight electrons each, so we can squeeze in seven electrons after lithium before we start on another shell, and thus find another atom with one electron in its outermost shell. This element is sodium, with eleven electrons. Not surprisingly, you will discover hydrogen, lithium, and sodium in the same column of the periodic table, along with potassium, rubidium, cesium, and francium—all of which have one electron in their outer Bohr shells.
Using quantum mechanics, the subject of the next chapter, we can predict the number of spaces available for electrons in each atomic shell. It is this calculation that ultimately justifies and explains the periodic table.
FRONTIERS
One of the most striking areas of modern research is the rapid development of “microscopes” capable of producing photographs of individual atoms in a material. The best developed of these instruments, the scanning tunneling microscope (or STM), works by measuring the electric current that flows between a tiny, precisely positioned point and the atoms on the surface of a material. The closer the point of the atom, the greater the current. A typical STM photograph shows the presence of individual atoms on a surface.
A scanning tunneling microscope reveals the locations of individual iodine atoms coated on a surface of platinum. The distance between atoms is only about a billionth of an inch
. PHOTO COURTESY OF BRUCE SCHARDT, PURDUE UNIVERSITY.
As scientists improve the resolution of these microscopes, and as they learn more and more about the surfaces of materials allaround us—metals, plastics, paper, and skin, to name just a few—we can expect to see more spectacular pictures of atoms.
SUPERHEAVY ELEMENTS
Although natural elements occur only up to atomic number 94 (the radioactive element plutonium), artificial “superheavy” elements up to 114 (with the inelegant but temporary name “ununquadium”), and as high as 118, have been created in the laboratory by colliding less massive atoms together and hoping that some
Similar Books
