The Periodic Table of the Elements - (From The Sciences, 6th ed., by Trefil and Hazen)
The periodic table of the elements, which systematizes all known chemical elements, provides a powerful conceptual framework for understanding the structure and interaction of atoms. Dimitri Mendeleev, the Russian scientist who studied the regularity or periodicity in the known chemical elements, related that periodicity to each element’s atomic properties. Today, each element is assigned an integer, called the atomic number, which defines the sequence of elements in the table. The atomic number corresponds to the number of protons in the atom, or, equivalently, if the atom is not charged, to the number of electrons surrounding the nucleus. If you arrange the elements as shown in Figure 8-18 (see below), with elements getting progressively heavier as you read from left to right and top to bottom as in a book, then elements in the same vertical column have very similar chemical properties.
Periodic Chemical Properties
The most striking characteristic of the periodic table is the similarity of elements in any given column. In the far left-hand column of the table, for example, are highly reactive elements called alkali metals (lithium, sodium, potassium, etc.). Each of these soft, silvery elements forms compounds (called salts) by combining in a one-to-one ratio with any of the elements in the next to last column (fluorine, chlorine, bromine, etc.). Water dissolves these compounds, which include sodium chloride, or table salt.
The elements in the second column, including beryllium, magnesium, and calcium, are metallic elements called the alkaline earth metals and they too display similar chemical properties among themselves. These elements, for example, combine with oxygen in a one-to-one ratio to form colorless compounds with very high melting temperatures.
Elements in the far right-hand column (helium, neon, argon, and so on), by contrast, are all colorless, odorless gases that are almost impossible to coax into any kind of chemical reaction. These so-called noble gases find applications when ordinary gases are too reactive. Helium lifts blimps, because the only other lighter-than-air gas is the dangerous, explosive element hydrogen. Argon fills incandescent light bulbs, because nitrogen or oxygen would react with the hot filament.
In the late nineteenth century, scientists knew that the periodic table “worked”—it organized the 63 elements known at that time and implied the existence of others—but they had no idea why it worked. Their faith in the periodic table was buttressed by the fact that, when Mendeleev first wrote it down, there were holes in the table—places where he predicted elements should go, but for which no element was known. The ensuing search for the missing kinds of atoms produced the elements we now call scandium (in 1876) and germanium (in 1886).
Why the Periodic Table Works: Electron Shells
With the advent of Bohr’s atomic model and its modern descendants, we finally have some understanding of why the periodic table works. We now realize that the pattern of elements in the periodic table mirrors the spatial arrangement of electrons around the atom’s nucleus—a concentric arrangement of electrons into shells.
The atom is largely empty space. When two atoms come near enough to each other to undergo a chemical reaction—a carbon atom and an oxygen atom in a burning piece of coal, for example—electrons in the outermost shells meet each other first. These outermost electrons govern the chemical properties of materials. We have to understand the behavior of these electrons if we want to understand the periodic table.
To do this, we need to know one more curious fact about electrons. Electrons are particles that obey what is called the Pauli exclusion principle, which says that no two electrons can occupy the same energy state at the same time. One analogy is to compare electrons to cars in a parking lot. Each car takes up one space, and once a space is filled, no other car can go there. Electrons behave in just the same way. Once an electron fills a particular niche in the atom, no other electron can occupy the same niche. A parking lot can be full long before all the actual space in the lot is taken up with cars, because the driveways and spaces between cars must remain empty. So, too, a given electron shell can be filled with electrons long before all the available space is filled.
In fact, it turns out that there are only two spaces that an electron can fill in the innermost electron shell, which corresponds to the lowest Bohr energy level. One of these spaces corresponds to a situation in which the electron “spins” clockwise on its axis, the other to a situation in which it “spins” counterclockwise on its axis. When we start to catalog all possible chemical elements in the periodic table, we have element one (hydrogen) with a single electron in the innermost shell, and element two (helium) with two electrons in that same shell. After this, if we want to add one more electron, it has to go into the second electron shell because the first electron shell is completely filled. This situation explains why only hydrogen and helium appear in the first row in the periodic table.
Adding a third electron yields lithium, an atom with two electrons in the first shell, and a single electron in the second electron shell. Lithium is the element just below hydrogen in the first column of the periodic table, because both hydrogen and lithium have a lone electron in their outermost shell (see Figure 8-19 in your textbook).
The second electron shell has room for eight electrons, a fact reflected in the eight elements of the periodic table’s second row, from lithium with three electrons to neon with 10. Neon appears directly under helium, and we expect these two gases to have similar chemical properties because both have a completely filled outer electron shell.
Thus, a simple counting of the positions available to electrons in the first two electron shells explains why the first row in the periodic table has two elements in it and the second row eight. By similar (but somewhat more complicated) arguments, you can show that the Pauli exclusion principle requires that the next row of the periodic table has 8 elements, the next 18, and so on. Thus, with an understanding of the shell-like structure of the atom’s electrons, the mysterious regularity that Mendeleev found among the chemical elements becomes an example of nature’s laws at work.
Figure 8-18. The periodic table of the elements. The weights of the elements increase from left to right. Each vertical column groups elements with similar chemical properties.