The Periodic Law

During the early part of the nineteenth century, a large body of chemical facts became available from exhaustive studies of the then known elements. With the hope of providing a systematic approach to the study of chemistry, scientists began to look for some form of order in the increasing amount of chemical information. They were spurred on in their efforts by the well known, but unexplained, fact that certain elements had properties that were very similar to those of other elements. Numerous attempts were made to understand these similarities and to use them as a means for classifying the elements.

These efforts resulted, in 1869, in the discovery of what is now called the periodic law. This law was proposed, independently, by both the Russian chemist Dmitri Ivanovich Mendeleev and the German chemist Julius Lothar Meyer, and is one of the most important chemical laws. Given in its modern form (in which atomic number replaces atomic weight), the periodic law states that when elements are arranged in order of increasing atomic numbers, elements with similar properties occur at periodic intervals.

The Periodic Table

A periodic table is a graphical representation of the behavior described by the periodic law. In this table the elements are arranged according to increasing atomic number in such a way that the similarities predicted by the periodic law is evident. This results in horizontal rows and vertical columns of elements. Each element is represented by a rectangular box, which contains the symbol, atomic number, and atomic weight of the element. The horizontal rows in the periodic table are called periods. In the table below the periods are designated by Arabic numerals on the left side of the table. The first period consists of hydrogen (H) and helium (He). The second and third period both contain 8 elements, while the fourth and fifth periods each contain 18 elements. The sixth period consists of 32 elements, but in order for it to fit on a page, part of it is placed at the bottom of the table. The seventh period, which is not complete, has some of its elements placed at the bottom of the table also. The elements in a column of the periodic table are known as a group or in certain cases as families. The groups are the more important divisions to a chemist, since it is in the groups that the close resemblances in properties are found. The labeling of the groups is arbitrary, and three different schemes are in common use, two of which are used in the table below. The bottom set of labels, which are Roman numerals with a suffix of A or B, is widely used in North America. In Europe a similar scheme is used, but some columns have the A's and B's interchanged. In an effort to eliminate this confusion, the International Union of Pure and Applied Chemistry (IUPAC) has proposed a new scheme that numbers the groups from 1 through 18 with no A or B designations. As stated above, the elements in any one group have similar properties. In four of the groups, the elements have such similar properties, that they are designated as families and given a family name. The elements in group IA are known as the alkali metals, those in group IIA are called the alkaline earth metals, those in group VIIA are known as the halogens, and the elements in group VIIIA are known as the noble gases (or rare gases). The elements in groups with an A suffix are called main group elements or representative elements.

In the periodic table above the elements are divided by a heavy red "staircase" line into metals on the left and nonmetals on the right. A metal is a good conductor of heat and electricity while a nonmetal is usually a poor conductor of heat and electricity. A metalloid has properties that are intermediate between those of metals and nonmetals. As you can see from the periodic table above, the majority of known elements are metals; only seventeen elements are nonmetals, and eight elements are metalloids. From left to right across any period, the properties of the elements change gradually from metallic to nonmetallic.