Periodic Table of the ElementsThe list of chemical elements and what we now call their atomic masses prepared by Dalton at the beginning of the 19th century contained a comparatively small number of elements and atomic masses. Most of the atomic masses given by Dalton were far from our modern values due to a variety of errors. Dalton assumed that hydrogen had an atomic mass of exactly one, since it was the lightest element, but did not know that hydrogen normally exists as a diatomic gas which has a molar mass of two. As a consequence water was considered to be HO rather than H2O. These errors were soon corrected by other workers, and growing tables of elements and their atomic masses became central to the careful and organized study of chemistry. The elements were not then arranged in any repeating or periodic pattern, but simply listed in order of increasing atomic mass together with their properties and the properties of their compounds.
As chemists studied this list and added to it, it began to appear as if there were some underlying periodic pattern to the chemical behavior of the different elements although the nature of the pattern was far from clear. Chlorine, bromine, and iodine have widely separated atomic masses but very similar properties are observed for their compounds. The first partially successful idea of a pattern was the suggestion that elements were grouped in threes, or triads, which was made in 1829 by Johann W. Doebereiner (1780-1849). The elements in these triads were separated by many mass units but had very similar chemical properties. Chlorine, bromine, and iodine are one triad and lithium, sodium, and potassium are another. The atomic mass of the middle element of the triad was supposed to be the mean of the atomic masses of the first and third members. Unfortunately, many of the metals cannot be grouped into reasonable triads.
A second pattern was suggested in 1864 by John A. R. Newlands, professor of chemistry at City College of London. Newlands suggested that the elements could be arranged into a periodic pattern of octaves, or groups of eight, in order of their atomic mass. While this pattern does place lithium, sodium, and potassium together it fails for the obvious group of chlorine, bromine, and iodine as well as for such common metals as iron and copper. Newlands' idea vanished in ridicule linking it to the notes on a musical staff, and the Chemical Society refused to publish his paper.
No simple numeric rule was found which could provide an overall organization of the chemical elements into a form consistent with both their chemical properties and their atomic masses. The theoretical basis on which we now arrange the chemical elements, atomic number and quantum theory, was then unknown and would remain so for decades to come. The overall organization of the periodic table of the chemical elements was developed not theoretically but on the basis of the observed chemistry of their compounds by a professor writing a chemistry textbook -- D. I. Mendeleev.
Periodic Chart of the Elements: MendeleevDmitri Ivanovich Mendeleev (1834-1907) was born in Siberia as the youngest of 17 children. His widowed mother moved to St. Petersburg (now Leningrad) to secure an education for her son. Mendeleev was educated in St. Petersburg and later in France and Germany. He secured a position as professor of chemistry at the University of St. Petersburg and in 1861 wrote a systematic textbook of organic chemistry. While writing a second textbook, this time on systematic inorganic chemistry, in 1872 he arranged the elements in the form of the periodic table (Figure). Mendeleev's development of the periodic table came within one vote of the award of the Nobel Prize in 1906.
Mendeleev prepared his periodic chart of the elements on the basis of the known chemical similarities of the elements arranged in order of their atomic mass. Since atomic masses were not in all cases accurately known, Mendeleev did not hesitate to make some corrections in the order. As a consequence of the arrangement he became aware of apparent gaps in the known elements. It was the prediction that not only would two new elements be found with atomic masses between zinc and arsenic (the elements we know as gallium and germanium) but that their properties were predicted as well that is the mark of his genius. The properties of gallium predicted by Mendeleev in 1869 match well its actual properties measured after its discovery in 1875.
Since none of the noble gas elements were then known, Mendeleev could not and did not predict them. With this exception, and the actual discovery of some of the less-common elements and the lanthanide and actinide groups, our modern periodic chart is the same as that produced by Mendeleev. A similar arrangement was devised at about the same time by the German chemist Julius Lothar Mayer, but Mayer did not venture to predict new or missing elements.
Modern Periodic ChartsMendeleev's periodic charts contained no atomic numbers, as our modern periodic charts do, because the atomic structure of elements was not yet known. Where Mendeleev ordered elements by atomic mass, modern periodic charts order elements by atomic number, which is the number of protons contained in the nucleus of the atom of an element. The order of atomic mass and the order of atomic number, however, turn out to be almost the same.
Modern periodic charts are often numbered across the tops of the columns, which are almost invariably laid out as shown (Table). Two different numbering schemes are in use, one of which uses Roman numerals and letters while the other uses Arabic (ordinary) numerals. In both numbering schemes the elements 57-71 form a separate series, the lanthanoids, as do the elements 89-103, the actinoids. Elements 104 and beyond are named systematically, the name following directly from the number. Element 104 is un-nil-quadium (Unq), for example, while element 106 is un-nil-hexium (Unh).
The columns of a periodic chart of the elements each contain several elements whose chemistry is very similar. Each row of a periodic chart is called a period. The elements listed in the same column are called a group. Some of these groups of elements have names which are useful in describing the chemical properties of elements. The group headed by lithium are the alkali metals; the group headed by beryllium are the alkaline earths; the group headed by oxygen are the chalcogens; the group headed by fluorine are the halogens; and the group headed by helium are called the noble gases or inert gases. The several groups headed by scandium (Sc, 21) through zinc (Zn, 30) are collectively known as the transition metals or transition elements. Elements other than those of the transition metals, lanthanoids, and actinoids are collectively known as the main group elements.
The two systems of designating the groups in the periodic chart by number are the more recent I.U.P.A.C. designation, which uses only Arabic numerals, and the traditional designation which uses Roman numerals and letters. The traditional system differs somewhat between North America and Europe. The two designations can be most easily compared using the elements potassium (19) through krypton (36). The I.U.P.A.C. form numbers the columns containing these elements left to right in numerical order, from group 1 (potassium) through group 18 (krypton). The North American version of the traditional order, also going from left to right, is IA, IIA, IIIB, IVB, VB, VIB, VIIB; the columns headed by iron, cobalt, and nickel are together designated VIIIB and constitute the ferrous metals. The columns continue from copper (29) as IB, IIB, IIIA, IVA, VA, VIA, VIIA, and finally the column including krypton as group 0. In the North American version of the traditional order, the designation B thus corresponds to the transition elements and the designation A corresponds to the main group elements.
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