For two thousand years it was generally accepted that all matter was composed of four elements - earth, air, fire and water - combined in varying proportions. This idea, credited to Empedocles, a Greek philosopher of the fifth century B.C., was popularized by Aristotle, and widely held until the Middle Ages. During those two millennia alchemists believed that one element could be transformed into another, and sought to convert less valuable metals, such as lead and copper, into gold.
It was only in the sixteenth and seventeenth centuries that scientists began to reconsider the nature of matter, and in 1661 a scientist called Robert Boyle overthrew the idea of the four elements, and undermined the alchemistsæ assumptions, by suggesting the modern conception of the element. Each element, he claimed, is a pure substance that cannot be broken down into any simpler ones.
At the end of the eighteenth century and the beginning of the nineteenth, as scientists developed the methods necessary for isolating elements, the volume of chemical knowledge began to grow, and it became useful to classify the elements according to their properties. The various systems designed to organize the data eventually evolved into the Periodic Table we know today.
John Dalton (1803)
John Dalton, an English scientist, made an important advance in chemical understanding when he realised that different elements possess different atomic weights. He used the weight of the lightest element, hydrogen, as a standard by which to express all the others. In 1803, he arranged the elements and their relative atomic weights into a table. This encouraged scientists to begin to seek numerical relationships between the elements.
Johann Wolfgang D÷bereiner (1817-1829)
D÷bereiner, a German professor of chemistry, was one of the first scientists to propose a system of classification for the elements. He suggested that certain elements, such as chlorine, bromine and iodine, and calcium, strontium and barium, could be arranged in groups of threes (known as triads). Each member of the triad had similar properties. When these elements were arranged in order of increasing atomic weight, the atomic weight of the middle element was shown to be an approximate average of the first and third. This suggested a relationship between the atomic weight of an element and its particular properties.
Alexandre Beguyer de Chancourtois (1862)
In 1862, de Chancourtois, a French professor, proposed a classification of the elements based on atomic weight. Using sixteen, the atomic weight of oxygen, as a standard, he divided the circumference of a vertical cylinder into sixteen equal parts, and then plotted the atomic weights of the elements on the cylinderæs surface. Elements with similar properties appeared together on the same vertical line, which prompted de Chancourtois to state that 'the properties of the elements are the properties of the numbers.'
John Newlands (1865)
In 1865, Newlands arranged the elements in order of increasing atomic weight, and noted similarities between each eighth element (the inert elements had not yet been discovered), just as the eighth note in a musical octave resembles the first. For this reason his groupings of the elements are known as Newlands' Octaves. Unfortunately, he left no gaps for undiscovered elements, and his work, despite its importance, was coolly received at the time.
Lothar Meyer (1869)
In 1869, two men, one of them Lothar Meyer, a German, and the other, Dmitri Mendeleyev, a Russian, independently produced similar arrangements of the elements in the form of tables. Meyer, who had produced a periodic table in 1864, extended it five years later to include 56 elements. He arranged the elements in order of increasing atomic weight, and noted a periodic repetition of similar properties. 'The properties of the elements,' he stated, 'are largely periodic functions of the atomic weight.' This idea is known as the Periodic Law.
Dmitri Mendeleyev (1869)
Mendeleyev made similar observations to Lothar Meyer, and the two men are considered co-discoverers of the Periodic Law. Mendeleyev arranged the elements in order of increasing atomic weight, with account taken of their behaviour patterns, and stated that 'the elements arranged according to the magnitude of atomic weights show a periodic change of properties.' That is, certain chemical properties repeat at regular intervals, causing elements to fall into markedly different groups, with the members of each group resembling one another. What was particularly impressive about Mendeleyev's work was that he left gaps in his table for undiscovered elements, and using their position in the table he predicted, with great accuracy, what their properties would be. A modified version of Mendeleyevæs table is still used today.
Rutherford, Moseley and Bohr
One of the problems with the periodic tables produced by Mendeleyev and Meyer was that some of the elements, when placed in the positions dictated by their properties, disrupted the order of increasing atomic weights: argon is placed before potassium, cobalt before nickel, and tellurium before iodine, even though the first member of each pair has a greater atomic weight than the second. It was only when the work of Sir Ernest Rutherford, H.G.J. Moseley and others had led to a better understanding of atomic structure that this difficulty was overcome.
In about 1910, Rutherford's experiments with alpha particles led to the determination of the nuclear electrical charge, the ratio of which to that of the electron was seen to be roughly half the atomic weight. It was suggested by A. van den Broek that this number, known as the atomic number, was identical to the elements' ordinal number in the Periodic Table. This was confirmed by H.G.J. Moseley in 1913, whose studies of the wavelengths of the characteristic X-ray spectral lines of many elements demonstrated a clear identification between the ordinal number of an element and its nuclear charge. It was realised that the atomic number, rather than the atomic weight, was the true determinant of an element's chemical properties, and in 1922 a scientist called Niels Bohr devised a new Periodic Table based upon the electronic structure of atoms.
The modern Periodic Table, therefore, is organised according to a modification of Mendeleyev's and Meyer's Periodic Law. The modern Periodic Law states that the properties of the elements are periodic functions of their atomic numbers, rather than of their atomic weights.This eliminates the problem of some of the elements being out of order, as they seemed to be when arranged according to weight. Although great progress has been made since the beginning of the twentieth century in understanding how atomic structure relates to periodicity, the essential idea of periodic properties, as described by Mendeleyev and Meyer, remains unchanged.
Now And The Future
Continuing scientific developments ensure that the periodic table is always in a state of evolution. In recent years, the work of scientists at Berkeley in California, Dubna in Russia and Darmstadt in Germany has resulted in the synthesis of several new elements. Since 1937, when technetium was produced artificially, scientists have been able to create new elements in machines called cyclotrons and in nuclear reactors. The American scientist, Glenn Seaborg, won a Nobel Prize for his work in this field. All of the elements following uranium in the periodic table are man-made, although minute amounts of neptunium and plutonium have been detected in nature. One hundred and nine elements now exist, and attempts are being made to create element 110.
Until their existence has been generally accepted by the scientific community, new elements are given systematic names deriving from the Latin words for their proton numbers. Thus, element 104 was named unnilquadium, element 105 unnilpentium, and so on. Recently, however, The International Union of Pure and Applied Chemistry has ruled on disputes over who first synthesized certain elements, and has attributed names chosen by the elementsæ discoverers. Element 104 is now to be known as rutherfordium (Ru), element 105 as hahnium (Hn), element 107 as nielsbohrium (Ns), element 108 as hassium (Hs) and element 109 as meitnerium (Mt). No name has been chosen for element 106.
Scientists have predicted the properties of elements up to at least element 184, and although artificial elements tend to be extremely unstable, it is believed that elements with around 114 protons and 184 neutrons will have half-lives measured in years.
