PRESENTATION OUTLINE
SCIENTIST IN THE DEVELOPMENT OF THE PERIODIC TABLE
Lavoisier
Antoine-Laurent de Lavoisier is one of the most important scientists in the history of chemistry. He discovered elements, formulated a basic law of chemistry and helped create the metric system.
During his time, people believed that when an object burns, a mysterious substance called ‘phlogiston’ was released. This was called the ‘phlogiston theory’. Lavoisier’s experiments demonstrated the contrary, i.e. when something burned, it actually absorbed something from the air, instead of releasing anything. He later named the ‘something’ from the air as oxygen, when he found
That it combined with other chemicals to form acid. (In Greek, ‘oxy’ means sharp, referring to the sharp taste of acids.)
Henry Cavendish had earlier isolated hydrogen, but he called it inflammable air. Lavoisier showed that this inflammable air burned to form a colourless liquid, which turned out to be water. The Greek word for water is ‘hydro’, so the air that burned to form water was hydrogen!
Lavoisier was known for his painstaking attention to detail. Whenever he made a chemical reaction, he weighed all the substances carefully before and after the reaction. He
discovered that in a chemical reaction, though substances may change their chemical nature, their total mass remains the same. This is called the law of conservation of mass.
His love for accuracy led to the formulation of the metric system of weights and measures – which is still in use today.
Lavoisier’s attention to detail and habit of recording everything is perhaps his most important contribution – for that is now the way science is done.
Dobereiner
A German scientist called Johann Dobereiner put forward his law of triads in 1817. Each of Dobereiner's triads was a group of three elements. The appearance and reactions of the elements in a triad were similar to each other.Atomic masses At this time, scientists had begun to find out the relative atomic masses of the elements. Dobereiner discovered that the relative atomic mass of the middle element in each triad was close to the average of the relative atomic masses of the other two elements. This gave other scientists a clue that relative atomic masses were important when arranging the elements.
Chancourtois
In 1862, before Newlands announced his Law of Octaves and Mendeleev described his Periodic System, de Chancourtois presented a paper to the French Academy of Sciences which was then published in this Society own Journal, Comptes Rendus. However, the concept was poorly presented and difficult to understand. The following diagram which would have made de Chancourtois ideas much clearer was omitted although it did later appear in a less widely-read geological pamphlet. It is not surprising then, that chemists in other countries were unaware of de Chancourtois efforts. Indeed they were unrecognised until after Mendeleev's more detailed ideas of a Periodic Table had become accepted. (reference)de Chancourtois called his idea "vis tellurique" or telluric spiral because the element tellurium came in the middle. It was also somewhat appropriate coming from a geologist as the element tellurium is named after the Earth. He plotted the atomic weights on the outside of a cylinder such that one complete turn corresponded to an atomic weight increase of 16.
Newland
An English scientist called John Newlands put forward his law of octaves in 1864. He arranged all the elements known at the time into a table in order of relative atomic mass. When he did this, he found that each element was similar to the element eight places further on. For example, starting at Li, Be is the second element, B is the third and Na is the eighth element.Regular repeats Newlands' table showed a repeating or periodic pattern of properties, but it had problems. For example, he put iron in the same group as oxygen and sulphur, which are two non-metals. As a result, his table was not accepted by other scientists.
Mendeleev
In 1869, just five years after John Newlands put forward his Law of Octaves, a Russian chemist called Dmitri Mendeleev published a periodic table. Mendeleev also arranged the elements known at the time in order of relative atomic mass, but he did some other things that made his table much more successful.Mendeleev realized that the physical and chemical properties of elements were related to their atomic mass in a 'periodic' way, and arranged them so that groups of elements with similar properties fell into vertical columns in his table.
Gaps and predictions Sometimes this method of arranging elements meant there were gaps in his horizontal rows or 'periods'. But instead of seeing this as a problem, Mendeleev thought it simply meant that the elements which belonged in the gaps had not yet been discovered. He was also able to work out the atomic mass of the missing elements, and so predict their properties. And when they were discovered, Mendeleev turned out to be right. For example, he predicted the properties of an undiscovered element that should fit below aluminum in his table. When this element, called gallium, was discovered in 1875 its properties were found to be close to Mendeleev's predictions. Two other predicted elements were later discovered, lending further credit to Mendeleev's table.
Modern day periodic tables are expanded beyond Mendeleev's initial 63 elements. Most of the current periodic tables include 108 or 109 elements. It is also important to notice how the modern periodic table is arranged. Although we have retained the format of rows and columns, which reflects a natural order, the rows of today's tables show elements in the order of Mendeleev's columns. In other words the elements of what we now call a 'period' were listed vertically by Mendeleev. Chemical 'groups' are now shown vertically in contrast to their horizontal format in Mendeleev's table. (reference)
It is also worthy to note that Mendeleev's 1871 arrangement was related to the atomic ratios in which elements formed oxides, binary compounds with oxygen whereas today's periodic tables are arranged by increasing atomic numbers, that is, the number of protons a particular element contains. Although we can imply the formulas for oxides from today's periodic table, it is not explicitly stated as it was in Mendeleev's 1871 table. The oxides ratio column was not shown in earlier Mendeleev versions.
Mendeleev rewrote each edition of Principles of Chemistry, including all new scientific data-particularly confirmations of the periodic law-and reanalyzing difficulties that had arisen to hinder its confirmation (inert gases, radioactivity, radioactive and rare-earth elements)"
Ramsay
The British physicist John William Strutt (better known as Lord Rayleigh) showed in 1892 that the atomic weight of nitrogen found in chemical compounds was lower than that of nitrogen found in the atmosphere. He ascribed this discrepancy to a light gas included in chemical compounds of nitrogen, while Ramsay suspected a hitherto undiscovered heavy gas in atmospheric nitrogen. Using two different methods to remove all known gases from air, Ramsay and Rayleigh were able to announce in 1894 that they had found a monatomic, chemically inert gaseous element that constituted nearly 1 percent of the atmosphere; they named it argon. The following year, Ramsay liberated another inert gas from a mineral called cleveite; this proved to be helium, previously known only in the solar spectrum. In his book The Gases of the Atmosphere (1896), Ramsay showed that the positions of helium and argon in the periodic table of elements indicated that at least three more noble gases might exist. In 1898 he and the British chemist Morris W. Travers isolated these elements—called neon, krypton, and xenon—from air brought to a liquid state at low temperature and high pressure. Working with the British chemist Frederick Soddy in 1903, Ramsay demonstrated that helium (together with a gaseous emanation called radon) is continually produced during the radioactive decay of radium, a discovery of crucial importance to the modern understanding of nuclear reactions. In 1910, using tiny samples of radon, Ramsay proved that it was a sixth noble gas, and he provided further evidence that it was formed by the emission of a helium nucleus from radium. This research demonstrated the high degree of experimental skill that Ramsay had developed, but it also marked his last notable scientific contribution. Intrigued by the new science of radiochemistry, he made many unsuccessful attempts to further explore the phenomenon.
Moseley
Mendeleev ordered his elements in order of their relative atomic mass, and this gave him some problems. For example, iodine has a lower relative atomic mass than tellurium, so it should come before tellurium in Mendeleev's table - but in order to get iodine in the same group as other elements with similar properties such as fluorine, chlorine and bromine, he had to put it after tellurium, so breaking his own rules.
Using atomic number instead of atomic mass as the organizing principle was first proposed by the British chemist Henry Moseley in 1913, and it solved anomalies like this one. Iodine has a higher atomic number than tellurium - so, even though he didn't know why, Mendeleev was right to place it after tellurium after all!
