The history of the periodic table is also a history of the discovery of chemical elements
The history of the periodic table is also a history of the discovery of chemical elements. Hennig Brand, a bankrupt German merchant was the first person to discover a new element. Brand tried to discover the Philosopher’s Stone — a mythical object that was supposed to turn inexpensive base metals into gold. In 1669 (or later), his experiments with distilled human urine resulted in the production of a glowing white substance, which he called “cold fire” (kaltes Feuer). He kept his discovery secret until 1680, when Robert Boyle rediscovered phosphorus and published his findings. The discovery of phosphorus helped to raise the question of what it meant for a substance to be an element.
Lavoisier’s Traité Élémentaire de Chimie (Elementary Treatise of Chemistry), which was written in 1789 and first translated into English by the writer Robert Kerr, is considered to be the first modern textbook about chemistry. Lavoisier defined an element as a substances that cannot be broken down into a simpler substance by a chemical reaction. This simple definition served for a century and lasted until the discovery of subatomic particles. Lavoisier’s book contained a list of elements or substance that Lavoisier believed could not be broken down further, which included oxygen, nitrogen, hydrogen, phosphorus, mercury, zinc and sulfur, which formed the basis for the modern list of elements. Lavoisier’s list also included light and caloric. Lavoisier had classified the element into metals and non-metals.
In 1815, the English physician and chemist William Prout noticed that atomic weights seemed to be multiples of that of hydrogen. In 1828,Jons Jacob Berzelius, a Swedish chemist complied a table of relative atomic weights, where oxygen was set to 100, and which included all of the elements known at the time. Of significance for the periodic table is that he invented the modern system of chemical notation and established the basic symbols for the elements as used today. Students working in Berzelius laboratory are credited with discovering lithium, and vanadium. Other elements attributed to Berzelius are silicon, selenium, thorium, and cerium.
Johann Wolfgang Döbereiner, a chemist, began to formulate one of the earliest attempts to classify the elements. In 1829, he found that he could form some of the elements into groups of three, with the members of each group having related properties. He termed these groups triads. He identified groups of three chemically similar elements that today we understand fall in the same group of the periodic table.
Alexandre-Emile Béguyer de Chancourtois, a French geologist, was the first person to notice the periodicity of the elements. He devised an early form of periodic table, which he called the telluric helix. With the elements arranged in a spiral on a cylinder by order of increasing atomic weight, de Chancourtois saw that elements with similar properties lined up vertically. His chart included some ions and compounds in addition to elements.
John Newlands was an English chemist who wrote a paper in 1863 that classified the 56 elements that had been discovered at the time into 11 groups which were based on similar physical properties. He noted that many pairs of similar elements existed which differed by some multiple of eight in atomic weight. Newlands took Döbereiner’s ideas and expanded on them. He also organized his elements by mass and property, but he added a twist. Döbereiner had worked only in small groups, but Newlands wanted to relate all the elements to each other.
Newlands arranged the known elements in a table by atomic weights. In doing so, he noticed some recurring patterns, and the patterns were such that if he broke up his list of elements into groups of seven, the first elements in each of those groups were similar to one another, as was the second element in each group, and the third, and so on. By analogy with the tonic musical scale of seven notes, which form octaves, he called his discovery the Law of Octaves. Newlands also noticed that silicon and tin formed part of a triad and so predicted a third unknown element with atomic weight of about 73, anticipating Mendeleev’s prediction of germanium by six years, but did not leave a space for the new element in his table.
In 1864, Julius Lothar Meyer published a preliminary list of 28 elements classified into 6 families by their valence—this was the first time that the elements had been grouped and ordered according to their valence. Work on organizing the elements by atomic weight had hitherto been stymied by inaccurate measurements of the atomic weights.
Dmitri Mendeleev was a Siberian-born Russian chemist. Mendeleev was investigating the variation in the chemical properties of the elements (see image at right) and noticed their periodic variation. He arranged the elements in a table ordered by atomic mass.
Mendeleev’s work stated
1. The elements, if arranged according to their atomic weights, exhibit an apparent periodicity of properties.
2. Elements which are similar as regards to their chemical properties have atomic weights which are either of nearly the same value (e.g., Pt, Ir, Os) or which increase regularly (e.g., K, Rb, Cs).
3. The arrangement of the elements, or of groups of elements in the order of their atomic weights, corresponds to their so-called valencies, as well as, to some extent, to their distinctive chemical properties; as is apparent among other series in that of Li, Be, Ba, C, N, O, and Sn.
4. The elements that are the most widely diffused have small atomic weights.
5. The magnitude of the atomic weight determines the character of the element, just as the magnitude of the molecule determines the character of a compound body.
6. We must expect the discovery of many as yet unknown elements–for example, elements analogous to aluminium and silicon whose atomic weight would be between 65 and 75.
7. The atomic weight of an element may sometimes be amended by knowledge of those of its contiguous elements. Thus, the atomic weight of tellurium must lie between 123 and 126, and cannot be 128.
8. Certain characteristic properties of elements can be foretold from their atomic weights.
On the evening of April 19, 1894, William Ramsay attended a lecture given by Lord Rayleigh. Rayleigh had noticed a discrepancy between the density of nitrogen made by chemical synthesis and nitrogen isolated from the air by removal of the other known components. After a short discussion he and Ramsay decided to follow this up. By August Ramsay could write to Rayleigh to announce that he had isolated a heavy component of air previously unknown which did not appear to have any obvious chemical reactivity. He named the gas “argon.” In the years that followed he discovered neon, krypton, and xenon. He also isolated helium which had been observed in the spectrum of the sun but had not been found on earth. In 1910 he also isolated and characterized radon.
These noble gasses were initially incorporated into Mendeleev’s table as a zero group and placed before the group I elements of each period.
In 1913, Henry Moseley found a relationship between an element’s X-ray wavelength and its atomic number. Before this discovery, atomic numbers were just random numbers based on an element’s atomic weight. Moseley’s discovery showed that atomic numbers were not arbitrary but had an experimentally measurable basis. Mosley’s research demonstrated that the ordering of elements in the table should be according to atomic number not atomic weight.
In 1941 Glen Seaborg is credited with discovering and isolating plutonium, and subsequently the transuranic elements from atomic number 94 through 102. His work in the transuranic elements led to a reconfiguration of the periodic table where he placed the actinide series below the lanthanide series at the bottom of the table, to give the periodic table the look it has today.