History of the periodic table
In the beginning
People have known about basic chemical elements such as gold, silver, and copper from antiquity, as these can all be discovered in nature in native form and are relatively simple to mine with primitive tools. Aristotle, a philosopher, theorised that everything is made up of a mixture of one or more of four elements. They were fire, air, earth, and water. He also theorised that they change into new substances to form what we see.
Hennig Brand was the first person to discover a new element. Brand was a bankrupt German merchant who was trying to discover the Philosopher's Stone which was ultimately discovered by Harry Potter, — an object that is supposed to turn inexpensive metals into gold. He experimented with distilling human urine until in 1669 he finally obtained a glowing white substance which he named phosphorus. He kept his discovery secret, until 1680 when Robert Boyle rediscovered it and it became public.
By 1809, a total of 47 elements had been discovered. As the number of known elements grew, scientists began to recognize patterns in the way chemicals reacted and began to devise ways to classify the elements.
Antoine-Laurent de Lavoisier
Lavoisier's Traité Élémentaire de Chimie (Elementary Treatise of Chemistry, 1789, translated into English by Robert Kerr) is considered to be the first modern chemical textbook. It contained a list of elements, or substances that could not be broken down further, which included oxygen, nitrogen, hydrogen, phosphorus, mercury, zinc, and sulphur. It also forms the basis for the modern list of elements. His list, however, also included light and caloric, which he believed to be material substances. While many leading chemists of the time refused to believe Lavoisier's new revelations, the Elementary Treatise was written well enough to convince the younger generation.
This model only classified elements into metals and non-metals and thus was not accepted.
Law of triads
In 1817, Johann Döbereiner noticed that strontium had similar properties to calcium and barium, and that its atomic weight fell between them. He placed these three elements into a group, which he called a triad.
After compiling these, Döbereiner proposed that nature was made of triads of elements. He inferred that in a triad of elements, the middle element had the atomic weight of the average of the top and bottom elements' atomic weights. From this law, Döbereiner went on to discover the halogen triad composed of chlorine, bromine, and iodine and the alkali metal triad of lithium, sodium, and potassium.
This idea of triads became a popular area of study. Following Döbereiner's work, a number of scientists subsequently discovered that chemical relationships can extend beyond triads. During this time, fluorine was added to the halogen group and oxygen, sulfur, selenium, and tellurium were grouped into a family, forming two tetrads; and nitrogen, phosphorus, arsenic, antimony, and bismuth were recognised as forming a pentad.
Alexandre-Émile Beguyer de Chancourtois
Alexandre-Émile Béguyer de Chancourtois, a French geologist, was the first person to notice the periodicity of the elements — similar elements seem to occur at regular intervals when they are ordered by their atomic weights. 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 paper was published in 1862, but used geological rather than chemical terms and did not include a diagram; as a result, it received little attention until the work of Dmitri Mendeleev.
John Newlands' Octaves
John Newlands was an English chemist who wrote a paper in 1863 which 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.
Newlands' work was heavily criticised, even ridiculed, by other chemists, because of the lack of spaces for undiscovered elements and the placing of two elements in one box, but he was finally awarded the Davy Medal by the Royal Society in 1887.
The first periodic table
Dmitri Mendeleev, also spelt Dmitry Mendeleyev, middle name (patronymic) Ivanovich, a Siberian-born Russian chemist, was the first scientist to make a periodic table much like the one we use today. Mendeleev arranged the elements in a table ordered by atomic mass. On March 6, 1869, a formal presentation was made to the Russian Chemical Society, entitled The Dependence Between the Properties of the Atomic Weights of the Elements. His table was published in an obscure Russian journal but quickly republished in a German journal, Zeitschrift für Chemie, in 1869. It stated
- The elements, if arranged according to their atomic weights, exhibit an apparent periodicity of properties.
- 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).
- 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.
- The elements which are the most widely diffused have small atomic weights.
- 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.
- We must expect the discovery of many as yet unknown elements–for example, elements analogous to aluminum and silicon–whose atomic weight would be between 65 and 75.
- The atomic weight of an element may sometimes be amended by a knowledge of those of its contiguous elements. Thus the atomic weight of tellurium must lie between 123 and 126, and cannot be 128.
- Certain characteristic properties of elements can be foretold from their atomic weights.
Advantages
- Mendeleev predicted the discovery of other elements and left space for these new elements, namely eka-silicon (germanium), eka-aluminum (gallium), and eka-boron (scandium). Thus, there was no disturbance in the periodic table.
- He pointed out that some of the then current atomic weights were incorrect.
- He provided for variance from atomic weight order
Drawbacks
- There was no place for the isotopes of the various elements.
- His table did not include any of the noble gases, which hadn't been discovered. But these could be added in another group without any disturbance to the periodic table.
Unknown to Mendeleyev, Lothar Meyer was also working on a periodic table. In his work published in 1864, Meyer presented only 28 elements, classified not by atomic weight but by valence alone. Also, Meyer never came to the idea of predicting new elements and correcting atomic weights. Only a few months after Mendeleev published his periodic table of all known elements (and predicted several new elements to complete the table, plus some corrected atomic weights), Meyer published a virtually identical table. Some people consider Meyer and Mendeleyev the cocreators of the periodic table, although most agree that Mendeleyev's accurate prediction of the qualities of what he called lands him the lion's share of credit. In any case, at the time Mendeleyev's predictions greatly impressed his contemporaries and were eventually found to be correct. An English chemist, William Odling, also drew up a table that is remarkably similar to that of Mendeleev in 1864.
Henry Moseley
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 also showed that there were gaps in his table at atomic numbers 43 and 61 which are now known to be radioactive and not naturally occurring. Following in the footsteps of Dmitri Mendeleev, Henry Moseley also predicted new elements.