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 — an object that is supposed to turn silver 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.
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 similar elements 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, 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 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.
Mendeleev's paper was published only a few months before an independent paper by a German chemist, Julius Lothar Meyer, who had refined a more primitive table that he had originally drawn up in 1864. An English chemist, William Odling, also drew up a table that is remarkably similar to that of Mendeleev in 1864. Mendeleev predicted the discovery of other elements and pointed out that some of the then-current atomic weights were incorrect. He provided for variance from atomic weight order, left space for new elements, and predicted three undiscovered elements. His table did not include any of the noble gases, which hadn't been discovered.
Henry Moseley
In 1913, Henry Moseley found a relationship between an element's X-ray wavelength and its atomic number. Previous to this, 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.
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