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13th Mar 2020 @ 7 min read
Dmitri Mendeleev, a Russian chemist, is widely known for the development of the periodic table and is regarded by the father of the modern periodic table. He explicitly stated the periodic law: The properties of the elements are a periodic function of their atomic weights. Based on this principle, he not only corrected the properties of known elements but also accurately predicted the properties of undiscovered elements.
In the mid-1860s, Mendeleev began writing a book on chemistry. While working on his book The Principles of Chemistry, he accidentally discovered the periodic law. He observed that when the elements were arranged in the order of their atomic weights, the properties of elements would regularly repeat over an interval. He formulated his first periodic table by ordering the elements with atomic weights and grouping them based on their valency.
In 1869, his colleague Nikolai Menshutkin on the behalf of Mendeleev presented the paper The Dependence between the Properties of the Atomic Weights of the Elements to the Russian Chemical Society. The paper was published in the same year in the Russian language, and a German version of the paper consisting of the table and his eight remarks was circulated in Zeitschrift für Chemie.
His first table is given below.
|Ti = 50||Zr = 90||? = 180|
|V = 51||Nb = 94||Ta = 182|
|Cr = 52||Me = 96||W = 186|
|Mn = 55||Rh = 104.4||Pt = 197.4|
|Fe = 56||Ru = 104.4||Ir = 198|
|Ni = Co = 59||Pd = 106.6||Os = 199|
|H = 1||Cu = 63.4||Ag = 108||Hg = 200|
|Be = 9.4||Mg = 24||Zn = 65.2||Cd = 112|
|B = 11||Al = 27.4||? = 68||U = 116||Au = 197?|
|C = 12||Si = 28||? = 70||Sn = 118|
|N = 14||P = 31||As = 75||Sb = 122||Bi = 210?|
|O = 16||S = 32||Se = 79.4||Te = 128?|
|F = 19||Cl = 35.5||Br = 80||I = 127|
|Li = 7||Na = 23||K = 39||Rb = 85.4||Cs = 133||Tl = 204|
|Ca = 40||Sr = 87.6||Ba = 137||Pb = 207|
|? = 45||Ce = 92|
|?Er = 56||La = 94|
|?Y = 60||Di = 95|
|?In = 75.6||Th = 118?|
From the previous table, the elements were ordered from the top to bottom with increasing atomic weight. He grouped the elements according to their valency. For example, fluorine, chlorine, bromine, and iodine are placed in the same horizontal row. These elements have the valency equal to one and shared similar physical and chemical properties. This pattern that the elements in the same row have similar characteristics is what we called the periodicity of the elements.
The other thing to notice is the position of tellurium (Te = 128) in the table. Its position is interchanged with iodine (I = 127). Since iodine is lighter than tellurium, it should be placed immediately down to antimony (Sb = 122). Instead, Mendeleev reversed the order since he believed the atomic weight of tellurium was incorrectly measured and the iodine shared the properties with F, Cl, and Br, not with O, S, and Se. Based on this, he presumed the atomic weight of tellurium must be more than iodine. This partially came true. He was correct for the interchange of the position. In the modern periodic table, iodine is placed with other halogens in group 17, and tellurium is placed with O, S, and Se in group 16. But he was wrong for the prediction of the atomic weight of tellurium. The tellurium has an atomic mass of 127.6 u and iodine, 126.9 u. Even though tellurium is heavier than iodine, it is placed before iodine in the modern periodic table since the elements are ordered with the atomic number, not with the atomic weight, in the modern periodic table.
Mendeleev also stated the atomic weight of an element determined its properties, and the properties of an element could be predicted from its atomic weight. In the above table, there are two slots labeled with ?; one after aluminum (Al) and the other after silicon (Si). He predicted the existence of the two elements that filled these slots. He named them eka-silicon and eka-aluminum.
He refined his periodic table and published the new version in 1871. In this new table, the elements were ordered from the left to right similar to the modern periodic table.
|1||H = 1|
|2||Li = 7||Be = 9.4||B = 11||C = 12||N = 14||O = 16||F = 19|
|3||Na = 23||Mg = 24||Al = 27.3||Si = 28||P = 31||S = 32||Cl = 35.5|
|4||K = 39||Ca = 40||_ = 44||Ti = 48||V = 51||Cr = 52||Mn = 55||Fe = 56, Co = Ni = 59, Cu = 63|
|5||(Cu = 63)||Zn = 65||_ = 68||_ = 72||As = 75||Se = 78||Br = 80|
|6||Rb = 85||Sr = 87||?Y = 88||Zr = 90||Nb = 94||Mo = 96||_ = 100||Ru = Rh = 104, Pd = 106, Ag = 108|
|7||(Ag = 108)||Cd = 112||In = 113||Sn = 118||Sb = 122||Te = 125||I = 127|
|8||Cs = 133||Ba = 137||?Di = 138||?Ce = 140||_||_||_||_, _, _, _|
|10||_||_||?Er = 178||?La = 180||Ta = 182||W = 184||_||Os = 195, Ir = 197, Pt = 198, Au = 199|
|11||(Au = 199)||Hg = 200||Tl = 204||Pb = 207||Bi = 208||_||_|
|12||_||_||_||Th = 231||_||U = 240||_||_, _, _, _|
The elements with the similar characteristics were placed in the same column. As we can see from above table, he divided the columns into eight groups (group I to group VIII). The elements in each column shared similar physical and chemical properties and combined with other elements in the same molar proportion. For example, the elements under group I form oxides with the same chemical formula (R2O). Some of them are H2O, Li2O, Na2O, K2O. Also, one can see a number of empty slots; these were an indication of undiscovered elements.
He also revised an atomic weight of uranium to 240, which is very close to its present-day value of 238.03.
The primary reason that distinguishes Mendeleev from the rest of the scientists who contributed to the development of the periodic table was his accurate predictions of several unknown elements. The most accurate predictions were eka-boron (scandium), eka-aluminum (gallium), and eka-silicon (germanium), and the other predicted elements were eka-manganese (technetium), dvi-manganese (rhenium), eka-caesium (francium), eka-iodine (astatine), and eka-tantalum (dubnium).
The prefixes eka- and dvi- come from Sanskrit. It means one and two respectively. So, eka-aluminum is one place ahead of aluminum.
Gallium and germanium were discovered in the year 1875 and 1886 respectively. Mendeleev predicted their atomic weights as 68 and 72. The values are quite accurate to their present-day value of 69.72 and 72.63. He also foretold other properties of the two elements. They are listed in the following table.
|Property||Gallium or eka-aluminum||Germanium or eka-silicon|
|Density (g cm−3)||6.0||5.9||5.5||5.3|
|Melting point (°C)||Low||29.8||High||938|
|Formula of oxide||Ea2O3||Ga2O3||EaO2||GeO2|
|Formula of chloride||EaCl3||GaCl3||EaCl4||GeCl4|
Mendeleev also correctly predicted the atomic weight of eka-boron (scandium) and eka-manganese (technetium) as 44 and 100 respectively. The subsequent discoveries of these elements revealed their atomic mass as 44.96 u and 98 u.
Dmitri Mendeleev was born to Ivan Pavlovich Mendeleev and Maria Dmitrievna Mendeleeva in Siberia on 8 February 1834. He lived his childhood in financial constraints because his father lost his eyesight and became unemployed.
He became a professor at the Saint Petersburg Technological Institute in 1864. It was around this time that he started working on his chemistry book and accidentally found the periodic table. However, he was not the only one working on the periodic table. In 1870, after one year of Mendeleev, Lothar Meyer published a similar table. Like Mendeleev, Meyer also suggested the existence of undiscovered elements, but he hesitated to give any details on them. On the other hand, Mendeleev boldly foretold the characteristics of unknown elements, and they were proven reasonably accurate upon their discovery. Mendeleev's table was more scientific than all of his predecessors. This is why he is called the father of the modern periodic table.
He was nominated twice for the Nobel Prize but was never conferred. In 1955, the IUPAC officially named the element with the atomic number of 101 as mendelevium in honor of Mendeleev.
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