Mendeleev鈥檚 periodic table
Dmitri Mendeleev
Dmitri Mendeleev (1834-1907) was a Russian chemist. In the 19th century, he was one of several scientists who were looking for ways to organise the known elementA substance made of one type of atom only.. Mendeleev published his first periodic table of the elements in 1869.
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Features of Mendeleev鈥檚 tables
Mendeleev arranged the elements in order of increasing relative atomic massThe mean relative mass of the atoms of the different isotopes in an element. It is the number of times heavier an atom is than one-twelfth of a carbon-12 atom.. He then considered the propertiesThe characteristics of something. In chemistry, chemical properties include the reactions a substance can take part in. Physical properties include colour and boiling point. of the elements and their compoundA substance formed by the chemical union of two or more elements.. Mendeleev found that this allowed him to arrange elements with similar properties below each other into groups. However, to make this work, he had to:
- leave gaps for yet to be discovered elements
- swap the order of a few pairs of elements
Mendeleev鈥檚 1869 table showed groups of elements in horizontal rows, but from 1871 he showed groups of elements in vertical columns.
Predictions using gaps
Mendeleev left gaps in his table to place elements not known at the time. By looking at the chemical propertiesA description of how a substance reacts with other substances. For example, flammability, pH, reaction with acid, etc. and physical propertiesA description of the appearance of a substance or how it acts without involving chemical reactions. For example, state, melting point, conductivity, etc. of the elements next to a gap, he could also predict the properties of these undiscovered elements. For example, Mendeleev predicted the existence of 鈥榚ka-silicon鈥, which would fit into a gap next to silicon. The element germanium was discovered later. Its properties were found to be similar to the predicted ones, which supported the ideas behind Mendeleev鈥檚 periodic table.
Pair reversals
Iodine has a lower relative atomic mass than tellurium. So iodine should be placed before tellurium in Mendeleev鈥檚 tables. However, iodine has similar chemical properties to chlorine and bromine. To make iodine line up with chlorine and bromine in his table, Mendeleev swapped the positions of iodine and tellurium.