Mendeleev's periodic table
Dmitri Mendeleev
Dmitri Mendeleev was a Russian chemist. Like many scientists working at the end of the 19th century, he was 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. He then improved it.
Learn more on the periodic table in this podcast.
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Features of Mendeleev's table
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.. When he did this, he noted that the chemical propertiesA description of how a substance reacts with other substances. For example, flammability, pH, reaction with acid, etc. of the elements and their compoundA substance formed by the chemical union of two or more elements. showed a periodic trendA pattern in chemical or physical properties of elements in the periodic table.. He then arranged the elements by putting those with similar properties below each other into groups. To make his classification work, Mendeleev made a few changes to his order:
- he left gaps for yet to be discovered elements
- he switched the order of a few elements to keep the groups consistent
Predictions using gaps
Mendeleev left gaps in his table to place elements not known at the time. By looking at the chemical properties 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 'eka-silicon', 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. The other missing elements such as gallium and scandium were also then discovered, again with properties matching Mendeleev's predictions. This increased the confidence of other scientists in Mendeleev's periodic table, which was then developed further over the years.
Pair reversals
Iodine has a lower relative atomic mass than tellurium. So iodine should be placed before tellurium in Mendeleev's 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.
