The mystery of positions in the periodic table was not clarified until the 20th century – 06/29/2021 – Marcelo Viana

All substances in nature consist of around a hundred basic components, the chemical elements. For example, water molecules are made up of two atoms of the element hydrogen and one of oxygen.

The “Elementary Treatise on Chemistry”, published in 1789 by the Frenchman Antoine Lavoisier (1743–1794), the founder of chemistry, lists 33 elements (some incorrectly). Much was done over the next century to expand, understand, and organize the list.

In 1869, Russian chemist Dmitry Mendeleev (1834-1907) suggested listing the elements in ascending order of the weight of their atoms in a table with seven columns, noting that elements in the same column had similar chemical and physical properties.

But it didn’t work that well, Mendeleev had to “overbuild” a bit. In some cases, he reversed the order of the atomic weights: he placed iodine after tellurium, although it was lighter, so it would stay in the bromine column, whose properties are similar. Another time he left empty boxes in the table and stated that they corresponded to elements that were still unknown: Under aluminum and silicon there would be an “Eka-Aluminum” and an “Eka-Silicium”.

None of this was new, but Mendeleev’s periodic table, especially the revised version he published in 1871, summed up the facts very happily, and even its imperfections proved extremely fruitful to the advancement of science.

The periodic table not only predicted the existence of new elements, but also described their properties, which were critical to their identification. Eka-aluminum (gallium) and eka-silicon (germanium) were isolated in the laboratory in 1875 and 1886, respectively.

Another mystery concerned the position of each element in the table: hydrogen comes first, oxygen eighth, iron 26. If the order does not always correspond to the weight of the atom, what does this number mean? This mystery would not be cleared until the discovery of subatomic particles at the beginning of the 20th century.

In the following decades the periodic table underwent some adjustments and additions: neutral gases, rare earths, synthetic elements. But the big question was: why? Why is the periodic table the way it is? This is where math comes in, as we’ll see next week.

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