# Why is the periodic table like this? – 06.07.2021 – Marcelo Viana

In 1869, Dmitri Mendeleev (1834–1907) suggested listing the chemical elements in a table with seven columns in ascending order according to the weight of their atoms, and found that in this way elements in the same column have similar properties. The Periodic Table of the Elements directed much of the advancement of chemistry in the decades that followed as it went through several adaptations.

The first resulted from the discovery of an eighth column, made up of gases such as helium and neon, which hardly react with other elements called neutral gases. This made the table a little more complicated: two elements (hydrogen and helium) in the first row and eight elements each in the following.

In 1911 the New Zealander Ernest Rutherford (1871 – 1937) proposed a new model for the structure of the atom: a massive atomic nucleus with a positive electrical charge (later attributed to the presence of a certain number of positively charged particles, the protons) of an equal number orbited by electrons with a negative charge.

In the same year, the Dutchman Antonius van den Broek (1870-1926) suggested that the position of each element in the periodic table is determined by the charge on the atomic nucleus, the so-called atomic number, and not really by the weight of the atom.

It also became clear that the following rows in the table had to be rearranged: Currently, rows four and five, which correspond to ordinal numbers 19 to 54, are represented with 18 columns: the additional 10 columns contain the metals. And the discovery of the rare earths (lanthanides) with the ordinal numbers 57 to 70 made it even more complicated: Rows six and seven now have 32 columns.

In short, 2 items in row one; 8 on two and three; 18 on four and five; and 32 at six and seven. Because? Here’s a crazy observation: they’re always even numbers, and if we divide by two we get the sequence 1, 4, 9, 16 of perfect squares. Coincidence?

The answer came with the development of the quantum model of the atom and the mathematical resolution of that model. As is so often the case in the history of science, the mathematical ideas for this have long been developed, especially by Jacques Sturm and Joseph Liouville in the 1830s, and next week it will be the last installment in the series.

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