Mendeleev and the First Periodic Table

By the mid-1800s, chemists had discovered around 60 elements and had identified that many shared similar chemical properties. They suspected there was a way to organise the elements into a logical order that accounted for both their atomic properties and their chemical behaviour.

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Dmitri Ivanovich Mendeleev was a Russian chemist, known for his contribution to chemistry by creating the first periodic table of the elements. He organised the then-known elements in a table according to their relative atomic masses. This is because he believed that an element’s atomic mass defined its chemical properties.

Mendeleev released his periodic table in 1869. He arranged elements into rows based on their atomic mass, and into columns based on their chemical properties. The element with the smallest atomic mass was in the top left, and then for each row below, the atomic mass increased from left to right.

However, the table wasn’t quite as neat as Mendeleev would have liked. In some cases, there were no known elements in a given range of atomic masses that could fit into a certain column based on their chemical properties. He deduced that this was due to the existence of other elements that had not yet been discovered. To account for this, he left empty spaces in the table so that when new elements were discovered, they could be added – and this proved to be a powerful way of identifying new elements.

Undiscovered Elements

After creating the first periodic table of the elements, the chemist Dmitri Mendeleev was able to predict the existence and properties of elements that had not yet been discovered. He did this based on the sequences and properties of the first 60 elements contained in his table. When no known element had the right chemical properties and relative atomic mass to fit in a certain spot on his table, Mendeleev left it blank. His idea was that elements with these properties existed, but simply had not yet been discovered.

Mendeleev used the prefixes eka-, dvi-, and tri-, meaning one, two, and three, in addition to the name of existing elements with related properties to name these unknown elements. For example, Mendeleev predicted the existence of an element called eka-silicon based on a gap near silicon in his table and was able to estimate its atomic mass and chemical properties such as density and melting point. A corresponding element was later discovered with these predicted properties and was called germanium.

• Another example is eka-aluminium, which was one below aluminium on Mendeleev’s table) – Now named gallium

These gaps might at first have looked like a problem for Mendeleev’s periodic table. However, this ability to predict undiscovered elements turned his table from a sorting system into a useful scientific tool for discovery. By having a prior idea of what properties to expect, scientists could more easily isolate new elements.