In 1869, the Russian chemist Dmitri Mendeleev first proposed that the chemical elements exhibited a “periodicity of properties.”

Mendeleev had tried to organize the chemical elements according to their atomic weights, assuming that the properties of the elements would gradually change as atomic weight increased.

What he found, however, was that the chemical and physical properties of the elements increased gradually and then suddenly changed at distinct steps, or periods.  To account for these repeating trends, Mendeleev grouped the elements in a table that had both rows and columns.

The periodic table we use today is based on the one devised and published by Dmitri Mendeleev in 1869.

Mendeleev found he could arrange the 65 elements then known in a grid or table so that each element had:

1. A higher atomic weight than the one on its left. For example, magnesium (atomic weight 24.3) is placed to the right of sodium (atomic weight 23.0):

na mg

2. Similar chemical properties to other elements in the same column – in other words similar chemical reactions. Magnesium, for example, is placed in the alkali earths’ column, with other elements whose reactions are similar:

be sr

Mendeleev realized that the table in front of him lay at the very heart of chemistry. And more than that, Mendeleev saw that his table was incomplete – there were spaces where elements should be, but no-one had discovered them.

Just as Adams and Le Verrier could be said to have discovered the planet Neptune on paper, Mendeleev could be said to have discovered germanium on paper. He called this new element eka-silicon, after observing a gap in the periodic table between silicon and tin:

si sn

Similarly, Mendeleev discovered gallium (eka-aluminum) and scandium (eka-boron) on paper, because he predicted their existence and their properties before their actual discoveries.


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