Dmitri Ivánovich Mendeléiev was a Russian chemist who discovered a pattern that made it possible to order all chemical elements (natural or artificial) according to their atomic mass and chemical characteristicas. Thanks to the periodic nature of the values of the elements in their positions on the table, it was possible to predict the properties and atomic weight of some elements not yet discovered.
Mendeléiev (1834 - 1907) first became interested in chemistry after attending a German congress in 1960. At the time, there was no consensus regarding the formulae of known elements, although it was known that they shared similar properties.
In 1960, the English chemist John Newlands discovered that, when you order elements according to their atomic weights, the resulting table presents a periodicity. This meant that some similar characteristics were reproduced at regular intervals.
Mendeléiev detected errors in Newlands' model and grouped the elements according to their shared properties: horizontal rows in ascending order according to their atomic weight and vertical columns according to their chemical characteristics. That is how he created his table with 63 elements, leaving gaps for elements not yet identified but corresponding to those positions according to their properties.
This way, it was possible to predict the behaviour and characteristics of unknown or unclassified elements.
Three years later, the gaps were filled thanks to the discovery of gallium, scandium and germanium. They all presented the characteristics Mendeléiev had predicted.
The periodic table has been modified multiple times since its creation. The original 63 elements are now 109.
The periodic table of elements is a key tool to learn about the properties and behaviour of the matter that surrounds us and conforms our universe.