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The Periodic Table explained

It’s one of the few scientific tools available on mugs and t-shirts, but where did the periodic table come from?

Classifying the chemical elements has become easier the more we have discovered about them. For example, the discovery that different elements bond in different ratios meant that chemicals could be grouped according to this property (known as “valency”). But the remarkable thing about the first periodic table, designed by Dmitri Mendeleev in the 1860s, is that it predicted the properties of chemicals that hadn’t yet been discovered.

Supposedly inspired by the card game Patience, Mendeleev arranged the elements in rows by their atomic weight, starting a new row every time an element with similar properties came round so that each column contained elements with similar properties. This repetition of properties is the “period” in the “periodic table”.

The Periodic Table explained

This approach was similar to existing ways of arranging the elements – but Mendeleev went further. When it seemed like an element was missing because the next in the sequence did not have the right properties, he left a gap – and these gaps have since been filled in with newly-discovered elements, which behave the way the table says that they should.

Secondly, he didn’t always stick to ordering the elements by their atomic weight: if it made the properties line up better, he switched them around. Without realising it, he ended up putting them in order of their atomic number – the number of protons in the atom – long before the structure of atoms was understood well enough to explain why.

Modern periodic tables contain more information: they are coloured to show different chemical groups, like alkali metals and noble gases, and they list chemical symbols and whether an element is a solid, liquid or gas at standard temperature and pressure. But in principle, not much has changed since Mendeleev and his deck of cards.