The periodic table is the ultimate infographic. It presents the fabric of the Universe (at least the bits of it we can see) as 118 units, and we can learn a lot about these units – the chemical elements – just by looking at where they sit among the rest.The elements are substances that cannot be refined or purified into simpler ingredients. Each one is unique, with a set of physical and chemical properties that arises from the structure of its atoms. In 1869, Dmitri Mendeleev, a Russian chemist, created the periodic table as a way of organizing the known elements (around half of what is there today) into a system that related the increasing weights of each element with patterns in their chemical properties.Although he did not know it, Mendeleev’s system was based around the varying atomic structures of the elements. He was working 30 years before the first subatomic particle, the electron, was discovered, and 60 years before researchers had drawn a full picture of how atoms are constructed from smaller particles. Nevertheless, that picture revealed why the periodic table works so well. Every element has a unique collection of subatomic particles – electrons, protons and neutrons – and the way these particles are arranged gives an element its set of characteristics.