In 1869 the Russian chemist Dmitri Mendeleev tried to bring order to the elements that were known at the time. There were only about 60 of them. Nothing was known yet about the protons in the nucleus, so he had to use things that could be measured: the relative atomic mass of each element, and the properties of the elements and of their compounds.
He arranged the elements in order of increasing relative atomic mass. When the properties of the elements began to repeat, he started a new row. This lined up elements with similar properties in the same vertical column. For example, lithium, sodium and potassium are all soft, reactive metals that form chlorides with similar formulae, so he put them in the same column, with sodium under lithium.
Mendeleev did not rely on appearance alone. He also compared the formulae and behaviour of compounds, such as the oxides and chlorides, to decide which column an element belonged in.