Russian chemist Dmitri Mendeleev arranged the elements in the order of relative atomic mass. The modern periodic table still retains the overall structure devised by Mendeleev, with the addition of new elements.
Mendeleev published a periodic table in 1869 containing the elements known at the time. He observed patterns in the chemical and physical properties of elements that related in a periodic way to their atomic mass. This led to him arranging the elements in the periodic table in ascending order of their relative atomic mass.
Mendeleev's periodic table
Mendeleev's original periodic table features 63 elements arranged in groups. The elements with similar properties appear in vertical columns and the periods are arranged horizontally. There are gaps in Mendeleev's table because some elements were yet to be discovered. However, Mendeleev deduced the atomic mass and predicted the properties of the missing elements.
Modern periodic table
The modern periodic table features 118 elements, as listed by the American Chemical Society, with elements 113, 115, 117 and 118 without permanent names as of 2015. The modern version extends and refines Mendeleev's periodic table to reflect the discovery of new elements and theoretical models of chemical behavior. However, it still arranges the elements in familiar rows and columns.