A chemical element is a substance made up of one type of atom. The known chemical elements are arranged on a periodic table in order of their atomic numbers, which represent the number of protons in one atom of an element.
The first periodic table of the elements was created by Dmitri Mendeleev in 1869. Unlike the modern periodic table, Mendeleev's table was arranged by atomic weight, which represents the average mass of the isotopes in an element based on their frequency.
The modern periodic table arranges the elements into rows called periods and columns called groups. The periods show the number of energy levels an element has, and groups show the number of valence electrons in the outer shell of an element.