In the modern periodic table, elements are arranged by atomic number. They are placed with the lowest atomic number first, and elements with increasing atomic numbers run to the right. They are also arranged in vertical columns known as groups, with these groupings based on shared properties.
Each horizontal line of elements on the modern periodic table is known as a period. The setup of the numbering system is arranged by the International Union of Pure and Applied Chemistry. Former periodic table arrangements included grouping elements into triads with similar features and by ordering the elements according to their mass.