Mendeleev’s periodic law states that when elements are arranged in order of increasing atomic number, the physical and chemical properties of the elements are repeated periodically. This means that elements with similar properties tend to be grouped together in the periodic table, and that elements that have similar properties are arranged in vertical columns, called groups, in the periodic table. Mendeleev’s periodic law also predicts the properties of elements that had not yet been discovered at the time the law was proposed.