Periodni sustav elemenata je po imenu i naravi periodičan, no njegova se periodičnost ne može jednostavno, matematički definirati. Uz osnovnu periodičnost po skupinama uočavaju se i druge vrste pravilnosti, poput dijagonalne sličnosti, sličnosti među skupinama (n) i (n + 10) te povezanosti elemenata “konjićevim skokom”. Složenost PSE-a odražava se i u njegovim mnogobrojnim i raznovrsnim grafičkim prikazima (“tablicama”) koji naglašavaju specifične odnose među elementima.The periodic system of the elements is periodic, but its periodicity is far from simplicity, i.e. the properties of the elements and their compounds cannot be reduced to the “algebraic, periodic function of the values of atomic weights of the elements”, as Mendeleev thought. Besides the well-known trends along the columns (groups) of periodic system, there are also secondary trends such as diagonal behaviour (e.g. similarity of Li to Mg, of Be to Al, and of B to Si), similarities between groups (n) and (n + 10), secondary periodicity inside the group, and knight’s move relationship. These complex relationships are reflected in many attempts at an “ideal” periodic table
