Periods of the periodic table. Animation of the periodic table, showing the seven periods of the modern 118-element periodic table. The seven periods are highlighted in succession. The number of occupied electron shells of an element corresponds to its period number in the periodic table. The outer electron shells fill up moving from left to right across the period, and this largely accounts for the trends in the physical and chemical properties of the elements. Period 1 has two elements, periods 2 and 3 have eight elements each, and periods 4 and 5 have eighteen elements each. The final two periods (6 and 7) are more extensive (each have thirty-two elements) and are shown expanded across bottom forming the lanthanides (part of period 6) and the actinides (part of period 7).