Why do the chemical properties cycle through each period? every eighth element starting with hydrogen restarts the transition from metal to inert gas. elements with atomic numbers which are multiples of eight start a new period. the outermost shell of all atoms with two or more electron shells will not have more than 8 electrons. each period represents a cycle of completing the outer electron shell.

The answer is that; the outermost shell of all atoms with two or more electron shells will not have more than 8 electrons. 
Elements in the same group in the periodic table have similar chemical properties. This is because their atoms have the same number of electrons in the outermost energy level. On the other hand elements in the same period have the same number of energy levels and differ in chemical properties since they belong to different groups.