 Several exceptions, however, do exist, such as that of ionization energy in group 3, The electron affinity trend of group 17, the density trend of alkali metals aka group 1 elements and so on.The periodic trends are based on the Periodic Law, which states that if the chemical elements are listed in order of increasing atomic number, many of their properties go through cyclical changes, with elements of similar properties recurring at intervals.For example, after arranging elements in their increasing atomic numbers, many of the physical and chemical properties of Lithium, such as its vigorous reactivity with water, recur in sodium, potassium and caesium.