In Mendeleev’s periodic table, the elements are arranged in the increasing order of their atomic masses but some pairs of elements were placed such that elements with higher atomic masses were placed prior to those with lower atomic masses.Example:Ar (Z = 18, At. Wt. = 40) precedes K (Z = 19, At.wt. = 39.0)Te (Z = 52, At.wt. = 127.6) precedes I (Z = 53, At.wt = 126.9)Co (Z = 27, At. Wt. = 59.9) precedes Ni (Z = 28, At.wt. = 58.6)Th (Z = 90, At. Wt. = 232.12) precedes Pa (Z = 91, At.wt. = 231)Mendeleev has to arrange these elements in this order inevitably since otherwise by reversing their position they would fall into the groups with elements having altogether different properties.