\tElements are arranged in the periodic table in the increasing order of their relative atomic masses.\tMendeleev divided his periodic table in eight groups and seven periods.\tGroups from I to VII are meant for normal elements and group VIII is for transition elements.\tGroups from I to VII have been divided in two sub groups, while group VIII is meant for three elements.\tPeriods from 4th to 7th have been divided in two series: 1st series and 2nd series.\tElements having similar properties have been kept in the same group. For example; lithium, potassium, rubidium, etc. are in 1st group.