Mendeleev arranged the elements in his periodic table ordered by atomic weight or mass.

He arranged the elements in periods and groups in order of their increasing atomic weight.

He placed the elements with similar properties in the same group.

However, he did not stick to this arrangement for long. He found out that if the elements

were arranged strictly in order of their increasing atomic weights, then some elements did

not fit within this scheme of classification.

Therefore, he ignored the order of atomic weights in some cases. For example, the atomic

weight of iodine is lower than that of tellurium. Still Mendeleev placed tellurium (in Group

VI) before iodine (in Group VII) simply because iodine’s properties are so similar

tofluorine, chlorine, and bromine.