Write the drawbacks in Mendeleev’s periodic table that led to its modification.

• Some elements do not fit to his classification that was based on the atomic weights. For example, cobalt was placed before nickel, tellurium before iodine etc.

• He violated his periodic law at certain places to assign the correct position to elements on the basis of their properties.

• The position of hydrogen was not defined correctly as it resembled few characteristics of alkali metals and few of halogens.

• Position of isotopes would be different according to Mendeleev’s periodic table.

• No separate positions were given to lanthanoids and actinoids.

• Position of group VIII elements was not justified.