This is because hydrogen's characteristics are very similar to halogens and alkali metals. Other keylimitationsinMendeleev periodic tablewere that the atomic number in thetablewas not shown increasing in a regular pattern and the symmetry of thetablewas also disturbed.

(i) The position of hydrogen was not correctly defined. It was placed in Group I although its properties resembled both the Group I elements (the alkali metals) and the group VII elements (the halogens).

(ii) In some cases Mendeleev placed elements according to their similarities in properties and not in increasing order of their atomic masses, while some dissimilar elements were grouped together. Thus, the position of these elements was not justified.Example: Copper was placed in group I although it did not resemble the elements of this group.

(iii) In certain pairs of elements like, Ar (40) and K (39); Co (58.9) and Ni (58.6); Te (127.6) and I (126.9) the arrangement was not justified. For example, argon was placed before potassium whereas its atomic mass is more than potassium.

(iv) Isotopes have not been given separate places in the periodic table.

(v) Fourteen elements that follow lanthanum called lanthanides and fourteen elements following actinium called actinides were not given proper places in Mendeleev's periodic table.

(vi) Mendeleev's table was unable to explain the cause of periodicity among elements.