This table was helpful in the prediction of properties of elements as there was a regular gradation in properties of elements across periods and groups. The group numbers of the periodic table indicates the highest oxidation state of elements. Mendeleev’s left some vacant positions which later filled with newly discovered elements. These vacant positions provided the idea about the chemical and physical properties of those new elements. For example Eka-Boron,
Eka-Aluminium and Eka-silicone were later discovered as Sc, Ga and Ge respectively

the isotopes and no separate position also as they have different atomic mass but same atomic number. Since the change in atomic mass of two successive elements was not constant therefore it was difficult to predict the number of missing elements with the help of atomic number only. 
the atomic masses of all elements were not known therefore it was difficult to place the correct element in proper group of the periodic table such as Mendeleev’s placed iodine after tellurium and potassium and Ni after Co which was not following the periodic law but was following the order of properties      

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Advantages and Disadvantage of Mendeleev's PT >>>>>

Advantages--- Empty spaces for undiscovered element. 

ii)- It was based on atomic mass concept. Elements were placed acc. to their increasing order of mass.

Disadvantage - i) Wrong position of Hydrogen, ii) No place for the noble gases., iii)Probelm of inverted pair.

Advantages of Modern PT.>>>>>

>Physical and chemical properties of the element are the periodic function of their atomic no.
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