Question

What are
the
limitation of
Mendeleav's periodic.
table ?​

Answer 1

1.Elements with large differences in properties were included in the same group. ...

2.No proper position could be given to the element hydrogen. ...

3.The increasing order of atomic mass was not strictly followed throughout.

Answer 2

IN SHORT  :

i) The positions of the isotopes could not be explained

ii) Wrong order of atomic masses of some elements could not be explained

iii) A correct position could not be assigned to hydrogen in the periodic table

SAME THING BUT IN DETAIL :

i) The positions of the isotopes could not be explained

Isotopes are the atoms of the same element having similar chemical properties but different atomic masses. If the elements are arranged according to atomic masses, the isotopes should be placed in different groups of the periodic table (because they have different atomic masses). The isotopes were not given separate places in Mendeleev's periodic table.

for example the placing of these two isotopes of chlorine (having different atomic masses) in the same group of the periodic table could not be explained by  Mendeleev's periodic law.

ii) Wrong order of atomic masses of some elements could not be explained

according to Mendeleev's periodic law, the elements are arranged in the order of increasing atomic masses. so, the element with lower atomic mass should come first and the element with higher atomic mass should come next. The element Cobalt having higher atomic mass of 58.9 comes first and nickel element with slightly lower atomic mass of 58.7 comes later. Mendeleev's periodic law could not explain this abnormal situation of wrong order of atomic masses.

iii) A correct position could not be assigned to hydrogen in the periodic table

In Mendeleev's periodic table, Hydrogen (H) has been placed in group 1 with alkali metals. This is because like alkali metals , (say, sodium), Hydrogen also combines with halogens (chlorine, etc), oxygen and sulphur to form compounds having similar formulae. This means that Hydrogen resembles alkali metals in some of the properties.

Hydrogen also resembles halogens (fluorine, chlorine and bromine) in some of the properties. For example, just like halogens (F2, Cl2 and Br2), hydrogen also exist in the form of diatomic molecules (H2). Moreover, just like halogens, hydrogen combines with certain metals to form ionic compounds (called Hydrides) and reacts with non-metals to form covalent compounds. All these properites show that hydrogen could also be placed in group 7 of halogen elements.

From the above discussion we conclude that, on the basis of its properties, hydrogen element could be placed in alkali metal group as well as in halogen group. Thus, Mendeleev's periodic law could not assign a correct position to hydrogen in the periodic table.

(THE POINTS I HAVE USED ABOVE ARE FROM A GUIDE BOOK)