Page 5 - 3. Lesson Notes Ch-5 MENDELEEVES CLASSIFICATION

• Limitation of Mendeleev’s Periodic Table:

1. Position of Hydrogen in the Mendeleev’s periodic table-

Hydrogen exhibits properties similar to alkali metals as well as halogens.

a. Alkali metals- Hydrogen has been placed in 1st group with alkali metals,
since hydrogen makes compound in the same way as alkali metals do.

a. Halogens- Hydrogen also exists as diatomic molecules like halogens and
combines with metals to form salts and non-metals to form covalent compounds.
For example- H2, Cl2, Br2, I2

2. Position of Isotopes:
Elements having same atomic number but different atomic masses are known as
isotopes.

So they must have different places allotted to them in this periodic table as
elements are arranged according to their atomic masses.

But no separate places are there to accommodate these isotopes of elements.

3. Wrong Order of Elements:
Mendeleev placed many elements in wrong order of their increasing atomic
masses in order to place elements having similar properties in similar group.

Example: The atomic mass of nickel is less than that of cobalt; in spite of that cobalt
is placed before nickel.
The atomic mass of Chromium is 50.20 and the atomic mass of vanadium is 50.94.
In spite of this, chromium is placed after vanadium.

In spite of above limitations and anomalies, the Mendeleev’s Periodic Table was
one of the wonderful discoveries.

At a glance-

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