Question

The modern periodic table has been evolved through the early attempts of Dobereiner, Newland and Mendeleev. List one advantage and one limitation or all the three attempts.

Answer 1

Dobereiner's triads:

Advantage: It made chemists look at elements in terms of groups of elements with similar properties. This led to rigorous classification of elements.

Limitation: Dobereiner could identify only three triads from the elements known at that time. Eg: for F, Cl, Br. The atomic mass of Cl is not the arithmetic mean of atomic masses of F and Br. 

Newland's Law of Octaves:

Advantage: This law provided a basis for the classification of element into groups of elements having similar properties.

Limitation: Newland assumed that only 56 elements existed in nature, and no more elements would be further discovered in the nearer future. But later on several new elements were discovered, whose properties could not be defined as per the Law of Octaves.

Mendeleev's Periodic Table:

Advantage: He correctly predicted the properties of undiscovered elements .e.g. scandium, germanium, gallium.

Limitation: Some elements having higher atomic masses were placed before those having lower atomic masses. (Co with higher atomic mass was placed before Ni having lower atomic mass)