(a) The modern periodic table has been evolved through the early attempts of Dobereiner, Newland and Mendeleev. List one advantage and one limitation of all the three attempts.
(b) Name the scientist who first of all showed that atomic number of an element is a more fundamental property than its atomic mass.
State Modern periodic law.

Explanation

(a) Dobereiner's classification
Advantage. He was the first scientist to show the relationship between the properties of elements and their atomic masses.
Disadvantage . He failed to arrange all the then known elements in the form of triads of elements having similar chemical properties.
Newland's classification:
Advantage . Newland 's law of octaves gave a very important conclusion that there is some systematic relationship between the order of atomic masses and repetition of properties of elements.
Disadvatage . This law was applicable to the classfication of elements up to calcium only.
Mendeleev's classfication :
Advantage. Mendeleev's periodic law predicted the existance of some elements that had been discovered at that time and he left proper gaps in his periodic table for the then undiscovered elements like Ga, Sc and Ge.
Disadvantage . The position of isotopes could not be explained in Mendeleev's periodic table.
(b) The scientist who first of all showed that atomic number of an element is a more fundamental property than its atomic mass was Henry Moseley in 1913.
(c ) Modern Periodic Law. It states that " the physical and chemical properties of the elements are a periodic function of their atomic numbers".