Question

Assertion : According to Mendeleev’s periodic law:
The properties of elements is the periodic function of their atomic numbers.
Reason : Mendeleev's periodic law could not explain the phenomena of anomalous pairs.
A. Both assertion and reason are correct and reason is the correct explanation of the assertion
B. Both assertion and reason are correct, but reason is not correct explanation of assertion
C. Assertion is correct, but reason is incorrect
D. Assertion is incorrect, but reason is correct
E. Both assertion and reason are incorrect

Answer 1

Hint: The periodic table consists of a set of all the elements arranged according to their atomic number and atomic masses, in an organized manner.
-The atomic mass is the mass of an atom, and it is defined as the $1/12$ of the mass of a single carbon- $12$ atom.

Complete step by step answer:
Periodic properties of elements are classed into either chemical or physical.
Chemical properties usually come in chemical reaction, this is due to the distribution of electrons around the atom's nucleus, particularly valence electrons, and these are those electrons involved in chemical reaction. Physical properties include color, density, melting point, electrical and thermal conductivity etc. The periodic table is divided into three parts, metals, nonmetals and metalloids.The elements of the periodic table are arranged in order of increasing atomic weights in a way that the elements in any column have similar properties.
Now, coming to the question solving part, according to Mendeleev's periodic law it was the atomic weight on which all properties depend upon. Which implies that assertion is incorrect, which is that the properties of elements is the periodic function of their atomic numbers.
It failed because they did not conform completely to the order of atomic mass and also they were not able to locate hydrogen in the periodic table. Increase in atomic mass was found to be irregular while moving from one element to another element. They failed because of anomalous pairs, atoms are arranged according to the atomic masses in the periodic table before that of the atom that has a smaller atomic weight. These are called anomalous pairs which violate the rule.
Which means assertion is incorrect and reason is correct.
Hence, option D would be the correct option.

Note: Mendeleev's periodic law states that the physical and chemical properties of elements are a periodic function of their atomic mass. Meaning, the chemical and physical properties of elements are similar or repeats themselves in nature periodically, when they are arranged according to their atomic masses.
-But this law was given before we knew that it would be more appropriate if we arrange the periodic table according to the atomic numbers of the element and the elements would have periodic properties with lesser exceptions.