$(i)$ State Modern Periodic Law.
$(ii)$ How many :
$(a)$ groups and
$(b)$ periods are present in the modern periodic table?
$(iii)$ State how the problem of placing:
$(a)$ hydrogen,
$(b)$ isotopes of an element has been solved in this periodic table.

(N/A) $(i)$ Modern Periodic Law states that the properties of elements are a periodic function of their atomic numbers.
$(ii)$ $(a)$ There are $18$ groups in the modern periodic table.
$(b)$ There are $7$ periods in the modern periodic table.
$(iii)$ $(a)$ Hydrogen is placed in the $1st$ group because its electronic configuration resembles that of alkali metals.
$(b)$ Isotopes of an element have the same atomic number,so they occupy the same position in the modern periodic table,thus resolving the anomaly found in Mendeleev's periodic table.