Question

a) Define modern periodic law
b) The atomic number of the element ‘X’ is 20. To which group and period of the modern periodic table it belongs?

Answer 1

To answer this question we must have knowledge of modern periodic table and law. To find the group number check the number of electrons in the valence shell, that will be the group number of the element in this particular case. The number of shells it has determines the period.

Complete step by step solution:
Let us look at what is modern periodic law:
Mendeleev gave its periodic table and periodic law for the arrangement of various elements and for the effective studies of them. But there was some modification needed in the Mendeleev periodic table and hence modern periodic law come which states:
“The physical and chemical properties of the elements are the periodic functions of their atomic number”, whereas the Mendeleev table was based on the atomic masses rather than atomic number.
The modern periodic law was given by Dmitri Mendeleev and Lothar Meyer in 1869. The various properties like electronegativity, ionization energy, and radius follows a trend in the modern periodic table.
Now coming to part 2, it is given to us that the atomic number of the element is X. Let us write the electronic configuration for an atomic number 20.
$1{{\text{s}}^2}2{{\text{s}}^2}2{{\text{p}}^6}3{{\text{s}}^2}3{{\text{p}}^6}4{{\text{s}}^2}$
As we can see that the last electron is going into the s orbital, hence it belongs to the s block elements. And there are two electrons as valence electrons; hence the group number is 2.
Now we will calculate the number of shells present in the element to know the value of the period number. There are 4 shells present as 1, 2, 3 and 4. Hence the period is 4.
The element X belongs to group 2 and period 4.

Note: The name of element X is calcium and the atomic symbol is ${\text{Ca}}$. It is an alkaline earth metal that is its oxide and hydroxides are basic in nature in aqueous solutions. Its atomic mass is 40 amu.