Question

The cause of periodicity of the properties is:
(A) Increasing atomic radius
(B) Increasing atomic weight
(C) Number of electrons in the valency orbit
(D) The recurrence of similar outer electronic configuration

Answer 1

Hint: The long form of periodic table or the modern periodic table is the current form of the periodic table. It consists of a total of 118 elements. We can predict the properties of an element belonging to a particular group because of the periodicity of properties.

Complete step by step solution:
Before solving this question, we first need to know about the modern periodic table. In modern periodic tables the elements are arranged in increasing order of their atomic numbers. The modern periodic table has 18 vertical columns and 7 horizontal rows.
Properties of the Groups in the Modern Periodic Table
They are the vertical columns in the long form of the periodic table and the total number of these vertical columns is 18, with the first group denoted as Group 1. For each group, the elements belonging to a particular group will have the same type of outer shell electronic configuration where for every element the principal quantum number will increase as we move down the group. As we go down a group there are certain general trends such as the electronegativity will decrease, the atomic radii will increase, electropositive character will increase, and ionization energy will decrease.
Properties of the Periods in the Modern Periodic Table
They are the horizontal rows in the long form of the periodic table and the total number of these horizontal rows is 7. The first period only has 2 elements (Hydrogen and Helium), the second and third period have 8 elements, the fourth and the fifth period have 18 elements while the sixth and the seventh period have 32 elements. With each successive period, the principal quantum number increases. As we move across a period there are certain general trends such as the electronegativity will increase, the atomic radii will decrease, electropositive character will decrease, and ionization energy will increase.
Now let us explore the reasons behind the periodicity of properties in the periodic table.
Reasons for the periodicity in the Modern Periodic Table
The main reason behind the periodicity in properties is the recurrence of similar outer electronic configuration after certain regular intervals.
For example:
All the elements belonging to group 1 of the periodic table have a similar outer electronic configuration i.e. $ns^1$ where n is the Principal Quantum Number of the outermost shell.
Similarly, all the elements belonging to group 18 of the periodic table have an outer electronic configuration: $ns^2 np^6$ i.e. their outermost shell is completely filled. Therefore they are highly unreactive in nature and are called Inert gases.
Therefore option (D) the recurrence of similar outer electronic configuration is the correct answer.

Note: Due to the periodicity of the properties, the unknown properties of an element can be predicted but they are not always true. Certain exceptions within a group do exist. For example, the electron affinity trend of group 17 where the electron affinity of Fluorine is less than that of Chlorine.