Question

What is periodicity in properties of elements with reference to the Modern Periodic Table? Why do all the elements of the same group have similar properties? How does the tendency of elements to gain electrons change as we move from left to right in a period? State the reason for this change.

Answer 1

Earlier, Mendeleev gave periodic law which stated that periodic properties are the periodic function of their atomic mass. Later Modern periodic law was given by Henry Mosely. It states that periodic properties are the periodic function of their atomic numbers, which was internationally accepted.

Complete answer:
Mendeleev’s periodic law was overruled by Henry's modern periodic law. According to modern periodic law he gave a term called periodicity. Let us look at what periodicity means.
Periodicity means repeating after a certain interval of time. Like the hands of a clock are periodic in nature.
In the same way, let's define periodicity: the repetition or recurrence of certain elements after regular intervals of time when they are arranged in the increasing order of their atomic numbers is called periodicity.
Now let's answer the second part of this question about , why do all the elements of the same group have similar properties. The reason for this is simple. As there is periodicity among the elements , they show similarity due to the repetition of similar outer electronic configuration at regular intervals of atomic numbers.
Let's look at the third part of the question about how the tendency of elements to gain electrons changes as we move from left to right in a period. Along the period the tendency to gain electrons increases, the reason for this is that the effective nuclear charge increases along period and also atomic radius decreases along period.

Note:
The tendency of elements to gain electrons is also called electron affinity. This electron affinity increases because electrons are added to the existing energy levels and come closer to the nucleus, due to which there is more force of attraction between nucleus and its electrons.