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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 of this change.

Answer:

The occurance of the elements with similar properties after certain regular intervals when they are arranged in increasing order of atomic number is called periodicity. The periodic repetition of the properties is due to the recurrence of similar valence shell configuration after regular interval.

The elements in a group have same valence electrons thus similar chemical properties. In a period, tendency to gain electrons increases from left to right. This tendency increases because the hold of nucleus on the outermost electrons becomes weak thus it becomes easy eject the electron.