Why there are 2, 8 and 8 elements in first, second and third periodic of periods table respectively ? Explain.

In first period 1s is completed. Its capacity is of two electrons. In second period 2s 2p and in third period 3s 3p are completed. The capacity of these shells is of 8 electrons each. Thus, 2, 8 and 8 elements are present in first, second and third periods respectively.