Ans. 1. The number of elements in the first three periods depends on the electron capacity of the shells and the law of electron octet. The electron capacity of third shell is 18. So the third period should actually contain 18 elements. But it contains only 8 elements due to the law of electron octet.

2. The third shell i.e. the m- shell has n = 3. This implies that it can contain a maximum of 2 x 32 = 18 electrons. However, if you consider the first 18 elements of the periodic table, the maximum number of electrons found in the m - shell is 8. This is because of the fact that electrons are filled in these shells in the increasing order of electrons.

3. So, the shells are filled in the order K, then L, and finally M. The K and L shells together can hold 10 electrons. And hence electrons begins to occupy the m- shell only in the elements with atomic number 11 - 18. Since there are only 8 electrons left from 11 - 18, these 8 electrons occupy the m - shell.

4. Therefore, the third period contains only eight elements even through the electron capacity of the third shell is 18.