The Total solution for NCERT class 6-12
The first period is having 2 elements and the second period is having 8 elements. So, the third period begins with element Z = 11. Presently, the third period contains 8 elements. So, the 18th element is the last element of the third period and this 18th element is present in the 18th group. Thus, the element in the seventeenth group of the 3rd period is having atomic number 17 i.e. Z = 17.