3.6 Write the atomic number of the element present in the third period and seventeenth group of the periodic table.
Given that it is present in the 3rd period and a 17th group of the periodic table.
So, the first period has 2 elements and the second period has 8 elements. The 3rd period starts with the element with Z=11. And there are 8 elements present in the third period. Thus, the 3rd period ends with the element with Z=18. The element in the 18th group of the 3rd period has Z=18. Hence, the element in the 17th group of the 3rd period has atomic number Z=17.