3.6 Write the atomic number of the element present in the third period and seventeenth group of the periodic table.

 There are two elements in the 1st period and eight elements in the 2nd period. The third

period starts with the element with Z = 11. Now, there are eight elements in the third

period. Thus, the 3rd period ends with the element with Z = 18 i.e., the element in the 18th

group of the third period has Z = 18. Hence, the element in the 17th group of the third

period has atomic number Z = 17.