Question

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

Answer 1

There are two elements in the $1^{st}$ period and eight elements in the $2 ^{nd}$ 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 $18^{th}$ group of the third period has $Z = 18$.

Hence, the element in the $17^{th}$ group of the third period has atomic number $Z = 17$.