Write the atomic number of the element present in the third period

There are two elements in the $1^{\text {st }}$ period and eight elements in the $2^{\text {nd }}$ period., The third period starts with the element with $Z=11$. Now, there are eight elements in the third period. Thus, the $3^{\text {rd }}$ period ends with the element with $Z=18$ i.e., the element in the $18^{\text {th }}$ group of the third period has $Z=18$. Hence, the element in the $17^{\text {th }}$ group of the third period has atomic number $Z=17$.