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

(17) The $1^{st}$ period contains $2$ elements and the $2^{nd}$ period contains $8$ elements. The $3^{rd}$ period begins after the $2^{nd}$ period,starting with the element having atomic number $Z = 11$. The $3^{rd}$ period contains $8$ elements,ending at $Z = 18$ (which is in the $18^{th}$ group). Therefore,the element in the $17^{th}$ group of the $3^{rd}$ period has an atomic number of $Z = 17$.