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

General configuration for 17th group elements is ns2np5. In the third period, the principal quantum number for valence shell is three, so the electronic configuration of valence shell for the given element is 3s23p5. Third period starts from atomic number, Z = 11 and end at Z = 18. Hence, the atomic number of the given element is l0 + 7 = 17.