Chlorine (Cl) has an atomic number of 17, which means it has 17 electrons in total. To determine the number of valence electrons, we look at the electron configuration of chlorine.
The electron configuration of chlorine is 1s² 2s² 2p⁶ 3s² 3p⁵. Valence electrons are the electrons in the outermost energy level (highest principal quantum number, n). In the case of chlorine, the outermost energy level is the third energy level (n = 3).
The third energy level has three sublevels: 3s, 3p, and 3d. In the case of chlorine, the 3s sublevel is filled with 2 electrons, and the 3p sublevel is filled with 5 electrons. The electrons in the 3s² and 3p⁵ sublevels are considered the valence electrons. Therefore, chlorine (Cl¹⁷) has 7 valence electrons.