The atomic number of chlorine (Cl) is 17, which means it has 17 electrons. To determine the valence electron configuration, we look at the outermost energy level of the atom, which is the third energy level (designated as the 3s²3p⁵).
The electron configuration of chlorine can be written as follows:
1s² 2s² 2p⁶ 3s² 3p⁵
In this configuration, the first energy level (1s²) has two electrons, the second energy level (2s²2p⁶) has eight electrons, and the third energy level (3s²3p⁵) has seven electrons. Therefore, chlorine has 7 valence electrons in its outermost energy level (3s²3p⁵).
The presence of 7 valence electrons in chlorine makes it highly reactive, as it tends to gain one electron to complete its outermost energy level and achieve a stable electron configuration, resulting in a full complement of 8 valence electrons, similar to the noble gas configuration of argon (Ar).