An atom of chlorine has 17 electrons. These electrons are negatively charged and are arranged in orbits around the nucleus of the chlorine atom. The nucleus has 17 positively charged protons.
The electronic configuration of the chlorine atom is 2, 8, 7; this means that the first orbit has a complete set of two electrons, the second orbit has a complete set of eight electrons, and the third orbit has an incomplete set of seven electrons. The chlorine atom needs one more electron to complete this orbit with eight electrons. This gives chlorine a valence of 1 for chemical reactivity. The atomic number of chlorine is 17.