Argon is a neutral atom with atomic number of 18.  It has an electron configuration of 2 electrons in the first energy level, 8 in the second, and 8 in the third, for a total of 18 electrons. Because there are equal numbers of protons in the nucleus and electrons it has no net charge.

Chlorine is a neutral atom with atomic number of 17. It has an electron configuration of 2 electrons in the first energy level, 8 in the second, and 7 in the third, for a total of 17 electrons.

But the third energy level can hold up to 8 electrons. So when chlorine gets one more electron to fill that third level, it becomes a chloride ion with a net charge of -1.  This means it still has 17 protons in the nucleus (atomic number = number of protons) but now has a total of 18 electrons around the nucleus - just like argon its neighbor.