The difference between Chloride ion and Chlorine atom are:
Chloride ion
(a) Chloride ion (CI-) has a stable electronic configuration of (2, 8, and 8) of argon with 8 electrons in its outermost shell).
Image Source: drivetofive.files.wordpress.com/2013/11/chloride_billboard.jpg
ADVERTISEMENTS:
(b) Chloride ion is negatively charged.
(c) The chloride ion is stable. It does not react with sodium.
(d) In the chloride ion there are 17 protons but 18 electrons.
Chlorine atom
ADVERTISEMENTS:
A chlorine atom has an unstable electronic configuration of 2, 8, and 7
A chlorine atom is electrically neutral.
Chlorine atom is reactive; it reacts with sodium vigorously forming sodium chloride.
In the chlorine atom, there are equal number of protons (17) and electrons (17).