The phrase "chlorine atoms" refers to individual particles of the element chlorine. Chlorine atoms are the tiny building blocks that make up chlorine. Full definition
A single chlorine atom can tear apart thousands of ozone molecules. (discovermagazine.com)
These orderly patterns of dark blue dots indicate where individual chlorine atoms are missing from an otherwise regular grid of atoms. (sciencenews.org)
A single chlorine atom can destroy thousands of ozone molecules. (livescience.com)