What is the electron configuration of a chlorine atom?
In order to know the electron configuration of any atom, a person needs to know how many electrons are in the atom. The atomic number of chlorine is 17, which tells us there are 17 protons in the atom. The atomic number also identifies how many electrons are in the atom.
I'm not sure how detailed of an electron configuration the question is asking for, so I'll provide a broad configuration and a detailed configuration.
The electrons of an atom exist in different levels known as shells. The first shell can hold 2 electrons. The second can hold up to 8. The third can hold up to 18, and that is all that is needed for chlorine. Chlorine will hold 2 electrons in the first main level, which leaves 15 electrons remaining. Eight more will be in the second level, leaving 7 to go. Those 7 will be held in the third level.
Within those main levels are sublevels. For chlorine, sublevels s and p are needed. Using those, chlorine's electron configuration is as follows: 1S^2, 2S^2, 2P^6, 3S^2, 3P^5.