what do a chloride ion and an argon atom have in common? Why is a chlorine ion not called an argon atom?
The chloride ion and the argon atom have the same electronic structure. The reason why the chloride ion cannot be called argon atom is that the structure of their nuclei is different.
When the chlorine atom gains an electron, becoming the chloride ion, the structure of its nucleus does not undergo any changes. Yes, it is true that the electronic structure of the chloride ion can be recognized to the atom of the noble gas, argon, but the structure of nucleus of chloride ion consists of 17 protons and 18 neutrons, while the structure of nucleus of the atom of argon consists of 18 protons and 22 neutrons.