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.