Chlorine(Cl) has configuration 2,8,7 but Chlorine ion (Cl-) has 18 electrons and configuration 2,8,8.

Here is the reason:

Chlorine has 17 electrons, and these are distributed in shells (or energy levels) as follows: 2 in the first shell, 8 in the second, and 7 in the third.This is represented as 2,8 7. Chlorine can gain one electron to achieve a stable electron configuration, resembling that of the noble gas Argon. 

When chlorine gains an electron, its electron configuration becomes 2, 8, 8, making it a chloride ion (Cl⁻). The 2,8,8 configuration means the outermost shell is full with 8 electrons, which is the octet rule, making the ion stable.