Which of the following ions has an electron configuration different from the other?

The ion with an electron configuration different from the others is ₁₇CI^-. Chlorine (CI) has the electron configuration of 1s²2s²2p⁶3s²3p⁵ when neutral, but when it gains an electron to become an ion, it gains one more electron in the 3p subshell to have a full outer shell of 8 electrons, which gives it the electron configuration of 1s²2s²2p⁶3s²3p⁶. Oxygen (O), magnesium (Mg), and aluminum (AI) have electron configurations of 1s²2s²2p⁴, 1s²2s²2p⁶3s², and 1s²2s²2p⁶3s²3p¹, respectively, when neutral. When they become ions, they either lose or gain electrons to achieve a full outer shell of 8 electrons for stability, which does not change their electron configuration in the same shell. Therefore, the correct answer is ₁₇CI^-.