Here's how to determine the number of electrons in the outermost energy level of a chloride ion:

1. Understanding the Basics

* Neutral Chlorine (Cl): A neutral chlorine atom has 17 electrons. Its electron configuration is 2-8-7, meaning it has 7 electrons in its outermost energy level (the third energy level).

* Chloride Ion (Cl⁻): A chloride ion forms when a chlorine atom gains one electron. This gain gives it a full outer shell, making it more stable.

2. The Answer

Since a chloride ion gains an electron, its outermost energy level now has 8 electrons (2-8-8).