International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

How many electrons does a chloride ion have in its outer shell?

1. 7

2. 8

3. 6

4. 9

The correct answer is: 8

A chloride ion is formed when a chlorine atom gains one electron. Chlorine, in its neutral state, has 7 electrons in its outer shell, consistent with its position as a Group 17 element in the periodic table, often referred to as a halogen. By gaining an electron to achieve a full outer shell, a chloride ion, therefore, has 8 electrons in its outermost energy level. Having 8 electrons allows the chloride ion to attain a stable electronic configuration, similar to the noble gases, which are known for their lack of reactivity due to having full outer shells. This stability is a key principle driving the formation of ions in chemical reactions. Other choices do not represent the correct number of valence electrons for a chloride ion. The option implying that there are 7 electrons reflects the configuration of a neutral chlorine atom rather than the ion. Similarly, options suggesting 6 or 9 electrons are incorrect, as they do not accurately depict the electron gain or preserve the stability achieved by the chloride ion.