Which statement correctly describes halogens?

• A. Have a completely full outer shell
• B. Are noble gases
• C. Missing one electron in outer shell
• D. Are always unreactive

Answer: (C)

Halogens have seven electrons in their outermost shell, so they are one electron short of a full outer shell. That makes them want to gain one electron to complete their octet, leading to high reactivity and the common formation of -1 ions in reactions. That’s why describing halogens as missing one electron in the outer shell best fits their behavior.

In contrast, having a full outer shell is the trait of noble gases, not halogens, which is why they’re typically very unreactive. Saying halogens are always unreactive contradicts their well-known reactivity, especially with alkali metals.