Here's why:

* Electron Configuration: Halogens are in Group 17 (VIIA) of the periodic table. Their general electron configuration ends with *ns² np⁵*.

* Valence Electrons: Valence electrons are the electrons in the outermost energy level. The *np⁵* part of the configuration represents 5 electrons in the p subshell of that outermost energy level.

Let me know if you have any other questions about electron configurations!