What determines the reactivity of halogens?

The reactivity of halogens is primarily determined by the number of valence electrons in their outer shell. Halogens, which include fluorine, chlorine, bromine, iodine, and astatine, have seven valence electrons. This configuration means they are just one electron short of achieving a stable octet, which is a complete outer shell configuration.

Because of this, halogens readily gain or share an electron in chemical reactions to achieve stability, thus demonstrating high reactivity. This tendency to react is what characterizes halogens and differentiates them from other groups in the periodic table. The valence electron count directly influences their ability to form bonds and interact with other elements, making it the key factor in determining their reactivity.

Other factors such as the number of protons, atomic mass, and density may provide some context regarding the properties of halogens, but they do not directly dictate their chemical reactivity like the number of valence electrons does.