According to the octet rule, what is true about the reactivity of halogens? a halogen atom has seven electrons in its valence shell, so halogens are very reactive. a halogen atom has two electrons in its valence shell, so halogens are very unreactive. a halogen atom has two electrons in its valence shell, so halogens are very reactive. a halogen atom has seven electrons in its valence shell, so halogens are very unreactive.