Group 17 elements, also known as halogens, are the most reactive non-metals in the periodic table. These elements are highly electronegative, meaning they have a strong tendency to attract electrons from other atoms. This reactivity is due to the fact that halogens have seven valence electrons in their outer shell, and they need only one more electron to achieve a stable configuration. As a result, halogens readily form bonds with other elements to complete their outer electron shell.