What makes a halogen unique?

Answer 1

The halogens are a group of elements on the Periodic Table (group 17/7A) which are all highly reactive non-metals. They consist of fluorine (F), chlorine (Cl), Bromine (Br), and Iodine (I). Astatine (At) is also in that group, but is rarely encountered in chemistry. They are unique because the first four referred to above are all diatomic, which means they exist in nature as a diatomic molecule with a single covalent bond. Because they have seven valence electrons, the octet rule says they only need to acquire one more to achieve a full valence shell. This means they are one of the most reactive groups of elements, and they react readily with most metals.

Answer 2

Halogen elements are unique because they have seven electrons in their outermost energy level, making them highly reactive. They readily form compounds with other elements by gaining an electron to complete their outer energy level. Halogens include fluorine, chlorine, bromine, iodine, and astatine.