Group 7A (also known as Group 17) on the periodic table contains the most reactive nonmetals. This group includes the elements fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These elements are highly electronegative, meaning that they have a strong tendency to attract electrons from other atoms, making them very reactive. They are all diatomic molecules, meaning that they exist as pairs of atoms bonded together.

Fluorine is the most reactive nonmetal and the most electronegative element on the periodic table. It is a pale yellow gas that reacts vigorously with most other elements, including metals, nonmetals, and even noble gases. Chlorine is a greenish-yellow gas that is also highly reactive, but not as much as fluorine. Bromine is a reddish-brown liquid that is less reactive than chlorine, and iodine is a black solid that is even less reactive. Astatine is a radioactive element that is so rare that it has not been studied extensively.

The reactivity of the halogens (Group 7A elements) decreases down the group from fluorine to astatine. This is because the atomic size increases down the group, which means that the valence electrons are further from the nucleus. This makes the valence electrons less tightly held, and therefore more reactive.