* Helium (He) is a noble gas. Noble gases have a full outer shell of electrons, making them extremely stable and unreactive. They rarely form chemical bonds.

* Fluorine (F) is a halogen. Halogens have seven electrons in their outer shell, making them highly reactive. They readily gain an electron to achieve a stable octet configuration.

Reactivity is determined by the tendency of an element to gain, lose, or share electrons. Fluorine has a strong desire to gain an electron, while helium is perfectly content with its existing electron configuration.

Therefore, fluorine is one of the most reactive elements, while helium is one of the least reactive.