Final answer:
The most reactive group of nonmetals whose atoms gain or share one electron in chemical reactions are the halogens.
Step-by-step explanation:
The most reactive group of nonmetals whose atoms gain or share one electron in chemical reactions are the halogens. Group 17 on the periodic table contains the halogens, including elements like fluorine, chlorine, iodine, and bromine. These elements have one less electron than a neighboring noble gas and readily accept an extra electron, making them highly reactive chemically.