The correct answer is Hydrogen.Monatomic elements exist as single atoms under normal conditions. Noble gases (Group 18) are typically monatomic.Helium (He), Neon (Ne), and Argon (Ar) are noble gases and exist as stable, single atoms. Hydrogen (H), along with nitrogen (N), oxygen (O), fluorine (F), chlorine (Cl), bromine (Br), and iodine (I), exists as diatomic molecules (H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂) in its elemental form under standard conditions. Therefore, hydrogen is not monatomic.