Which one of the following is monatomic?

The correct answer is D, Helium. A monatomic element exists as single, separate atoms. Helium is a noble gas of group 18: its only shell, the first, is full with two electrons, so a helium atom has no tendency to share or transfer electrons and does not bond even with another helium atom. All the noble gases, helium, neon, argon, krypton, xenon and radon, are monatomic for the same reason. Their atomicity, the number of atoms in one molecule, is therefore 1. Option A is wrong because hydrogen exists as diatomic H2 molecules, two atoms joined by a single covalent bond. Option B is wrong because sulphur usually exists as S8 molecules, rings of eight atoms, so its atomicity is 8. Option C is wrong because white phosphorus exists as P4 molecules, four atoms arranged in a tetrahedron, so its atomicity is 4. Exam tip: noble gases = monatomic (1); H2, O2, N2 = diatomic (2); P4 = 4; S8 = 8.