Atomicity is defined as the total number of atoms in one molecule of a substance.
MoleculeConstituent AtomsAtomicityHe1 He oneO22 O twoNH31 N, 3 HfourN2O32 N, 3 OfiveH2SO41 S, 2 H, 4 Oseven
Thus, the ascending order of atomicity is: He < O2 < NH3 < N2O3 < H2SO4