Which one of the following pairs is not correctly matched?

Nitrogen has electronic configuration 1s² 2s² 2p³, with three unpaired electrons in 2p orbitals — correctly matched. Carbon has four valence electrons and is tetravalent — correctly matched. Chlorine has two stable isotopes, Cl-35 and Cl-37 — correctly matched. Helium, however, is a noble gas that exists as monoatomic gas (single atoms), not as a diatomic molecule, because its valence shell (1s²) is fully filled. Therefore, the pair 'Helium : Diatomic gas' is not correctly matched.