monoatomic

/M AA1 N AH0 AH0 T AA1 M IH0 K/
adjective
  1. 1

    Of an element, consisting of a single atom in the molecule, for example, the noble gases. Note: Strictly speaking, a molecule has at least two atoms.

Source: https://en.wiktionary.org/wiki/monatomic, https://en.wiktionary.org/wiki/monoatomic