  We perform a systematic first-principles study of phosphorene in the presence
of typical monovalent (hydrogen, fluorine) and divalent (oxygen) impurities.
The results of our modeling suggest a decomposition of phosphorene into weakly
bonded one-dimensional (1D) chains upon single- and double-side hydrogenation
and fluorination. In spite of a sizable quasiparticle band gap (2.29 eV), fully
hydrogenated phosphorene found to be dynamically unstable. In contrast, full
fluorination of phosphorene gives rise to a stable structure, being an indirect
gap semiconductor with the band gap of 2.27 eV. We also show that fluorination
of phosphorene from the gas phase is significantly more likely than
hydrogenation due to the relatively low energy barrier for the dissociative
adsorption of F2 (0.19 eV) compared to H2 (2.54 eV). At low concentrations,
monovalent impurities tend to form regular atomic rows phosphorene, though such
patterns do not seem to be easily achievable due to high migration barriers
(1.09 and 2.81 eV for H2 and F2, respectively). Oxidation of phosphorene is
shown to be a qualitatively different process. Particularly, we observe
instability of phosphorene upon oxidation, leading to the formation of
disordered amorphous-like structures at high concentrations of impurities.
