We demonstrate that the competition
between Pauli exclusion and
H-bonding dominates the adsorption of H<sub>2</sub>O on silicene through
first-principles calculations. It explains the bewildering problem
that isolated H<sub>2</sub>O is inert on silicene while isolated NH<sub>3</sub> tends to chemisorption. Moreover, Pauli exclusion can be
overcome by the synergetic effect of Si···O dative
bonding and intermolecular H-bonding. As a result, H<sub>2</sub>O
molecules are readily to chemisorb in clusters. It is expected that
the competition is in general polar molecule adsorption on silicene
and, thus, crucial for the adsorption mechanism
