The gas-phase adsorption of 1,2,3-triazole, benzotriazole, and naphthotriazole—considered as corrosion inhibitors—on copper surfaces was studied and characterized using density functional theory (DFT) calculations. We find that the molecule–surface bond strength increases with increasing molecular size, thus following the sequence: triazole<benzotriazole<naphthotriazole. This trend is explained in terms of molecular electronegativity and chemical hardness, which decrease monotonously as the molecular size increases. While the electronegativity of triazole is almost degenerate with the work function of Cu(111) surface, the electronegativity of larger acenotriazoles is smaller. The difference in electronegativity between the Cu(111) and the acenotriazoles thus increases with increasing the molecular size, which, together with decreasing the molecular hardness, results in larger molecule-to-metal electron charge transfer and stronger molecule–surface bonds.