We study the chemical reactivity of the sidewall of single-walled carbon nanotubes (SWNTs) (n, n), with n ranging from 5 to 16 using density functional theory calculations. We find that the exterior surface of SWNTs is reactive to amidogen, which can be chemically bound to a carbon atom, with the binding energy between 1.71 and 2.12 eV. The binding energy of SWNTs with amidogen decreases as the radius of the tubes increases. However, the interior surface of SWNTs is inert to amidogen, making them promising candidates as containers for the chemical reactions involving amidogens. The introduction of 7-5-5-7 topological defects into the wall of a (5, 5) SWNT improves the chemical reactivity of the tube wall. The bond between amidogen and the carbon atoms in the region of the topological defect can be further stabilized by about 1.05 eV. We also discuss the modification in the electronic properties of SWNTs caused by decoration of amidogen on the exterior surface.