The variations of hardness, polarizability, and valency of formamide and thioformamide on internal rotation are studied through calculations using the Kohn-Sham version of spin-polarized density functional theory. It is observed that a minimum energy structure is associated with maximum hardness and maximum molecular valency. Also the calculated C-N bond order is found to be larger in thioformamide in the energetically more stable planar structure which is consistent with its higher rotational, barrier. The observation that the preferable protonation site is at the oxygen (sulfur) atom and not the nitrogen atom in formamide (thioformamide) is explained through the calculated Fukui reactivity indices.