The resonance structures for RCOG are

or
Electron-release from G(H, $\mathrm{C}{\mathrm{H}}_3,\mathrm{O}\mathrm{C}{\mathrm{H}}_3$ or $\mathrm{N}{\mathrm{H}}_2$ ) to O stabilizes by diminishing the electrophilic character of carbonyl carbon. Since degree of delocalization is maximum in case of $-\mathrm{N}{\mathrm{H}}_2$ , therefore, the properties of is suppressed most in amides $\left(\mathrm{R}\mathrm{C}\mathrm{O}\mathrm{N}{\mathrm{H}}_2\right)$ .