Understanding the reactivity of small clusters and nanoparticles is a fascinating subject. There is a wide variability in reactivity depending on the chemical nature and the size of the cluster. Insight into the effects of the electronic structure and cluster geometry on the reactivity can have tremendous impact on such diverse areas as thin lm coating, catalysis, and the synthesis of new materials. Some chemical concepts such as electronegativity and chemical hardness, very intuitive and at the same time having a sound theoretical foundation, have been used to analyze the reactivity of molecules, but not so much for clusters. In this study, we apply these concepts in order to understand some aspects of the reactivity of atomic clusters. The clusters considered are mainly metallic, but a few examples for carbon clusters are also provided.