Most known molecules and compounds follow fixed stoichiometry and can be rationalized on the basis of classical valence theories. However, nonstoichiometric species, particularly in the gas phase, are common. These species cannot be easily understood by classical valence considerations because they do not have the full octet of valence electrons-they are valence unsaturated molecules with dangling bonds. We consider nonstoichiometric molecules consisting of only four or five atoms and show the great variety of molecules and bonding that can be derived from this class of seemingly simple species. We demonstrate that gas-phase photodetachment photoelectron spectroscopy using a laser vaporization source and ab initio quantum calculations provide an ideal approach to characterize and understand the structure and bonding of nonstoichiometric molecular and cluster species. Specifically, we review our recent progress in the design and characterization of the first pentaatomic tetracoordinate planar carbon molecules, CAl4-, CAl3Si-, CAl3Ge-, and a salt complex, Na+[CAl42-] containing a planar carbon building block. We also review our recent discovery of an all-metal aromatic species, Al-4(2-), in a series of bimetallic clusters, M+[Al-4(2-)] (M = Cu, Li, Na), as well as the Ga-4(2-) and In-4(2-) analogues. We also show the existence of aromaticity in a series of isoelectronic singly charged anions, MAl3- (M = Si, Ge, Sri, Pb), and how aromaticity helps stabilize the heterocyclic structure over a pyramidal isomer. We show how, by pursuing and understanding the concept of nonstoichiometry, one can extend the classical valence theory and discover new structures and new types of bonding.