The basic properties of alloys can be understood through considerations based on the cohesive energy of the solvent, energy fluctuations, atomic bond population (ABP), and atomic population (AP). Alloys are composed of combinations of two or more elements, and whether a solute element dissolves into a solvent element metal depends on the similarity of their bonding characteristics. These bonding characteristics can be determined by using ABP and AP. If the ABP and AP values are close to those of the solvent, the solute dissolves; if they differ significantly, solubility is poor. Furthermore, when solubility is high, cohesive energy changes linearly with composition. If compounds are formed, cohesive energy increases significantly, whereas in cases of insolubility, cohesive energy decreases markedly. By analyzing the solubility, insolubility, and compound formation of 15 practical alloys (Al, Ti, Zr, Fe, Co, Ni, Pd, Pt, Cu, Ag, Au, Mg, Zn, Sn, and Pb alloys) using ABP and AP, metallic bonding, covalent bonding, and electrostatic bonding were identified. Based on this examination, the alloys were classified into six types, such as solid-solution-hardening, age-hardening, work-hardening, corrosion-resistant, heat-resistant, and casting types. This classification helps to clarify the characteristics of each alloy and outlines their applications.