To discuss the stability of matter, two measures of stability, cohesive energy and energy fluctuation, are obtained using the extended Hückel method. Magnitude of cohesive energy is valid as a measure of “static stability,” and, on the other hand, energy fluctuation is valid as a measure of “dynamic stability.” Using atomic bond populations (ABP) and atomic populations (AP) obtained by the extended Hückel method, the four types of bonds, covalent, metallic, electrostatic, and molecular bonds, can be determined. Covalent bond is formed by the sharing of electron pairs. Metallic bond can be regarded as a covalent bond appearing in the case of with electron deficiency. Electrostatic bond is formed due to a transfer of electrons from one atom to the other. Molecular bond is due to the interaction between electric dipoles caused by electron-charge fluctuations. In terms of bonding nature, the presence of unoccupied states corresponds to metallic bonding, the absence of unoccupied states corresponds to covalent or electrostatic bonding, and in terms of structure, the presence of unoccupied states corresponds to a most closed-packed structure with isotropic delocalization, and the absence of unoccupied states corresponds to a sparse structure with anisotropic localization. In terms of properties, the presence of unoccupied states corresponds to good plastic deformability, low strength, low corrosion resistance and low heat resistance, while the absence of the unoccupied states corresponds to brittleness, high strength, high corrosion resistance and high heat resistance. With regard to reactivity, the presence of unoccupied states corresponds to instability, hence short lifetime and high reaction rate. The existence of unoccupied states fundamentally controls the direction and rate of reactions.