Solids: Thermodynamics and Bonding

· The unit cell in the diagram shows the relative positions of the metal ion-cores in metallic lithium.
· The covalent sharing of electrons between the ion-cores changes among the cores overlapped by the 2s wave function. The 2s electrons behave like a negative "ideal gas" in the presence of the positively charged ion-cores, the solid being electrically neutral.

· The interaction of the electrons with more than one ion-core reduces their potential energy, and the increased volume of space they occupy decreases their kinetic energy as compared to the bound 2s electrons.
· This reduction in energy of the condensed system is the source of its binding energy.