The Change In The Nature Of The Bonding Between A And Cl, Where A Is An Element In The Third Period

As we move across Period 3 from sodium to argon, the elements increase in electronegativity. Consequently, their attraction for electrons increases from left to right. The sodium atom has a low affinity for electrons and in fact loses an electron to the electronegative chlorine. The compound sodium chloride therefore contains ions. Aluminum has a greater affinity for electron density and therefore shares some electron density with chlorine, but the chlorine retains most of the electron density. By the time chlorine is reached, the bond is homogeneous; that is, both chlorines share equally in the electron density.

Schematic Representation Of Lattice Energy

As ions come together from a very large separation, such as they would have in the gas phase, the attractions between the ions outweigh the repulsions and the energy of the system decreases. The difference in the energy of the ions in the solid and the energy of the ions in the gas state is the lattice energy. More precisely, the lattice energy is the energy liberated when the ions collapse into the solid state from the gaseous state.

The Transformation Of 8-Coordination To 6-Coordination

The red flashing arrows indicate attractions between the anions and the cation. The blue arrows represent repulsions between the negatively charged anions surrounding the cation. When the smaller sodium ion takes the place of the cesium ion, the anions move closer to the cation and closer to one another. The magnitude of the repulsions therefore increases. A lower potential energy for the system can be obtained if the number of anions surrounding the cation decreases to six. This change in geometry reduces the repulsions and allows the anions to move slightly closer to the cation. In general, the ratio of the size of the cation to the size of the anion controls the geometry of a lattice.