Solides moléculaires et ioniques

Molecular solids are held together by weak intermolecular forces including hydrogen bonds, dispersion forces, and dipole-dipole interactions. The strength of these forces determines the solid's properties. Nonpolar molecules like solid nitrogen rely primarily on weak dispersion forces, while polar molecules like ice exhibit stronger hydrogen bonds and dipole-dipole interactions, resulting in higher melting points.

Ionic solids have high melting points due to strong electrostatic attractions between oppositely charged ions. According to Coulomb's law, ionic interactions strengthen with increased charge on ions or decreased distance between them. For example, calcium oxide melts at 2572°C because its higher ionic charges create stronger coulombic forces than sodium chloride, which melts at 801°C.

Smaller ions pack more closely in the crystal lattice, reducing the distance between charges and strengthening electrostatic attractions. Sodium chloride melts at 801°C, while caesium chloride melts at only 645°C because sodium's smaller cation enables closer packing and stronger ionic interactions. This demonstrates how ion size directly influences melting point and overall solid properties.

Nonpolar molecular solids like dry ice sublime easily because they are held together by weak dispersion forces. These weak intermolecular forces require minimal energy to overcome, allowing molecules to transition directly from solid to gas. Solid nitrogen exhibits similar behavior, with very low melting points and easy sublimation due to the weakness of dispersion forces between nonpolar molecules.

Molecular solids are generally soft, have low melting points, and exhibit low thermal and electrical conductivity. These properties result from weak intermolecular forces holding the constituent molecules together. Examples include ice, sucrose, and iodine. The specific properties depend on the strength of intermolecular forces present, which varies based on molecular polarity and size.

Higher symmetry in molecules enables close, compact packing within the crystal structure, increasing intermolecular attractions and raising the melting point. Lower molecular symmetry prevents efficient packing, weakening intermolecular forces and lowering the melting point. This packing efficiency is crucial because intermolecular forces are contact-dependent, making molecular geometry a key determinant of solid properties.

Ionic solids do not conduct electricity because ions are immobilized by strong coulombic forces and cannot move freely through the crystal lattice. However, when ionic solids melt or dissolve in water, ions become mobile and can conduct electricity. This difference between solid and molten or dissolved states reflects the role of ion mobility in electrical conductivity.