Dispersion, dipole interactions and hydrogen bonding influence molecular materials.
Key knowledge
Dispersion interactions arise from correlated fluctuations in electronic distributions. Permanent dipoles and suitable hydrogen-bond donors and acceptors add other interactions. Their combined effects help explain condensation and differences in physical properties of molecular substances.
Conditions and scope
Molecular shape, polarizability and environment matter; interaction names alone do not give a universal strength ranking.
Intermolecular interactions are distinguished from covalent connectivity within a molecule.
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References
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Crystalline solids can be classified by dominant bonding patterns: ionic arrays, metallic bonding, discrete molecular units, or extended covalent networks. Bonding and structure help explain conductivity, mechanical behaviour and phase properties, with important exceptions such as graphite.
Molecular solids depend on intermolecular interactions.
A semicrystalline polymer contains ordered crystallites and amorphous material. Melting disrupts the crystalline regions, whereas the glass transition concerns the changing mobility and response of amorphous chain segments. Chain architecture, packing, permanent crosslinks, temperature and observation timescale influence mechanical behavior. Permanent network junctions restrict long-range chain flow; an uncrosslinked melt can instead flow as chains rearrange. Crystallinity and chain orientation can alter stiffness and directional strength without uniquely fixing all material properties.
Packing and intermolecular interactions contribute to ordered-region cohesion.
Electronegativity describes an atom's tendency to attract bonding electron density. Unequal sharing can produce partial positive and negative regions in a covalent bond. A neutral molecule's permanent dipole depends on its whole charge distribution and geometry. In the bond-dipole model, the contributions combine as vectors: the two C–O contributions cancel in linear CO2, whereas the O–H contributions in bent water do not.
Permanent molecular polarity is one input to intermolecular interactions; induced interactions remain distinct.