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12.1 Intermolecular Forces.docx

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CHEM 110
Ariel Fenster

12.1 Intermolecular Forces Atomic Forces: at the center of all bonding (at the heart its all Coulombic interaction)  Opposite charges attract (both covalent and ionic) Van der Waals Forces: very weak  Dispersion forces exist between all molecules  Forces associated with permanent dipoles involve displacements of electron pairs in bonds rather than in molecules as a whole  When comparing substances of roughly comparable molecular masses, dipole forces produce significant differences in properties  When comparing substances of widely different molecular masses, dispersion forces are usually more significant than dipole forces London Dispersion Forces: Instantaneous Dipoles: electrons are concentrated in one region of an atom or molecule. This displacement of electrons causes a normally nonpolar species to become momentarily polar Induced Dipoles: electrons in a neighboring atom or molecule may be displaced to also produce a dipole (process of induction) Polarizability: describes the relative tendency for a charge distribution to distort from its normal shape in an atom or a molecule  Greater the tendency  the more polarizable the atom or molecule  Increases with atomic or molecular size (volume of electron cloud)  Large molecules, electrons are farther from the nuclei, therefore they are more easily displaced and their polarizability increases Dispersion forces become stronger as polarizability increases  melting and boiling points increase with increasing molecular mass The strength of dispersion forces also depends on molecular shape. Electrons in elongated molecules (linear) are more easily displaced than those small, compact and symmetrical molecules. Dipole-Dipole Interactions: in polar substances, the molecules have permanent dipole moments, therefore molecules tend to line up with the positive end of one dipole directed toward the negative ends of neighboring dipoles. Hydrogen Bonding: the boiling points increase regularly as molecular mass increases  Electrostatic attraction, not an actual chemical bond  Bonded to a highly electronegative atom, which attracts electron density away from the nucleus  A proton is attracted to a lone pair of
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