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intermolecular forces (IMF)

an intermolecular force occurs between molecules. it is the force that must be overcome to have a phase change.


intermolecular forces occur between four types of substances

molecular substances (2 or more nonmetals)
ionic substances (one metal and one non metal)
metallic substances (metals)
network solids (diamonds)


molecular substances

three types of intermolecular forces:
hydrogen bond imf
London dispersion



occurs between polar molecules (polar bonds, no symmetry)
the + end of one molecule is attracted to the - end of another molecule
examples: HBr, PH3, H2S


hydrogen bond IMF

special type of dipole-dipole force (stronger because of the greater polarity of the molecules)
occurs between molecules that have H bonded to N, O, or F - (highest electronegativities)
examples: H2O, HF, NH3


van der waals, temporary dipole-dipole, or london dispersion IMF

occurs between non polar molecules (non polar bonds or polar bonds with symmetry)
occurs as a result of movement of electrons and one molecule becomes temporarily polar and in turn induces another molecule to become polar
example: CO2


ionic substances

ionic IMF
occurs between metal and nonmetal
composed of ions in a crystalline structure, + ions attracted to - ions to form a unit cell
examples: NaCl, CaI2, KNO3


metallic substances

metallic IMF
occurs between metals or alloys
"sea of electron" theory
explains the properties of metals
conducts electricity
tends to lose electrons
malleable, ductile


network solids

network solid or covalent network IMF
each molecule is bonded in a 3 dimensional network of covalent bonds
only examples: C (in diamond form), SiC, SiO2


the strength of the intermolecular force

is directly related to melting point and boiling point
the stronger the IMF the higher the m.p. or b.p.


strength of IMF

london dispersion - dipole-dipole - H bond - ionic - metallic - network solid