Properties of Covalent (Molecular) Substances

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Properties of Covalent (Molecular) Substances
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Properties

• depend on strength of IMF between particles or
  separate units
• covalent substances
• units are molecules

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Intermolecular Forces

• Dispersion forces occur between nonpolar
  molecules (Van der Waals)
• Dipole-dipole forces occur between polar
  molecules
• Hydrogen bonding occurs between molecules with an
  H-F, H-O, or H-N

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• weakest IMF dispersion forces
• - occur between nonpolar molecules
• monatomic molecules
• diatomics of same element
• symmetric molecules
• hydrocarbon molecules

He, Ne, Ar, Kr
O2, H2, N2
CO2, CCl4, CF4
CH4, C4 H10
Reminder Dispersion forces ? as size molecule ?
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Dipole-dipole forces occur between - polar
molecules (permanent charge separation) HCl and
HBr are examples of polar diatomic molecules
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• hydrogen bonds occur between one molecule
  containing an H with a neighboring molecule
  containing an F, O or N
• ? H-bonds strongest IMF

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Properties Covalent (Molecular) Substances

• poor conductors heat electricity
• no charged particles!
• low mp low bp
• easy to pull molecules apart from each other
• low Hf and Hv compared to ionic metallic
  substances
• high VP compared to ionic metallic substances
• majority of solids are soft

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mp, bp, Hf and Hv VP depend on how hard it is
to separate particles from each other
strong IMF difficult to separate particles
weak IMF easy to separate particles
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Which substance has the strongest IMF? The
weakest? How know which is which?