Polarity of Molecules

• Polarity


  Refers to the overall charge of a molecule
• Molecule


  A group of elements bonded together
• Positive Charge


  Refers to an atom or a molecule with an excess proton
• Negative Charge


  Refers to an atom or a molecule with an excess, unpaired electron
• Electronegativity (EN)


  The tendency of an atom/element to attract an electron, resulting in a negative charge
• Valence Electron


  An electron of an atom, located in the outermost shell (valence shell) of the atom, that can be transferred to or shared with another atom
• Dipole


  A pair of equal and opposite electric charges or magnetic poles of opposite sign separated especially by a small distance
• Covalent bond


  A chemical bond that involves the sharing of electrons to form electron pairs between atoms
• Ionic bond


  Formed between two or more atoms by the transfer of one or more electrons between atoms
• How to determine the polarity of molecules

  • The polarity of the bonds between atoms which can be studied based on electronegativity
  • The geometrical shape of the molecule which can be predicted via the valence shell electron pair repulsion (VSEPR) theory
• Electronegativity (EN)


  A measure of the relative tendency of an atom to attract electrons to itself when chemically combined with another atom. The higher the value of electronegativity, the more it tends to attract electrons toward itself.
• Electronegativity Trend in Periodic Table

  • Horizontal Trend: Electronegativity increases from left to right across a period
  • Vertical Trend: Electronegativity increases from bottom to top
• Electronegativity Difference


  The difference in the electronegativity values of two elements. This value is an essential quantity as it determines the type of chemical bond formed between two atoms.
• Types of Chemical Bonds based on Electronegativity Difference

  • Nonpolar covalent (0)
  • Slightly polar covalent (0.1 to 0.4)
  • Polar covalent (0.5 to 2)
  • Ionic (> 2)
• Molecular Geometry


  The three-dimensional structure or arrangement of atoms in a molecule
• VSEPR Theory


  The valence shell electron pair repulsion theory helps predict the spatial arrangement of atoms in a polyatomic molecule. The shapes are designed to minimize the repulsion within a molecule.
• Key Ideas of the VSEPR Theory

  • Electron pairs stay as far apart from each other as possible to minimize repulsions
  • Molecular shape is determined by the number of bond pairs and lone pairs around the central atom
  • Treat multiple bonds as if they were single bonds
  • Lone pairs occupy more volume than bond pairs. Lone pair-lone pair repulsions are greater than lone-pair-bond pair repulsions which in turn are greater than bond pair-bond pair repulsions.
• Molecular Shape & Polarity

  • Polar molecules have an asymmetrical shape and contain lone pairs of electrons around the central atom
  • Non-polar molecules have a symmetrical shape with no unshared electrons
• Properties of Polar vs Non-Polar Molecules

  • Polar molecules have high boiling and melting points, low vapor pressure, high surface tension
  • Non-polar molecules have low boiling and melting points, high vapor pressure, low surface tension
• Polar Molecules


  • Water, HF, CHCl3
• Non-Polar Molecules


  • Pentane, Hexane, Carbon Dioxide