Chemical Bonding (Section 2 of 2)

Contents of this page

1. Intermolecular forces of attraction
1. Hydrogen Bonding
2. Vander Waals Forces
1. Temporary Dipole induced dipole Forces
2. Dipole Dipole forces of attraction

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Van der Waals Forces and Hydrogen Bonding

Hydrogen Bonding (The strongest Force)
What is Hydrogen Bonding?

• In hydrogen bonds, the positively charged hydrogen end of one molecule is attracted to the negatively charged end of another molecule which must be an extremely electronegative element (fluorine, oxygen, or nitrogen - FON)
  • e.g.  H₂O, HF, and NH_3.
  • Alcohols can also form hydrogen bonds with water in which the O in the OH group of the alcohol bonds to the positively charged H end of the water molecule and the H in the OH group of the alcohol bonds to the negatively charged O of the water molecule.
  
• Hydrogen bonds are the strongest weak bond because the H atom essentially gives its single electron to form a bond and is therefore left unshielded.  The relatively strength of hydrogen bonds results in higher melting and boiling point temperatures than those in molecules with other van der Waals forces of attraction.
• Hydrogen bonding can explain why water is less dense in the solid phase than it is in the liquid phase (contrary to most other substances).  The hydrogen bonds between water molecules in ice to form a crystal structure, keeping them further apart than they are in the liquid phase.

Dipole dipole forces of attraction (The Medium Weak Force)

• Dipole-dipole forces exist between neutral, polar molecules where the positive end of one molecule is attracted to the negative end of another molecule.

• The greater the polarity (difference in electronegativity of the atoms in the molecule), the stronger the dipole-dipole attraction.
• Dipole-dipole attractions are very weak and substances held together by these forces have low melting and boiling point temperatures. Generally, substances held together by dipole-dipole attractions are gases at room temperature.

Dipole Induced Dipole Forces of attraction/London Dispersion forces (The weakest)

• London dispersion forces (LDF) occur between neutral, nonpolar molecules.  LDF occur due to the "random motion of electrons."  At any moment, one atom may be surrounded by an extra electron from a neighboring atom, resulting in an instantaneous polarity on the atom.  During that instant, the "polarized" atom will act as a very weak dipole.
• Since LDF are dependent upon the random motion of electrons, the more electrons an atom or molecule has, the greater the LDF between them.
• LDF as a whole are extremely weak, so substances held together by these forces have extremely low melting and boiling point temperatures.  These substances tend to be gases at room temperature.

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