Intermolecular forces

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Intermolecular forces
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How are ionic compounds held together?

• Each ion is strongly attracted to the other ions
  in the crystal lattice because of alternating
  positive and negative charges

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How are covalently bonded compounds (molecular
compounds) held together?
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Molecular compounds

• The molecules are held together by
• Intermolecular forces

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What does inter mean?

• Interstate highway a highway between 2 different
  states
• Interscholastic sports sports competition
  between 2 different schools
• International treaty an agreement between 2
  different nations

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What does intermolecular forces mean?

• Forces of attraction between 2 different molecules

Molecule 1
Molecule 2
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This is different than the BOND that hold the
atoms in each individual molecule
Atom 1
Bond
Atom 2
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If the force of attraction between two molecules
is strong, the molecules will tend to stick
together
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If the force of attraction between two molecules
is weak, the molecules willtend to drift apart
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Weak Intermolecular Forces

• The weakest intermolecular force is called
• Van Der Waals Forces
• Non Polar molecules (diatomic elements)
• are held together by these weak forces

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Weak Intermolecular Forces

• The weakest intermolecular force is called
• Van Der Waals Forces
• Most compounds held together by Van Der Waals
  forces are gases at room temperature O2, N2 ,
  H2 , F2 etc

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Strong Intermolecular Forces

• A strong intermolecular force is called
• Dipole-dipole Force
• Polar molecules are held together by
• dipole dipole forces

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Strong Intermolecular Forces

• A strong intermolecular force is called
• Dipole-dipole Force

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The molecules stick together like magnets

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Molecules held together by dipole-dipole forces
have higher melting and boiling points

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HCl is a liquid at room temperature
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The strongest intermolecular force

• ..unfortunately has a very misleading name
• Hydrogen Bonding

It is NOT an ionic, covalent, or metallic BOND
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Hydrogen Bonding

• Is the attraction between a Hydrogen atom in one
  molecule, and an Oxygen, Nitrogen, or Fluorine
  atom in another molecule

N O F
H
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Hydrogen bonding gives water its unusually high
boiling point for such a small molecule