• Recall: Ionic compounds form a LATTICE
• Ex: NaCl

• Have INTER molecular forces and INTRA molecular forces
• Intramolecular forces:
• Attractive and repulsive forces WITHIN a molecule or a polyatomic ion.
• Are what bond atoms together within a molecule
• Are very strong
• Ex: a covalent bond
• Intermolecular forces
• Attractive and repulsive forces BETWEEN molecules/atoms/ions
• Are very weak

• Example: H
2
O

Forces and Bond Strength
• Different molecular compounds have a different strength of intermolecular forces.
• As this strength increases, the compound’s melting and boiling point also increases.
• A solid has stronger intermolecular forces than a liquid or a gas.

Types of Intermolecular Forces
• form between any entities , including polar and non-polar molecules and unbounded atoms
• caused by the temporary imbalance of electrons within entities, forming a temporary dipole
• very weak

Types of Intermolecular Forces
• form between the slightly positive end of one polar molecule and the slightly negative end of an adjacent polar molecule.
• occur between all polar molecules.
• the more polar a substance is, the stronger the dipoledipole attraction
• are relatively strong

Types of Intermolecular Forces
• a very strong dipole-dipole force between a hydrogen atom attached to a highly electronegative atom (like N, O, or F) and a highly electronegative atom in another molecule.
• happens because of the large electronegativity difference between hydrogen and the other atom
• Strongest of the three forces

Types of Intermolecular Forces
Recap: Relative strength of bonds
Highest
Intramolecular
Covalent bonds
Intermolecular
Hydrogen bonds
Dipoledipole
Lowest
LDF
Collectively called
“Van der Waals forces”

Types of Intermolecular Forces
• See flow chart on pg 114

• Water is a very unusual substance!
• This is due to hydrogen bonds
• Water is one of the most polar molecules on earth
• Water’s shape is actually BENT . The two lone pairs of electrons push the hydrogen atoms closer together

• Hydrogen bonds between molecules are responsible for:
• water’s relatively high melting and boiling points
• the solid form being less dense than the liquid form, meaning ice floats on water. In most substances, the solid form sinks in the liquid form.
• Water’s unusually high surface tension (it is difficult to break through the surface)
• Water’s high specific heat capacity

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