Intermolecular Forces

Assumptions for “Ideal Gases”
High Temperature with Low Pressure
No intermolecular forces
Elastic Collisions
Molecules have no mass or volume
Van der Waals Forces
Weak forces between nonpolar or polar molecules.
Van der Waals:
Dipole-Dipole
Between 2 polar molecules.
A negative to positive attraction.
Van der Waals:
Dipole-Induced Dipole
A nonpolar molecule is affected by the polarity of another molecule. A dipole is created int he nonpolar molecule.
Van der Waals:
London Dispersion
The electron cloud is stretched: (-) electrons are attracted to the (+) nuclei of a neighboring molecule.
Occurs in nonpolar molecules.

Effect increases with greater mass/electrons/protons. With more electrons, outer electrons are held loosely.

Ion-Dipole
Between an ion and polar molecule. An attraction of charge.
Ex: salt dissolving in water.

Cations have stronger attraction than anions. Smaller cations have stronger attraction that larger ones (fewer (-) electrons)

Ionic vs. Molecular forces
Ionic forces are always stronger than molecular ones.
Hydrogen Bonding
(Van der Walls)
Lone pairs of electrons are attracted to neighboring hydrogen.
Special, strong Van der Walls.

Occurs with large electronegative atoms: F, O, N

Solids:
Molecular Solid
Usually a gas, but at low temperatures, solid.
Becomes a solid when IMFs increase.
Has dispersion, dipole and/or hydrogen bonds.
Solids:
Metallic Bonding
Sea of electrons–electrons are freely shared. Delocalized bonding.
Causes metallic flexibility.
Solids:
Network Bonding
An array of covalent bonds.
Material comes off in sheets like mica.
Solids:
Ionic Bonding
Cations and anions are held together by charge (electrostatic) attraction. Lattice pattern with alternating + and – ions.