3.4 and 3.5: Forces and
Bonding
Ionic Compounds
• Recall: Ionic compounds form a LATTICE
• Ex: NaCl
Molecular Compounds
• Have INTERmolecular forces and INTRAmolecular 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
Molecular Compounds
• Example: H2O
Molecular Compounds
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
1. London Dispersion 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
2. Dipole-dipole 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
3. Hydrogen Bonds
• 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.
• Why N, O, or F? These are the three most
electronegative elements of all, partly because they are
so small.
• happens because of the large electronegativity difference
between hydrogen and the other atom
• Strongest of the three forces
Types of Intermolecular Forces
3. Hydrogen Bonds
Types of Intermolecular Forces
Recap: Relative strength of bonds
Highest
Intramolecular
Covalent
bonds
Intermolecular
Hydrogen
Dipolebonds
dipole
Lowest
LDF
Collectively called
“Van der Waals forces”
Types of Intermolecular Forces
• See flow chart on pg 114
Water and Hydrogen Bonding
•
•
•
•
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
Water and Hydrogen Bonding
• 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