1. Intermolecular Forces

2. Review Substances can be placed into four possible categories.
1. Ionic
2. Metallic
3. Network covalent
4. Molecular

3. Ionic Ions are held together by strong ionic bonds.
These bonds are caused by electrostatic forces (opposites attract).
As a result, ALL ionic compounds have high melting points and boiling points.
Solid at room temperature.
Ex: NaCl

4. Metallic Formed by the attractions of metal atoms.
Atoms are held together by metallic bonds.
As a result, metals have relatively high melting points & boiling points.
ALL metals are solid at room temperature (except Hg).

5. Network Covalent
3-D chains of carbon or silicon atoms covalently bonded to each other.
Covalent bonds are strong
Network of covalent substances have extremely high melting points & boiling points.
EX: Carbon (graphite or diamond), Silicon, Silicon carbide

6. Molecular Substances
“intra”= within the molecule. vs. “inter”= between molecules.
For molecular compounds, the intramolecular forces are STRONG covalent bonds.
The intermolecular forces are WEAK; they are not bonds.

8. Types of Intermolecular Forces
1. Van der Waal’s Forces
A. Londons (or dispersion) forces
B. Dipole-dipole forces
2. Hydrogen bonding forces

9. London’s (dispersion) Forces
Caused by instantaneous dipoles. This is caused by the constant motion of electrons. At any instant, there will be an uneven distribution of the electrons in a molecule.
ALL molecular compounds exhibit dispersion forces.
The strength of dispersion forces depends on 2 factors.
# of electrons.
How spread out a molecule is.

10. Dipole-dipole forces
Dipole: one side of a molecule is positive and the other side is negative.
Dipole-dipole forces are caused by a permanent dipole in a molecule.
Ex: H-Cl
Chlorine has a stronger
attraction for the electrons
that are “shared”.

11. Dipole-dipole forces
Dipole-dipole forces only occur in polar molecules. The dipole in one molecule is attracted to the dipole in a nearby molecule.

12. Hydrogen Bonding Forces
A unique force that ONLY occurs between Hydrogen in one molecule and a highly electronegative element in a neighboring molecule.
Not as strong as a chemical bond, but is significantly stronger than the London’s dispersion forces.

14. Strength of Attraction
Ionic Bonds
Covalent Bonds
Hydrogen Bonds
Dipole-dipole
London’s Dispersion
Increasing Strength

15. Example – Which will have the higher boiling point? Kr or Ne
Both Kr and Ne are noble gases that only have dispersion forces attracting the atoms.
Kr is much larger with more electrons making it more polarizable. Therefore, Kr has the higher boiling point due to its stronger dispersion forces.

16. Example – Which will have the higher melting point? C or Cl2
Carbon atoms make networks of covalent bonds (graphite, diamond). To melt carbon requires breaking covalent bonds.
Cl2 molecules are nonpolar, so they are attracted by dispersion forces only.
Carbon has much stronger forces that need to be weakened so it has a much higher melting point.

17. Example – Which will have the higher boiling point? H2O or H2S
The attractive forces between molecules of H2O (dispersion, dipole-dipole, and hydrogen bonding) are stronger than the attractive forces between molecules of H2S (dispersion and dipole-dipole).
Stronger forces require higher temperatures to break, so H2O will have the higher boiling point.