1. Intermolecular Forces

2. Intra- versus Inter-molecular Forces
So far in this unit we have been discussing intramolecular forces
Intramolecular forces = forces within the molecule (chemical bonds)
Now let’s talk about intermolecular forces
Intermolecular forces = forces between separate molecules

3. Breaking Intramolecular forces
Breaking of intramolecular forces (within the molecule) is a chemical change
2 H2 + O2  2 H2O
Bonds are broken within the molecules and new bonds are formed to form new molecules

4. Breaking Intermolecular forces
Breaking of intermolecular forces (between separate molecules) is a physical change
Breaking glass is breaking the intermolecular connections between the glass molecules to separate it into multiple pieces.
Boiling water is breaking the intermolecular forces in liquid water to allow the molecules to separate and be individual gas molecules.

5. London Dispersion Forces
This lop-sidedness of electrons creates a partial negative charge in one area and a partial positive charge in another.
Electrons move around the nuclei. They could momentarily all “gang up” on one side
All molecules have electrons +
Positively charged nucleus -
Negatively charged electron + - + -
Electrons are fairly evenly dispersed. + -
As electrons move, they “gang up” on one side.

6. London Dispersion Forces
Once the electrons have “ganged up” and created a partial separation of charges, the molecule is now temporarily polar.
The positive area of one temporarily polar molecule can be attracted to the negative area of another molecule. + - + -

7. Strength of London Dispersion Forces
Electrons can gang up and cause a non-polar molecule to be temporarily polar
The electrons will move again, returning the molecule back to non-polar
The polarity was temporary, therefore the molecule cannot always form LDF.
London Dispersion Forces are the weakest of the intermolecular forces because molecules can’t form it all the time.

8. Strength of London Dispersion Forces
All molecules have electrons…all molecules can have London Dispersion Forces
The more electrons that gang-up, the larger the partial negative charge.
Larger molecules have more electrons
The larger the molecule, the stronger the London Dispersion Forces
Larger molecules have stronger London Dispersion Forces than smaller molecules.

9. Dipole Forces
Polar molecules have permanent partial separation of charge.
The positive area of one polar molecule can be attracted to the negative area of another molecule. + - + -

10. Strength of Dipole Forces
Polar molecules always have the ability to form attractions with opposite charges
Polar molecules always have a partial separation of charge.
Dipole forces are stronger than London Dispersion Forces

11. Hydrogen Bonding Hydrogen has 1 proton and 1 electron.
There are no “inner” electrons. It bonds with the only one it has.
When that electron is shared unevenly (a polar bond) with another atom, the electron is farther from the hydrogen proton than usual.
This happens when Hydrogen bonds with Nitrogen, Oxygen or Fluorine
This creates a very strong dipole (separation of charges) since there’s no other electrons around the hydrogen proton to counter-act the proton’s positive charge.

12. Strength of Hydrogen Bond
This very positively-charged hydrogen is highly attracted to a lone pair of electrons on another atom.
It’s an extreme example of polar bonding with the hydrogen having a large positive charge.
Hydrogen has no inner electrons to counter-act the proton’s charge
This is the strongest of all the intermolecular forces.

13. Hydrogen Bond N H
Hydrogen bond N H