1. Intermolecular Forces

2. What Holds A Liquid Or Solid Together?
In ionic compounds, all atoms are equally bonded together in the crystal
What about covalent?

3. Two Types of Covalent Compounds
Covalent Network Crystal
- all atoms covalently bonded together equally
- very strong and hard
- ex: diamond, graphite

4. Covalent Molecular Substance
- individual molecules attracted to one another through intramolecular forces
- not anywhere near as strong as networks
- ex: water

5. Intermolecular Forces
Force of attraction between molecules in a covalent molecular substance
Three types:
Dipole – Dipole forces
Hydrogen Bond
London Dispersion forces

6. Dipole- Dipole Between polar molecules
Positive pole of one ion attracted to the negative pole of another

7. Hydrogen Bonding
Attraction between hydrogen atom in one molecule and an unshared pair of electrons on the atom of another molecule

8. London Dispersion or Van der Walls Force
Hold non-polar molecules together
Caused by motion of electrons

9. Strength of Interaction
Substances held together by ionic bonds or covalent network bonds are very rigidly connected: mostly solid at room temp and melt at very high temps
Those held by dipole-dipole or hydrogen bonds are fairly strongly connected: liquid at room temp or solids that melt at low temps
Those held by london dispersion forces are weakly connected: gases at room temp; need to be very cold to become liquid

10. Ionic vs. Covalent Ionic Covalent Molecular Melting Point High Low
Ionic
Covalent Molecular
Melting Point
High
Low
Vaporize No
Maybe
Crystalline
Yes, hard & brittle
Maybe, soft
Dissolve in Water
Yes
If polar covalent
Conduct electricity when liquid
Conduct electricity when dissolve in water

11. Example
Substance A melts at 600oC, dissolves well in water, and does not conduct electricity when solid but does when dissolved in water. Is the substance held together by ionic or covalent bonds?

12. Example Identify the interparticle forces present in each substance
a) HI
b) SO2
c) Ne
d) CF4
e) CH3OH

13. Stronger the intermolecular forces,
- the more solid in nature the substance
- higher the melting and boiling points
- higher the viscosity and surface tension
- lower the vapor pressure

14. Metallic Bond
Bond that exists between metal atoms

15. Electron Sea Model
Metal atoms give up valence electrons and form + ions
The released electrons move freely around the + metal ions

17. Properties of Metallic Bonds
Good conductors of electricity – free electrons
Malleable and ductile – not in rigid position so ions can be shaped and drawn into wires
Conduct heat because of free electrons
Lusterous – absorb and emit light in regular pattern due to free electrons

18. Alloy
Mixtures of metals

19. Interstitial Alloy
One metal in the mixture is a much smaller atom than the other
Increases the overall density of the metal
Tends to decrease malleability and ductility
Steel is an example: carbon between iron

20. Substitutional Alloy Atoms of similar radius mixed
Density is an average of the pure metal density
Stays malleable and ductile
Brass is an example: Copper and Zinc