1. Warm-up #26 Compare CCl4 and MgCl2
1. Which compound is ionic and which is molecular? _____________________Explain_______________
2. What would you call the ionic compound? ________ The molecular compound?____________
3. What type of bond is in the molecular compound? ______________The ionic compound? __________
4. Is the ionic compound polar or nonpolar? _______
5. The molecular compound?_________________

2. Review-Properties of Molecular Compounds
Called molecules (Nonmetal)(Nonmetal)
Have covalent bonds
Do not conduct electricity
Solids, liquids and gases
Low melting points and boiling points
The molecules can also form polar or nonpolar bonds.

3. Polar Covalent Bonds Bonds between nonmetal compounds
Electrons are shared, unequally
More electronegative atoms pull e- towards it = more negative
weaker electronegative atoms the e- are pulled away = more positive.

4. Increasing electronegativity

5. Example 1: Oxygen - Hydrogen
This shared pair of atoms has a large difference in electronegativities.
Oxygen is more electronegative than hydrogen.
This means the e- pair will spend their time closer to the oxygen.

6. This oxygen has more e- around it for most of the time, so it has a slight negative charge and the hydrogen has less e- so it has s slight positive charge.
Over all neutral charge

8. Polar Bonds Ex. 2 NH3 Is this molecule ionic or covalent? covalent
Which atom is more electronegative? N
Which atom will pull e- towards itself?
What is the partial charge of N? δ-
What is the partial charge of each H? δ+

9. Which is more electronegative?
H F HF CO I2
BF3
H=+, F=-
C= +, O=-
Equal (nonpolar)
B=+, F=-

10. Non-polar Covalent Bonds
Non-polar Covalent bond — a covalent bond in which the bonding electrons are shared equally by the bonded atoms.
H2, N2, H2, F2 all have non-polar covalent bonds
Each element has the same electronegativity so both atoms attract electrons equally.

11. Polar & Non-polar Covalent Bonds
What if two different atoms with different electronegativities are bonded? HF
H = 2.2, F = 4.0 so F will attract electron better
An electron pair is more likely to reside closer to the nucleus of the atom with the greater electronegativity and forms a polar covalent bond. The greater the difference between the electronegativites, the more polar the bond.
H F

12. Polar & Nonpolar Covalent Bonds
What if the difference between the electronegativities is really large?
Na and F
Na = 0.93, F = Difference is 3.07!
Called an ionic bond.

13. Polarity of Molecules
Just as bonds can be polar (electronegativity of atoms) so can molecules
If the atoms attract electrons around the central atom symmetrically, the molecule is non-polar C O B F O H
If the atoms pull electrons around the central atom asymmetrically, the molecule is polar (cut in half)

14. Intermolecular Forces: a force of attraction between different molecules

15. Three types of intermolecular forces
INTERmolecular forces do not chemically combine anything. They do help molecules “stick” to one another
All intermolecular forces are weaker than actual bonds (covalent, ionic)
Three types of intermolecular forces
London Dispersion
Dipole-Dipole
Hydrogen Bonding

16. London Dispersion Forces
Weakest intermolecular forces
Caused by random and temporary movement of electrons causing temporary “mini-dipoles”
Present among all molecules, but only are significant in nonpolar molecules

17. London Dispersion - A Short Visual
Three Molecules + e- + e- + e-
When one molecule has a temporary imbalance in electrons + e- δ- δ+ + e- + e- + e- δ- δ+ + e- δ- δ+ + e-
the other molecules respond to the imbalance + e- + e- δ- δ+ + e- δ- δ+ + e- + e- + e- δ- δ+

18. Dipole-Dipole
Still a very weak force, but stronger than London dispersion
Found between polar molecules- with one positive and one negative side
The positive side of one molecule attracts the negative of another molecule

19. Dipole-Dipole - A Short Visual
One end of a molecule is permanently positive and one end is permanently negative
Opposites attract δ+
δ - δ+
δ -

20. Dipole-Dipole - Another Short Visual
A quick visual of what this might look like…
Notice there is not a regular pattern of the molecules - the attraction is not strong enough to hold the molecules in a fixed position.

21. Hydrogen Bonding
A weak force of attraction, but strongest of the intermolecular forces
Occurs when the most electronegative elements – F, O, and N– are directly bonded to H.
High electronegative elements attract electrons so strongly that the molecule becomes very polar
Think of it as a dipole –dipole interaction on steroids!

22. Hydrogen Bonding – A Short Visualδ-
The very polar molecule has a very negative end and a very positive end. O H
δ -
δ + O H
The negative electron pairs are attracted to the positive hydrogen.
δ +

23. Hydrogen bonds lead to higher boiling points!