1. Week 4, Section Bonds: Tug of war for electrons
9/30/05

2. H2
H-H
Do both atoms contribute equally to the bond?

3. HCl Do both atoms contribute equally to the bond?
Are the electrons in the bond shared equally by both atoms?
Have 2 volunteers grab a chain and pull in opposite directions. Ask if the students are representing the H-H bond or the H-Cl bond. If the HCl bond, which student would represent Cl in the bond?

4. Polarity Polarity is an unequal sharing of electrons
What causes polarity in HCl?

5. Electronegativity
the ability of an atom in a covalent bond to attract shared electrons to itself

6. Is the molecule polar? CO2 NH3 SF4
In order to determine polarity, you must visualize the tug-of-war going on in the molecule for the electrons.
Have students draw Lewis structure, name the molecular geometry, and demonstrate with ropes the polarity of a molecule.
Introduce the arrow notation to denote dipoles.

7. For a molecule to be polar it must:
1. have at least 1 polar bond or 1 lone pair
2. and polar bonds or lone pairs must be arranged in space so that the dipoles do not cancel out

8. Are they polar? C2H2 XeF2 BF3 SF6 CHCl3 XeF4 ICl3 BrF5 Report back:
#Valence electrons
Lewis structure
Molecular structure
Dipole (if any)

9. Polarity of molecules Electron density maps (color) Plus-arrows CHCl3
BrF5
SCN-
SCN- (do dipoles cancel out)
H20 SO2 opposite dipole
BrF5 dipole even in large molecule!
SO2 or
H2O?