1. Polarity, Lewis Structures, and Resonance
Sections 4-6

2. Bond Polarity Ionic and covalent bonding is not black and white
Sharing is not usually equal
Electron tug of war
Must look at electronegativity to see how equally electrons are shared

3. Electronegativity
The ability of an atom in a molecule to attract electrons to itself
Linus Pauling

4. Electronegativity: Increases as you go… …from left to right
…from bottom to top

5. Polar Covalent Bonds
When two atoms share electrons unequally, a bond dipole results.
The dipole moment, , produced by two equal but opposite charges separated by a distance, r, is calculated:
 = Qr
It is measured in debyes (D).

6. Polar Covalent Bonds
The greater the difference in electronegativity, the more polar is the bond.

7. Writing Lewis Structures
PCl3
Find the sum of valence electrons of all atoms
- For anions, add 1 for each negative charge.
For cations, subtract 1 for each positive charge.
(7) = 26

8. Writing Lewis Structures
The central atom is the least electronegative that isn’t hydrogen. Connect the outer atoms to it by single bonds.
Keep track of the electrons:
26  6 = 20

9. Writing Lewis Structures
Fill the octets of the outer atoms.
Keep track of the electrons:
26  6 = 20  18 = 2

10. Writing Lewis Structures
Fill the octet of the central atom.
Keep track of the electrons:
26  6 = 20  18 = 2  2 = 0

11. Writing Lewis Structures
If you run out of electrons before the central atom has an octet…
…form multiple bonds until it does.

12. Formal Charge
1) For each atom, count the electrons in lone pairs and half of its shared electrons.
2) Subtract that from the number of valence electrons
3) The difference is its formal charge.

13. The Best Lewis Structure
Has the fewest non-zero formal charges
Puts negative charge on the most electronegative atom

14. Resonance
Lewis structure for ozone, O3. +1 -1

15. Resonance At odds with true, observed structure, in which…
…both O—O bonds are the same length
…both outer O’s have a charge of 1/2

16. Resonance One Lewis structure cannot accurately depict some molecules
Use resonance structures to describe these molecules.

17. Resonance Just as green is a synthesis of blue and yellow…
…ozone is a synthesis of these two resonance structures.

18. Homework
30, 34, 36, 38, 40 (a only), 42, 46, 50, and 52 starting on page 337