1. Chemistry 141 Friday, November 10, 2017 Lecture 28 Bond Enthalpy
Chemistry 11 - Lecture 11
9/30/2009
Chemistry 141
Friday, November 10, 2017
Lecture 28
Bond Enthalpy
Periodic Trends

2. Your questions:
What does a double bond between two atoms actually look like? Can you explain how a covalent bond comes into existence?
Can we go over what the formal charge means? 
For Li, Be, and B, are they more likely to form bonds which result 8 valence electrons or 2 valence electrons?
What do we do when the structures do not satisfy an octet?

3. Drawing Lewis Structures
Sum the valence electrons in the molecule (or ion – correct for charge)
Arrange and write the atomic symbols. Connect atoms with a single line to indicate a bond.
Use dots to complete octets around the peripheral atoms (except H, which has 2).
Place any remaining electrons as lone pairs on the central atom
If there are not enough electrons for the central atom to have an octet, try forming multiple bonds.
Calculate formal charges:
Formal charge = valence electrons – ½ (bonding electrons) – all nonbonding electrons

4. Average Bond Enthalpies

5. Bond lengths

6. Order of filling of atomic orbitals

7. Sizes of Atoms

8. Sizes of Ions

9. Ionization Energy

10. Trends in ionization energy
decrease down a column
increase across a row
decrease down column

11. Ionization Energy

12. Trends in electron affinity (EA)
N (EA >0)
low affinity
becomes more negative across a row
high affinity
Trends in EA are less predictable than those in IE
Must look carefully at electron configurations
Why is the EA of N positive but that of C negative? [C has a greater ‘affinity’]
N: 1s22s22p3 C: 1s22s22p2

13. Electron Affinities