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

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?

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

Average Bond Enthalpies

Bond lengths

Order of filling of atomic orbitals

Sizes of Atoms

Sizes of Ions

Ionization Energy

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

Ionization Energy

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

Electron Affinities