AP Chemistry Ch 8 Bonding

Vocab (Ch 8) Isoelectronic series Dipole Dipole moment Formal charge Resonance structure Bond enthalpy

Lewis Dot Structures Create structures that satisfy the Octet Rule. This shows us how the electrons are distributed within a molecule. Page 278

Formal Charge The Formal Charge of an atom is the charge that an atom in a molecule would have if all atoms had the same electronegativity. There may be instances when you can draw 2 structures for the same molecule. Then you would want to use the Formal Charge Method to determine which structure is the most feasible.

Calculating Formal Charge 1. All of the unshared (nonbonding) electrons are assigned to the atom on which they are found. 2. Half of the bonding electrons are assigned to each atom in the bond. 3. Formal Charge = # of valence e- in the isolated atom minus # of electrons assigned to the atom in the Lewis Structure. See pages 280-281 for examples.

Resonance Structures These are individual Lewis dot structures in cases where 2 or more Lewis structures are equally good descriptions of a single molecule. The resonance structures are equivalent except for the placement of electrons.

Resonance Structures Use double-headed arrows to indicate that the structures are resonance structures. Molecules that have resonance structures can not be described accurately using only one Lewis structure. The true arrangement of the electrons must be considered as a “blend” of 2 or more Lewis structures. The molecule does not “oscillate” between 2 or more Lewis structures. See page 283

Resonance Structures commons.wikimedia.org/wiki/File:Nitrate_ion_r...

Exceptions to the Octet Rule There are 3 main types of exceptions: Molecules with an odd number of electrons. Molecules in which an atom has less than an octet. Molecules in which an atom has more than an octet. Page 285-287

Strengths of Covalent Bonds The stability of a molecule is related to the strengths of the covalent bonds it contains. The strength of a covalent bond between 2 atoms is determined by the energy required to break that bond. We will use bond enthalpy to determine the strength of the bonds.

Enthalpy and Bond Strength The bond enthalpy is always a positive value. Energy is always required to break chemical bonds. Energy is released when a bond forms between 2 gaseous atoms or molecular fragments. The greater the bond enthalpy, the stronger the bond. A molecule with strong chemical bonds generally has less tendency to undergo chemical change than a molecule with weak bonds.

Bond Enthalpy and Bond Strength ΔH = Σ(bond enthalpies of bonds broken) minus Σ(bond enthalpies of bonds formed) One method of doing this is to break all of the bonds on the reactant side and add them up. Then break all of the bonds formed on the product side and add them up. Subtract products from reactants. Use the chart on page 289 for Average Bond Enthalpy values.

Exam Topics Concept Ionic vs Covalent vs Metallic bonding Lattice energy problems Isoelectronic series (nuclear charge) Electronegativity, bond polarity, dipole, dipole moment Bond length Formal charge Resonance structures Exceptions to the Octet Rule Bond Enthalpy Calculations Draw Lewis Dot Structures

Problems to Try Ch 8 # 9-11, 19, 20, 23, 27, 33, 34, 40, 41, 43, 44, 46-48, 50, 51, 54, 60, 61, 63, 64, 66-68, 79, 81, 85, 87 AP Exam Problems to Try 1999 # 8 2000 # 7 (last section) 2002 # 6 2003 # 8 2004 # 7 & # 8