WRITING LEWIS STRUCTURES
(1)Total valence electrons » Include charge (2) Skeleton POLAR OR NOT? » Clues (3) Complete octet around terminals » Octet Rule: Nonmetals, except for H, achieve a noble-gas structure by sharing in an “octet” of electrons (4) Complete octet of central » Not enough, multiple bonds
EXCEPTIONS TO OCTET RESONANCE RULE
Resonance Structures Exceptions to Octet Rule – Both are right and neither is correct – Odd # of valence electrons (free radicals) – True is hybrid – Incomplete octet – Differ in arrangement of electrons » Groups 2 and 13 Isomers: Differ in arrangement of atoms, – Expanded octet different skeleton » Period 3 or physically lower
ELECTRONEGATIVITY ELECTRONEGATIVITY
Ability of an atom to attract electrons in a chemical bond Unequal sharing
Electron Affinity - measurable, Cl is highest
X (g) + e- X-(g)
Electronegativity - relative, F is highest
ELECTRONEGATIVITY ELECTRONEGATIVITY
Polar covalent bond or polar bond is a covalent bond with Classification of bonds by difference in electronegativity greater electron density around one of the two atoms Difference Bond Type 0 Covalent > 2 Ionic electron rich electron poor region 0 < and < 2 Polar Covalent region e- poor e- rich H F H F Increasing difference in electronegativity δ+ δ- Covalent Polar Covalent Ionic
share e- partial transfer of e- transfer e-
ELECTRONEGATIVITY MOLECULAR GEOMETRY
Valence Classify the following bonds as ionic, polar covalent, Shell or covalent: The bond in CsCl; the bond in H2S; and the NN bond in H2NNH2. Electron Pair Cs – 0.7 Cl – 3.0 3.0 – 0.7 = 2.3 Ionic Repulsion VSEPR Theory H – 2.1 S – 2.5 2.5 – 2.1 = 0.4 Polar Covalent In VSEPR theory, multiple bonds behave like a single electron pair N – 3.0 N – 3.0 3.0 – 3.0 = 0 Covalent
All have tetrahedral electronic geometry Different molecular geometry
lone-pair vs. lone pair lone-pair vs. bonding bonding-pair vs. bonding repulsion > pair repulsion > pair repulsion
MOLECULAR GEOMETRY Predicting Molecular Geometry 1. Draw Lewis structure for molecule. 2. Count number of lone pairs on the central atom and number of atoms bonded to the central atom. 3. Use VSEPR to predict the geometry of the molecule.
What are the molecular geometries of SO2 and SF4? O S O F AB4E
AB2E F S F distorted bent tetrahedron 10.1 F
DIPOLE MOMENTS DIPOLE MOMENTS
Bonds and molecules may be polar or Dipole Moments and Polar Molecules nonpolar Relative to distribution of electrons electron rich electron poor Dipole moment (µ= Q x r) region region Bonds F – Compare EN’s of 2 atoms H Molecule – Consider bonds + shape δ+ δ-
DIPOLE MOMENTS DIPOLE MOMENTS
H2O vs CO2 Does CH2Cl2 have BF3 vs NH3 a dipole moment?
cis-C2H2Cl2 vs trans- C2H2Cl2
Yes