General Chemistry M. R. Naimi-Jamal Faculty of Chemistry Iran University of Science & Technology

فصل نهم: شکل هندسی‌ مولکول اوربیتال مولکولی

The Shapes of Molecules H O H

Terminology VSEPR Theory Valence Shell Electron Pair Repulsion theory Valence Shell Electron Pair Repulsion theory Electron pairs repel each other whether they are in chemical bonds (bond pairs) or unshared (lone pairs). Electron pairs assume orientations about an atom to minimize repulsions.

Electron group geometry – distribution of e - pairs. Molecular geometry – distribution of nuclei. Most important factor in determining geometry is relative repulsion between electron pairs. Molecular Geometry

Balloon Analogy

H H H H tetrahedral 109  C  planar trigonal FF B F  linear 2 Geometry Example No. of e- Pairs Around Central Atom F—Be—F

Ammonia, NH 3 1. Draw electron dot structure 2. Count BP’s and LP’s of the central atom: 4 pairs 3. The 4 electron pairs are at the corners of a tetrahedron. Structure Determination by VSEPR

Ammonia, NH 3 The electron pair geometry is tetrahedral. The MOLECULAR GEOMETRY — the positions of the atoms — is PYRAMIDAL. Structure Determination by VSEPR

Water, H 2 O 1. Draw electron dot structure 2. Count BP’s and LP’s = 4 3. The 4 electron pairs are at the corners of a tetrahedron. The electron pair geometry is TETRAHEDRAL. Structure Determination by VSEPR

Water, H 2 O The electron pair geometry is TETRAHEDRAL. The molecular geometry is bent. Structure Determination by VSEPR

Formaldehyde, CH 2 O 1. Draw electron dot structure 2. Count BP’s and LP’s = 3 3. There are 3 electron pairs are at the corners of a planar triangle. The electron pair geometry is PLANAR TRIGONAL with 120 o bond angles. Structure Determination by VSEPR

Formaldehyde, CH 2 O The electron pair geometry is PLANAR TRIGONAL The molecular geometry is also planar trigonal. Structure Determination by VSEPR

Methanol, CH 3 OH 1. Draw electron dot structure 2. Define bond angles 1 and 2 Structure Determination by VSEPR

Methanol, CH 3 OH Define bond angles 1 and 2 Angle 1 = 109 o Angle 2 = 109 o H H 109  H—C—O—H Structure Determination by VSEPR In both cases the atom is surrounded by 4 electron pairs.

Acetonitrile, CH 3 CN Draw the electron dot structure Structure Determination by VSEPR

Acetonitrile, CH 3 CN Define bond angles 1 and 2 Angle 1 = 109 o Angle 2 = 180 o One C is surrounded by 4 electron “lumps” and the other by 2 “lumps” H H 109  H—C—CN 180  Structure Determination by VSEPR

Phenylalanine, an amino acid

Phenylalanine

STRUCTURES WITH CENTRAL ATOMS THAT DO NOT OBEY THE OCTET RULE

Compounds with 5 or More Pairs Around the Central Atom Trigonal bipyramid 120  90  F F F F FP F Octahedron F F F FF 90  S 5 electron pairs 6 electron pairs

Number of valence electrons = 34 Central atom = S Dot structure Electron pair geometry = ? Sulfur Tetrafluoride, SF 4 F F F F S

Number of valence electrons = 34 Central atom = S Dot structure Electron pair geometry = trigonal bipyramid (because there are 5 pairs around the S) F F F F 120  90  S Sulfur Tetrafluoride, SF 4 F F F F S

Lone pair is in the equator because it requires more room. 90  F F F F 120  S Sulfur Tetrafluoride, SF 4 F F F F S

Hybridization of Atomic Orbitals

sp 3 Hybridization

Bonding in Methane

sp 3 Hybridization in Nitrogen

Bonding in Nitrogen

sp 2 Hybridization in Boron

Orbitals in Boron

sp Hybridization in Be

Orbitals in Beryllium

sp 3 d and sp 3 d 2 Hybridization

Hybrid Orbitals and VSEPR Write a plausible Lewis structure. Use VSEPR to predict electron geometry. Select the appropriate hybridization.

Multiple Covalent Bonds Ethylene (H 2 C=CH 2 ) has a double bond in its Lewis structure. VSEPR says trigonal planar at carbon.

Ethylene

Acetylene Acetylene, C 2 H 2, has a triple bond. VSEPR says linear at carbon.

Applying VSEPR Theory Draw a plausible Lewis structure. Determine the number of e - groups and identify them as bond or lone pairs. Establish the e - group geometry. Determine the molecular geometry. Multiple bonds count as one group of electrons. More than one central atom can be handled individually.

Molecular Orbital Theory Atomic orbitals are isolated on atoms. Molecular orbitals span two or more atoms. LCAO: –Linear Combination of Atomic Orbitals. Ψ 1 = φ 1 + φ 2 Ψ 2 = φ 1 - φ 2

Combining Atomic Orbitals

Molecular Orbitals of Hydrogen

Basic Ideas Concerning MOs Number of MOs = Number of AOs. Bonding and antibonding MOs formed from AOs. e - fill the lowest energy MO first. Pauli exclusion principle is followed. Hund’s rule is followed

Bond Order Stable species have more electrons in bonding orbitals than antibonding. Bond Order = # e - in bonding MOs - # e - in antibonding MOs 2

Diatomic Molecules of the First-Period BO = (1-0)/2 = ½ H2+H2+ BO = (2-0)/2 = 1 H2H2 BO = (2-1)/2 = ½ He 2 + BO = (2-2)/2 = 0 He 2 BO = (e - bond - e - antibond )/2

Molecular Orbitals of the Second Period First period use only 1s orbitals. Second period have 2s and 2p orbitals available. p orbital overlap: –End-on overlap is best – sigma bond (σ). –Side-on overlap is good – pi bond (π).

Molecular Orbitals of the Second Period

Combining p orbitals

Expected MO Diagram of A 2

MO Diagram of A 2, Z ≥ 8

Modified MO Diagram of A 2, Z < 8

MO Diagrams of 2 nd Period Diatomics

Benzene

Ozone

Chapter 9 Questions 7, 16, 22, 25, 33 34, 35, 43