Ch. 6.5 Molecular Structure (p. 183 - 187)

6.5 Molecular Geometry Properties of molecules depend upon: 1) Kinds of elements present & how much of each. 2) Bonding - types & strengths 3) Geometry - arrangement or structure

Molecular Geometry 2 Ways to predict Molecular Geometry 1) VSEPR - Valence Shell Electron Pair Repulsion This indicates that electrostatic repulsion between valence shell electrons will cause them to be as far apart from each other as possible. 2) Hybridization – mixing of 2 or more orbitals (usually S & P) of similar energies on the same atom to give new orbitals of equal energies.

A. VSEPR Theory Valence Shell Electron Pair Repulsion Theory Electron pairs orient themselves in order to minimize repulsive forces.

Lone pairs repel more strongly than bonding pairs!!! A. VSEPR Theory Types of e- Pairs Bonding pairs –shared pairs that form bonds Lone pairs - nonbonding e- pairs Lone pairs repel more strongly than bonding pairs!!!

A. VSEPR Theory Bond Angle Lone pairs reduce the bond angle between atoms. Bond Angle

VSEPR THEORY DEFINITION VSEPR Theory classifies molecules using LETTERS ~{“A” FOR CENTRAL ATOM,”B” for what is bonded to central atom “a” & #’s to indicate electron pairs surrounding a central atom, whether in a bond OR as a “lone pair” of e-

MOLECULAR GEOMETRY

AB2, A is central atom, 2 B’s are the surrounding atoms, shape is linear. AB3 , A is central atom, B are again the surrounding atoms, 3 this time, and to fully separate shape is trigonal-planar AB4, A remains as the central atom, B again are the 4 surrounding atoms, shape to maximize distance apart becomes tetrahedral.

Molecular Geometry

Molecular Geometry AB5, Triangular Bipyramidal AB6, Octahedral

Molecular Geometry Sometimes, molecules have lone pair(s) of electrons. These unshared pairs can cause bending (repulsion) of bonded pairs, (E stands for unshared pair). .. AB2E - Bent shape, SnCl2 Sn Cl Cl AB3E - Triangular pyramidal, NH3 N H H H .... AB2E2 - Bent, H2O O H H

B. Determining Molecular Shape Draw the Lewis Diagram. Tally up e- pairs on central atom. double/triple bonds = ONE pair Shape is determined by the # of bonding pairs and lone pairs. Know the 8 common shapes & their bond angles!

C. Common Molecular Shapes 2 total 2 bond 0 lone BeH2 LINEAR 180°

C. Common Molecular Shapes 3 total 3 bond 0 lone BF3 TRIGONAL PLANAR 120°

C. Common Molecular Shapes 3 total 2 bond 1 lone SnF2 BENT <120°

C. Common Molecular Shapes 4 total 4 bond 0 lone CH4 TETRAHEDRAL 109.5°

C. Common Molecular Shapes 4 total 3 bond 1 lone NH3 TRIGONAL PYRAMIDAL 107°

C. Common Molecular Shapes 4 total 2 bond 2 lone H2O BENT 104.5°

C. Common Molecular Shapes 5 total 5 bond 0 lone PCl5 TRIGONAL BIPYRAMIDAL 120°/90°

D. Examples F P F F PF3 4 total 3 bond 1 lone TRIGONAL PYRAMIDAL 107°

Molecular Geometry 2) Hybridization - mixing of 2 or more orbitals(usually S & P) of similar energies on the same atom to give new orbitals of equal energies. Methane CH4 - provides a good example(1s22s22p2) __↓ __↓ __ __ _ C 1s 2s 2px 2py 2pz 4 valence electrons(2s2 & 2p2) combine or hybridize into 4 new identical orbitals __ __ _ __ sp3

Molecular Geometry Each sp3 orbital has equal energy and are 109.5o apart in a tetrahedron. Hybridization also explains bonding & geometry in molecules formed by Group 15 & 16 elements.

Each sp3 orbital has equal energy and are 109 Each sp3 orbital has equal energy and are 109.5o apart in a tetrahedron.

D. Examples O C O CO2 2 total 2 bond 0 lone LINEAR 180°

6.5 MOLECULAR GEOMETRY Intermolecular Forces Forces of attraction between molecules: 1) usually weaker than bonds a) Dipole-dipole forces b) Induced dipole c) Hydrogen Bonding d) London Dispersion Forces

Dipole-dipole forces Strongest intermolecular forces are between polar molecules. Equal but opposite charges separated by a small distance creates a dipole. Dipole-dipole forces are the attraction between polar molecules. Negative region of one molecule attracts positive region of the other. Sometimes causes higher boiling pts.

Induced dipole A polar molecule can induce a dipole in a nonpolar molecule by momentarily attracting electrons. **These forces are less or weaker than true dipole-dipole forces. These forces are responsible for O2, a nonpolar molecule to dissolve in water, a polar molecule.

INDUCED DIPOLE H+ O- +O = O-

Hydrogen Bonding Type of bonding between a hydrogen atom and an unshared pair of electrons of a strongly electronegative atom of another molecule Causes the high boiling point and surface tension of water. +H O-----+H +H +H O-

London Dispersion Forces Weak intermolecular forces caused by constantly moving electrons and the creation of instantaneous and induced dipoles. Present in all atoms & molecules. The only force in noble gases & nonpolar molecules. Increases with atomic mass, molecular mass.

MOLECULAR POLARITY Molecular Polarity is determined by 1) polarity of bonds - diatomic and 2) orientation(Geometry) of atoms – Linear, Bent, Triangular, Pyramidal, Tetrahedral

NONPOLAR MOLECULE In some molecules, bond dipoles oppose one another causing zero polarity. Cl CCl4 Cl-C-Cl also CO2 O=C=O

+ - H Cl A. Dipole Moment Direction of the polar bond in a molecule. Arrow pointing from + to –. H Cl + -

B. Determining Molecular Polarity Depends on: bond polarity molecular shape Possibilities: NP bonds  NP molecule Polar bonds  NP molecule Polar bonds  Polar molecule

B. Determining Molecular Polarity Polar bonds  NP molecule Dipole moments are symmetrical and cancel out. BF3

B. Determining Molecular Polarity Polar bonds  Polar molecule Dipole moments are asymmetrical and don’t cancel . H2O

B. Determining Molecular Polarity Polar molecules have... polar bonds AND asymmetrical shape &/or atoms CCl3F

STRUCTURAL FORMULA HCl H2O C2H6 Indicates KIND’S, NUMBERS’S & ARRANGEMENT(GEOMETRY) of bonds in a molecule …. H - Cl O H - C - C- H H H HCl H2O C2H6

MULTIPLE COVALENT BONDS DOUBLE BOND H H H H C : : C or C = C

Triple covalent bond N2 : N ::: N : or :N = N: C2H2 H: C ::: C :H or H - C = C - H