2Chemical Bonding Problems and questions — How is a molecule or polyatomic ion held together?Why are atoms distributed at strange angles?Why are molecules not flat?Can we predict the structure?How is structure related to chemical and physical properties?
3Structure & Bonding NN triple bond. Molecule is unreactive Phosphorus is a tetrahedron of P atoms. Very reactive!Red phosphorus, a polymer. Used in matches.
4Forms of Chemical Bonds There are 2 extreme forms of connecting or bonding atoms:Ionic—complete transfer of 1 or more electrons from one atom to anotherCovalent—some valence electrons shared between atomsMost bonds are somewhere in between.
5Ionic CompoundsMetalNonmetal2 Na(s) + Cl2(g) ---> 2 Na Cl-
6Covalent BondingThe bond arises from the mutual attraction of 2 nuclei for the same electrons. Electron sharing results.Bond is a balance of attractive and repulsive forces.
7Note that each atom has a single, unpaired electron. Bond FormationA bond can result from a “head-to-head” overlap of atomic orbitals on neighboring atoms.ClH•••+Overlap of H (1s) and Cl (3p)Note that each atom has a single, unpaired electron.
8Chemical Bonding: Objectives Objectives are to understand:1. valence e- distribution in molecules and ions.2. molecular structures3. bond properties and their effect on molecular properties.
9Electron Distribution in Molecules Electron distribution is depicted with Lewis electron dot structuresValence electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS.G. N. Lewis
10Bond and Lone PairsValence electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS.•••HClshared orbond pairlone pair (LP)This is called a LEWIS ELECTRON DOT structure.
12Rules of the GameNo. of valence electrons of a main group atom = Group number•For Groups 1A-4A, no. of bond pairs = group number.• For Groups 5A -7A, BP’s = 8 - Grp. No.
13Rules of the Game•Except for H (and sometimes atoms of 2nd and 3rd families and 3rd and higher periods),BP’s + LP’s = 4This observation is called theOCTET RULE
14Building a Dot Structure Ammonia, NH31. Decide on the central atom; never H.Central atom is atom of lowest affinity for electrons.Therefore, N is central2. Count valence electronsH = 1 and N = 5Total = (3 x 1) + 5= 8 electrons / 4 pairs
15Building a Dot Structure 3. Form a single bond between the central atom and each surrounding atomHN4. Remaining electrons form LONE PAIRS to complete octet as needed.H••N3 BOND PAIRS and 1 LONE PAIR.Note that N has a share in 4 pairs (8 electrons), while H shares 1 pair.
16Sulfite ion, SO32- Step 1. Central atom = S Step 2. Count valence electrons S = 63 x O = 3 x 6 = 18Negative charge = 2TOTAL = 26 e- or 13 pairsStep 3. Form bonds10 pairs of electrons arenow left.
17Each atom is surrounded by an octet of electrons. Sulfite ion, SO32-Remaining pairs become lone pairs, first on outside atoms and then on central atom.•••OS••Each atom is surrounded by an octet of electrons.
18Carbon Dioxide, CO2 1. Central atom = _______ 2. Valence electrons = __ or __ pairs3. Form bonds.This leaves 6 pairs.4. Place lone pairs on outer atoms.
19Carbon Dioxide, CO2 4. Place lone pairs on outer atoms. 5. So that C has an octet, we shall form DOUBLE BONDS between C and O.The second bonding pair forms a pi (π) bond.
20Double and even triple bonds are commonly observed for C, N, P, O, and S H2COSO3C2F4
21Sulfur Dioxide, SO2 O S 1. Central atom = S 2. Valence electrons = 18 or 9 pairs3. Form double bond so that S has an octet — but note that there are two ways of doing this.bring inleft pairOR bring inright pair•OS••
22Sulfur Dioxide, SO2 This leads to the following structures. These equivalent structures are calledRESONANCE STRUCTURES. The true electronic structure is a HYBRID of the two.
23Formal Atom ChargesAtoms in molecules often bear a charge (+ or -).The predominant resonance structure of a molecule is the one with charges as close to 0 as possible.Formal charge = Group number – 1/2 (no. of bonding electrons) - (no. of LP electrons)
25Thiocyanate Ion, SCN- S N C 6 - (1/2)(2) - 6 = -1 5 - (1/2)(6) - 2 = 0 •SNC4 - (1/2)(8) - 0 = 0
26Which is the most important resonance form? Thiocyanate Ion, SCN-•SNC•SNC•SNCWhich is the most important resonance form?
27Calculated Partial Charges in SCN- All atoms negative, but most on the S•SNC
28Violations of the Octet Rule Usually occurs with B and elements of higher periods.SF4BF3
29Boron Trifluoride Central atom = _____________ Valence electrons = __________ or electron pairs = __________Assemble dot structureThe B atom has a share in only 6 pairs of electrons (or 3 pairs). B atom in many molecules is electron deficient.
30Boron Trifluoride, BF3 F B •••B+1-1What if we form a B—F double bond to satisfy the B atom octet?
31Sulfur Tetrafluoride, SF4 Central atom =Valence electrons = ___ or ___ pairs.Form sigma bonds and distribute electron pairs.5 pairs around the S atom. A common occurrence outside the 2nd period.
33VSEPR MOLECULAR GEOMETRY Valence Shell Electron Pair Repulsion theory. Molecule adopts the shape that minimizes the electron pair repulsions.VSEPRValence Shell Electron Pair Repulsion theory.Most important factor in determining geometry is relative repulsion between electron pairs.
39Structure Determination by VSEPR H••NAmmonia, NH31. Draw electron dot structure2. Count BP’s and LP’s = 43. The 4 electron pairs are at the corners of a tetrahedron.
40Structure Determination by VSEPR Ammonia, NH3There are 4 electron pairs at the corners of a tetrahedron.The ELECTRON PAIR GEOMETRY is tetrahedral.
41Structure Determination by VSEPR Ammonia, NH3The electron pair geometry is tetrahedral.The MOLECULAR GEOMETRY — the positions of the atoms — is PYRAMIDAL.
42Structure Determination by VSEPR Water, H2O1. Draw electron dot structure2. Count BP’s and LP’s = 43. The 4 electron pairs are at the corners of a tetrahedron.The electron pair geometry is TETRAHEDRAL.
43Structure Determination by VSEPR Water, H2OThe electron pair geometry is TETRAHEDRALThe molecular geometry is BENT.
45Structure Determination by VSEPR •CHOFormaldehyde, CH2O1. Draw electron dot structure2. Count BP’s and LP’s at C3. There are 3 electron “lumps” around C at the corners of a planar triangle.•CHOThe electron pair geometry is PLANAR TRIGONAL with 120o bond angles.
46Structure Determination by VSEPR Formaldehyde, CH2OThe electron pair geometry is PLANAR TRIGONALThe molecular geometry is also planar trigonal.
47Structure Determination by VSEPR Methanol, CH3OHH••H—C—O—HDefine H-C-H and C-O-H bond angles109˚109˚H-C-H = 109oC-O-H = 109oIn both cases the atom is surrounded by 4 electron pairs.
48Structure Determination by VSEPR Acetonitrile, CH3CN••HH—C—CNDefine unique bond anglesH-C-H = 109oC-C-N = 180o109˚180˚One C is surrounded by 4 electron “lumps” and the other by 2 “lumps”
51Structures with Central Atoms with More Than or Less Than 4 Electron Pairs Often occurs with Group 3A elements and with those of 3rd period and higher.
52Boron Compounds Consider boron trifluoride, BF3 The B atom is surrounded by only 3 electron pairs.Bond angles are 120oGeometry described as planar trigonal
53Compounds with 5 or 6 Pairs Around the Central Atom 5 electron pairs
54Molecular Geometries for Five Electron Pairs Figure 9.11 All based on trigonal bipyramidMolecular Geometries for Five Electron Pairs Figure 9.11
55Sulfur Tetrafluoride, SF4 Number of valence electrons = 34Central atom = SDot structureElectron pair geometry--> trigonal bipyramid (because there are 5 pairs around the S)
56Sulfur Tetrafluoride, SF4 Lone pair is in the equator because it requires more room.
57Molecular Geometries for Six Electron Pairs Figure 9.14 All are based on the 8-sided octahedronMolecular Geometries for Six Electron Pairs Figure 9.14
58Compounds with 5 or 6 Pairs Around the Central Atom 6 electron pairs
59Bond PropertiesWhat is the effect of bonding and structure on molecular properties?Free rotation around C–C single bondNo rotation around C=C double bond
60# of bonds between a pair of atoms Bond Order# of bonds between a pair of atomsDouble bondSingle bondAcrylonitrileTriple bond
61Bond Order The N—O bond order = 1.5 Fractional bond orders occur in molecules with resonance structures.Consider NO2-The N—O bond order = 1.5
62Bond Order 414 kJ 123 pm 745 kJ 110 pm (a) bond strength Bond order is proportional to two important bond properties:(a) bond strength(b) bond length745 kJ414 kJ110 pm123 pm
63Bond Length H—F H—Cl H—I Bond length depends on size of bonded atoms. Bond distances measured in Angstrom units where 1 A = 10-2 pm.
64Bond Length Bond length depends on bond order. Bond distances measured in Angstrom units where 1 A = 10-2 pm.
65Bond Strength—measured by the energy req’d to break a bond. See Table 9.10.BOND STRENGTH (kJ/mol)H—H 436C—C 346C=C 602CCNN 945The GREATER the number of bonds (bond order) the HIGHER the bond strength and the SHORTER the bond.
66Bond Strength 1 142 pm 210 kJ/mol 2 121 498 1.5 128 ? Bond Order Length StrengthHO—OHO=O1142 pm210 kJ/mol21214981.5128?
67Molecular Polarity Water Boiling point = 100 ˚C Methane Boiling point = -161 ˚CWhy do water and methane differ so much in their boiling points?Why do ionic compounds dissolve in water?
68Bond PolarityHCl is POLAR because it has a positive end and a negative end.Cl has a greater share in bonding electrons than does H.Cl has slight negative charge (-d) and H has slight positive charge (+ d)
69Bond Polarity Three molecules with polar, covalent bonds. Each bond has one atom with a slight negative charge (-d) and and another with a slight positive charge (+ d)
70Bond PolarityThis model, calc’d using CAChe software for molecular calculations, shows that H is + (red) and Cl is - (yellow). Calc’d charge is + or
71Bond Polarity BOND ENERGY “pure” bond 339 kJ/mol calc’d Due to the bond polarity, the H—Cl bond energy is GREATER than expected for a “pure” covalent bond.BOND ENERGY“pure” bond 339 kJ/mol calc’dreal bond 432 kJ/mol measuredDifference = 92 kJ. This difference is proportional to the difference in ELECTRONEGATIVITY, .
72Electronegativity, is a measure of the ability of an atom in a molecule to attract electrons to itself.Concept proposed by Linus Pauling
73Linus Pauling,The only person to receive two unshared Nobel prizes (for Peace and Chemistry).Chemistry areas: bonding, electronegativity, protein structure
75Electronegativity, F has maximum . See Figure 9.14F has maximum .Atom with lowest is the center atom in most molecules.Relative values of determine BOND POLARITY (and point of attack on a molecule).
76Bond Polarity Which bond is more polar (or DIPOLAR)? O—H O—F OH is more polar than OFand polarity is “reversed.”
77Molecular PolarityMolecules—such as HI and H2O— can be POLAR (or dipolar).They have a DIPOLE MOMENT. The polar HCl molecule will turn to align with an electric field.
78Molecular PolarityThe magnitude of the dipole is given in Debye units. Named for Peter Debye ( ). Rec’d 1936 Nobel prize for work on x-ray diffraction and dipole moments.
79Why are some molecules polar but others are not? Dipole MomentsWhy are some molecules polar but others are not?
80Molecular Polarity All above are NOT polar Molecules will be polar if a) bonds are polarANDb) the molecule is NOT “symmetric”All above are NOT polar
81Compare CO2 and H2O. Which one is polar? Polar or Nonpolar?Compare CO2 and H2O. Which one is polar?
82Carbon DioxideCO2 is NOT polar even though the CO bonds are polar.CO2 is symmetrical.+1.5-0.75Positive C atom is reason CO2 and H2O react to give H2CO3
83Polar or Nonpolar?Consider AB3 molecules: BF3, Cl2CO, and NH3.
84B—F bonds in BF3 are polar. But molecule is symmetrical and NOT polar Molecular Polarity, BF3B atom is positive and F atoms are negative.B—F bonds in BF3 are polar.But molecule is symmetrical and NOT polar
85Molecular Polarity, HBF2 B atom is positive but H & F atoms are negative.B—F and B—H bonds in HBF2 are polar. But molecule is NOT symmetrical and is polar.
86Is Methane, CH4, Polar?Methane is symmetrical and is NOT polar.
87Is CH3F Polar?C—F bond is very polar. Molecule is not symmetrical and so is polar.
88CH4 … CCl4 Polar or Not?Only CH4 and CCl4 are NOT polar. These are the only two molecules that are “symmetrical.”
89Substituted Ethylene C—F bonds are MUCH more polar than C—H bonds. Because both C—F bonds are on same side of molecule, molecule is POLAR.
90Substituted Ethylene C—F bonds are MUCH more polar than C—H bonds. Because both C—F bonds are on opposing ends of molecule, molecule is NOT POLAR.