CHAPTER 9 Bonding and Molecular Structure: Fundamental Concepts Chemistry and Chemical Reactivity 6th Edition John C. Kotz Paul M. Treichel Gabriela C. Weaver CHAPTER 9 Bonding and Molecular Structure: Fundamental Concepts Lectures written by John Kotz © 2006 Brooks/Cole Thomson

Chemical 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?

Structure & Bonding NN triple bond. Molecule is unreactive Phosphorus is a tetrahedron of P atoms. Very reactive! Red phosphorus, a polymer. Used in matches.

Forms 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 another Covalent—some valence electrons shared between atoms Most bonds are somewhere in between.

Ionic Compounds Metal Nonmetal 2 Na(s) + Cl2(g) ---> 2 Na+ + 2 Cl-

Covalent Bonding The 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.

Note that each atom has a single, unpaired electron. Bond Formation A bond can result from a “head-to-head” overlap of atomic orbitals on neighboring atoms. Cl H •• • + Overlap of H (1s) and Cl (3p) Note that each atom has a single, unpaired electron.

Chemical Bonding: Objectives Objectives are to understand: 1. valence e- distribution in molecules and ions. 2. molecular structures 3. bond properties and their effect on molecular properties.

Electron Distribution in Molecules Electron distribution is depicted with Lewis electron dot structures Valence electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS. G. N. Lewis 1875 - 1946

Bond and Lone Pairs Valence electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS. • •• H Cl shared or bond pair lone pair (LP) This is called a LEWIS ELECTRON DOT structure.

Valence Electrons Br [Ar] 3d10 4s2 4p5 Electrons are divided between core and valence electrons B 1s2 2s2 2p1 Core = [He] , valence = 2s2 2p1 Br [Ar] 3d10 4s2 4p5 Core = [Ar] 3d10 , valence = 4s2 4p5

Rules of the Game No. 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.

Rules of the Game •Except for H (and sometimes atoms of 2nd and 3rd families and 3rd and higher periods), BP’s + LP’s = 4 This observation is called the OCTET RULE

Building a Dot Structure Ammonia, NH3 1. Decide on the central atom; never H. Central atom is atom of lowest affinity for electrons. Therefore, N is central 2. Count valence electrons H = 1 and N = 5 Total = (3 x 1) + 5 = 8 electrons / 4 pairs

Building a Dot Structure 3. Form a single bond between the central atom and each surrounding atom H N 4. Remaining electrons form LONE PAIRS to complete octet as needed. H •• N 3 BOND PAIRS and 1 LONE PAIR. Note that N has a share in 4 pairs (8 electrons), while H shares 1 pair.

Sulfite ion, SO32- Step 1. Central atom = S Step 2. Count valence electrons S = 6 3 x O = 3 x 6 = 18 Negative charge = 2 TOTAL = 26 e- or 13 pairs Step 3. Form bonds 10 pairs of electrons are now left.

Each 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. • •• O S •• Each atom is surrounded by an octet of electrons.

Carbon Dioxide, CO2 1. Central atom = _______ 2. Valence electrons = __ or __ pairs 3. Form bonds. This leaves 6 pairs. 4. Place lone pairs on outer atoms.

Carbon 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.

Double and even triple bonds are commonly observed for C, N, P, O, and S H2CO SO3 C2F4

Sulfur Dioxide, SO2 O S 1. Central atom = S 2. Valence electrons = 18 or 9 pairs 3. Form double bond so that S has an octet — but note that there are two ways of doing this. bring in left pair OR bring in right pair • O S ••

Sulfur Dioxide, SO2 This leads to the following structures. These equivalent structures are called RESONANCE STRUCTURES. The true electronic structure is a HYBRID of the two.

Formal Atom Charges Atoms 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)

Carbon Dioxide, CO2 +6 - ( 1 / 2 ) 4 = • O C +4 - ( 1 / 2 ) 8 =

Thiocyanate Ion, SCN- S N C 6 - (1/2)(2) - 6 = -1 5 - (1/2)(6) - 2 = 0 • S N C 4 - (1/2)(8) - 0 = 0

Which is the most important resonance form? Thiocyanate Ion, SCN- • S N C • S N C • S N C Which is the most important resonance form?

Calculated Partial Charges in SCN- All atoms negative, but most on the S • S N C

Violations of the Octet Rule Usually occurs with B and elements of higher periods. SF4 BF3

Boron Trifluoride Central atom = _____________ Valence electrons = __________ or electron pairs = __________ Assemble dot structure The B atom has a share in only 6 pairs of electrons (or 3 pairs). B atom in many molecules is electron deficient.

Boron Trifluoride, BF3 F B •• • B +1 -1 What if we form a B—F double bond to satisfy the B atom octet?

Sulfur 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.

MOLECULAR GEOMETRY

VSEPR MOLECULAR GEOMETRY Valence Shell Electron Pair Repulsion theory. Molecule adopts the shape that minimizes the electron pair repulsions. VSEPR Valence Shell Electron Pair Repulsion theory. Most important factor in determining geometry is relative repulsion between electron pairs.

Electron Pair Geometries Active Figure 9.8

Electron Pair Geometries Active Figure 9.8

Structure Determination by VSEPR H •• N Ammonia, NH3 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.

Structure Determination by VSEPR Ammonia, NH3 There are 4 electron pairs at the corners of a tetrahedron. The ELECTRON PAIR GEOMETRY is tetrahedral.

Structure Determination by VSEPR Ammonia, NH3 The electron pair geometry is tetrahedral. The MOLECULAR GEOMETRY — the positions of the atoms — is PYRAMIDAL.

Structure Determination by VSEPR Water, H2O 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, H2O The electron pair geometry is TETRAHEDRAL The molecular geometry is BENT.

Geometries for Four Electron Pairs Figure 9.9

Structure Determination by VSEPR • C H O Formaldehyde, CH2O 1. Draw electron dot structure 2. Count BP’s and LP’s at C 3. There are 3 electron “lumps” around C at the corners of a planar triangle. • C H O The electron pair geometry is PLANAR TRIGONAL with 120o bond angles.

Structure Determination by VSEPR Formaldehyde, CH2O The electron pair geometry is PLANAR TRIGONAL The molecular geometry is also planar trigonal.

Structure Determination by VSEPR Methanol, CH3OH H •• H—C—O—H Define H-C-H and C-O-H bond angles 109˚ 109˚ H-C-H = 109o C-O-H = 109o In both cases the atom is surrounded by 4 electron pairs.

Structure Determination by VSEPR Acetonitrile, CH3CN •• H H—C—C N Define unique bond angles H-C-H = 109o C-C-N = 180o 109˚ 180˚ One C is surrounded by 4 electron “lumps” and the other by 2 “lumps”

Phenylalanine, an amino acid

Phenylalanine

Structures 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.

Boron Compounds Consider boron trifluoride, BF3 The B atom is surrounded by only 3 electron pairs. Bond angles are 120o Geometry described as planar trigonal

Compounds with 5 or 6 Pairs Around the Central Atom 5 electron pairs

Molecular Geometries for Five Electron Pairs Figure 9.11 All based on trigonal bipyramid Molecular Geometries for Five Electron Pairs Figure 9.11

Sulfur Tetrafluoride, SF4 Number of valence electrons = 34 Central atom = S Dot structure Electron pair geometry --> trigonal bipyramid (because there are 5 pairs around the S)

Sulfur Tetrafluoride, SF4 Lone pair is in the equator because it requires more room.

Molecular Geometries for Six Electron Pairs Figure 9.14 All are based on the 8-sided octahedron Molecular Geometries for Six Electron Pairs Figure 9.14

Compounds with 5 or 6 Pairs Around the Central Atom 6 electron pairs

Bond Properties What is the effect of bonding and structure on molecular properties? Free rotation around C–C single bond No rotation around C=C double bond

# of bonds between a pair of atoms Bond Order # of bonds between a pair of atoms Double bond Single bond Acrylonitrile Triple bond

Bond 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

Bond 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 length 745 kJ 414 kJ 110 pm 123 pm

Bond 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.

Bond Length Bond length depends on bond order. Bond distances measured in Angstrom units where 1 A = 10-2 pm.

Bond Strength —measured by the energy req’d to break a bond. See Table 9.10. BOND STRENGTH (kJ/mol) H—H 436 C—C 346 C=C 602 CC 835 NN 945 The GREATER the number of bonds (bond order) the HIGHER the bond strength and the SHORTER the bond.

Bond Strength 1 142 pm 210 kJ/mol 2 121 498 1.5 128 ? Bond Order Length Strength HO—OH O=O 1 142 pm 210 kJ/mol 2 121 498 1.5 128 ?

Molecular Polarity Water Boiling point = 100 ˚C Methane Boiling point = -161 ˚C Why do water and methane differ so much in their boiling points? Why do ionic compounds dissolve in water?

Bond Polarity HCl 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)

Bond 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)

Bond Polarity This model, calc’d using CAChe software for molecular calculations, shows that H is + (red) and Cl is - (yellow). Calc’d charge is + or - 0.20.

Bond 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’d real bond 432 kJ/mol measured Difference = 92 kJ. This difference is proportional to the difference in ELECTRONEGATIVITY, .

Electronegativity,   is a measure of the ability of an atom in a molecule to attract electrons to itself. Concept proposed by Linus Pauling 1901-1994

Linus Pauling, 1901-1994 The only person to receive two unshared Nobel prizes (for Peace and Chemistry). Chemistry areas: bonding, electronegativity, protein structure

Electronegativity Figure 9.14

Electronegativity,  F has maximum . See Figure 9.14 F 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).

Bond Polarity Which bond is more polar (or DIPOLAR)? O—H O—F  3.5 - 2.1 3.5 - 4.0  1.4 0.5 OH is more polar than OF and polarity is “reversed.”

Molecular Polarity Molecules—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.

Molecular Polarity The magnitude of the dipole is given in Debye units. Named for Peter Debye (1884 - 1966). Rec’d 1936 Nobel prize for work on x-ray diffraction and dipole moments.

Why are some molecules polar but others are not? Dipole Moments Why are some molecules polar but others are not?

Molecular Polarity All above are NOT polar Molecules will be polar if a) bonds are polar AND b) the molecule is NOT “symmetric” All above are NOT polar

Compare CO2 and H2O. Which one is polar? Polar or Nonpolar? Compare CO2 and H2O. Which one is polar?

Carbon Dioxide CO2 is NOT polar even though the CO bonds are polar. CO2 is symmetrical. +1.5 -0.75 Positive C atom is reason CO2 and H2O react to give H2CO3

Polar or Nonpolar? Consider AB3 molecules: BF3, Cl2CO, and NH3.

B—F bonds in BF3 are polar. But molecule is symmetrical and NOT polar Molecular Polarity, BF3 B atom is positive and F atoms are negative. B—F bonds in BF3 are polar. But molecule is symmetrical and NOT polar

Molecular 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.

Is Methane, CH4, Polar? Methane is symmetrical and is NOT polar.

Is CH3F Polar? C—F bond is very polar. Molecule is not symmetrical and so is polar.

CH4 … CCl4 Polar or Not? Only CH4 and CCl4 are NOT polar. These are the only two molecules that are “symmetrical.”

Substituted 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.

Substituted 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.