Bonding and Molecular Structure: Fundamental Concepts Chapter 9 Bonding and Molecular Structure: Fundamental Concepts Dr. S. M. Condren

Chemical Bonding How is a molecule or polyatomic ion held together? 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? Dr. S. M. Condren

Structure & Bonding NN triple bond. Molecule is unreactive White phosphorus is a tetrahedron of P atoms. Very reactive! Red phosphorus, a polymer. Used in matches. Less reactive! Dr. S. M. Condren

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. Dr. S. M. Condren

Ionic Compounds 2 Na(s) + Cl2(g) ---> 2 Na+ + 2 Cl- Metal of low IE Nonmetal of high EA 2 Na(s) + Cl2(g) ---> 2 Na+ + 2 Cl- Dr. S. M. Condren

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. Dr. S. M. Condren

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 •• • + Note that each atom has a single, unpaired electron. Overlap of H (1s) and Cl (3p) Dr. S. M. Condren

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. Dr. S. M. Condren

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 Dr. S. M. Condren

Bond and Lone Pairs Valence electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS. • •• H Cl lone pairs shared or bond pair This is called a LEWIS ELECTRON DOT structure. Dr. S. M. Condren

Valence Electrons 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 Dr. S. M. Condren

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. Group 3A Group 5A Dr. S. M. Condren

Rules of the Game No. of valence electrons of an atom = Group number For Groups 1A-4A, no. of bond pairs = group number For Groups 5A -7A, BP’s = 8 - Grp. No. •Except for H (and sometimes atoms of 3rd and higher periods), BP’s + LP’s = 4 This observation is called the OCTET RULE Dr. S. M. Condren

Hydrophobic vs. Hydrophilic Hydrophobic - translation of Greek – water fear Hydrophilic – translation of Greek – water friendship Dr. S. M. Condren

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 Dr. S. M. Condren

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. Dr. S. M. Condren

Lewis Structures CH4 methane Step 1. Central atom = C Step 2. Count valence electrons C = 4 4 x H = 4 x 1 = 4 TOTAL = 8 e- or 4 pairs Step 3. Form bonds Dr. S. M. Condren

Lewis Structures C2H6 ethane Dr. S. M. Condren

Multiple Covalent Bonds double bond => 2 pairs shared triple bond => 3 pairs shared normally occurs between: C atoms; N atoms; O atoms; a C atom and a N, O or S atom a N atom and a O or S atom a S atom and an O atom Dr. S. M. Condren

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

Carbon Dioxide, CO2 C 4 2 1. Central atom = _______ 2. Valence electrons = __ or __ pairs 3. Form bonds. This leaves 6 pairs. Dr. S. M. Condren

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. Dr. S. M. Condren

Steps to form Lewis Electron Dot Structure 1. Central atom = _______ 2. Valence electrons = __ or __ pairs Form bonds. Place lone pairs on outer atoms. Form multiple bonds as necessary to obey Lewis’ “Octet Rule”. Remember that there are MANY compounds that do not obey the Octet Rule. Dr. S. M. Condren

Lewis Structures CO carbon monoxide Dr. S. M. Condren

Exceptions to Octet Rule NO nitric oxide Dr. S. M. Condren

Exceptions to Octet Rule NO2 nitrogen dioxide resonance Dr. S. M. Condren

Exceptions to Octet Rule PF5 expanded octet Dr. S. M. Condren

Exceptions to Octet Rule expanded octet SF4 Dr. S. M. Condren

Exceptions to Octet Rule SF6 expanded octet Dr. S. M. Condren

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) Dr. S. M. Condren

Carbon Dioxide, CO2 O C +6 - ( 1 / 2 ) 4 = +4 - ( 1 / 2 ) 8 = • • O C +4 - ( 1 / 2 ) 8 = Dr. S. M. Condren

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 Dr. S. M. Condren

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? Dr. S. M. Condren

MOLECULAR GEOMETRY Dr. S. M. Condren

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

Electron Pair Geometries Dr. S. M. Condren

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 •• Dr. S. M. Condren

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. Dr. S. M. Condren

Geometries for Four Electron Pairs Dr. S. M. Condren

Structure Determination by VSEPR Ammonia, NH3 1. Draw electron dot structure 2. Count BP’s and LP’s = 4 H •• N 3. The 4 electron pairs are at the corners of a tetrahedron. Dr. S. M. Condren

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

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. The molecular geometry is BENT. Dr. S. M. Condren

Consequences of H2O Polarity Dr. S. M. Condren

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. Dr. S. M. Condren

Molecular Geometries for Five Electron Pairs All based on trigonal bipyramid Molecular Geometries for Five Electron Pairs Dr. S. M. Condren

Molecular Geometries for Six All are based on the 8-sided octahedron Molecular Geometries for Six Electron Pairs Dr. S. M. Condren

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 Dr. S. M. Condren

# of bonds between a pair of atoms Bond Order # of bonds between a pair of atoms Double bond Single bond Acrylonitrile Triple bond Dr. S. M. Condren

Bond Order Fractional bond orders occur in molecules with resonance structures. Consider NO2- 1 1 The N—O bond order = 1.5 Dr. S. M. Condren

Bond Order (a) bond strength (b) bond length Bond order is proportional to two important bond properties: (a) bond strength (b) bond length 745 kJ 414 kJ 110 pm 123 pm Dr. S. M. Condren

Dr. S. M. Condren

stronger, weaker, or the same strength? Compare O-O and O=O. Is O=O expected to be stronger, weaker, or the same strength? Dr. S. M. Condren

longer, shorter, or the same length? Is O=O expected to be longer, shorter, or the same length? Dr. S. M. Condren

Using Bond Energies Estimate the energy of the reaction H—H + Cl—Cl ----> 2 H—Cl H—H = 436 kJ/mol Cl—Cl = 242 kJ/mol H—Cl = 432 kJ/mol Net = ∆H = [S bondsbroken] – [S bondsformed] Sum of H-H + Cl-Cl bond energies = 436 kJ + 242 kJ = +678 kJ 2 mol H-Cl bond energies = 864 kJ Net = ∆H = +678 kJ - 864 kJ = -186 kJ ∆Hfo (HCl(g)) = -92.31 kJ/mol or -184 kJ Dr. S. M. Condren

Water Boiling point = 100 ˚C Methane Boiling point = -161 ˚C 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? Dr. S. M. Condren

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) Dr. S. M. Condren

Bond Polarity Three molecules with polar, covalent bonds. Each bond has one atom with a slight negative charge (-d) and another with a slight positive charge (+ d) Dr. S. M. Condren

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

Electronegativity,   is a measure of the ability of an atom in a molecule to attract electrons to itself. Dr. S. M. Condren

Bond Polarity 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, . Dr. S. M. Condren

Electronegativity Pauling Scale relative attraction of an atom for electrons, its own and those of other atoms same trends as ionization energy, increases from lower left corner to the upper right corner fluorine: E.N. = 4.0 Dr. S. M. Condren

Electronegativities of the Elements Dr. S. M. Condren

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.” Dr. S. M. Condren

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

Compare CO2 and H2O. Which one is polar? Polar or Nonpolar? Compare CO2 and H2O. Which one is polar? Dr. S. M. Condren

Covalent Bond Properties electronegativity nonpolar bonds => diff. EN = 0 polar bonds => diff. EN > 0 ionic bonds => diff. EN > 1.5 Dr. S. M. Condren

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