Chapter 8. onding : General Concepts Chemical ondings create Diversity in the Universe Why and how do they make chemical bonds? and what do they make?
Types of Chemical onds Chemical bonds: orces that hold groups of atoms together and make them function as a unit. Why are chemical bonds formed? Total energy = PE + KE Coulombic repulsion e - e - + + Coulombic interaction Stabilization ond Energy ond Energy: energy required to break a bond. strength of a bonding interaction ond Length: The distance where the system energy is a minimum.
Types of Chemical onds Metallic ond Ionic ond Covalent ond
Types of Chemical onds Metallic ond Na: 1s 2 2s 2 2p 6 3s 1 orming sea of electrons (freely-moving valence electrons) Na + Na + Na + e - e - e - e - Na + Na + e -
Types of Chemical onds Ionic ond NaCl No conductivity Melting conductivity Why conductivity? Na + and Cl - exist in NaCl. Ionic bond ormed from electrostatic attractions of closely packed, oppositely charged ions. ormed when an atom that easily loses electrons reacts with one that has a high electron affinity. Motive: Coulombic interaction ( Q e)( Q2e) 19 Q1Q 2 E 1 (2.31 10 J nm 4 e r ) r 0 Q 1 and Q 2 = numerical ion charges r = distance between ion centers (in nm) E Na +, Cl - bond energy = 8.37 x 10-19 J per NaCl NaCl 0.276 nm : bond length
Types of Chemical onds Covalent ond In covalent bonding, electrons are shared between bonding partners.
Ionic ond Ions: Electronic Configurations and Sizes Achieving Noble Gas Electron Configurations (NGEC) 1. Two nonmetals react: They share electrons to achieve NGEC. => covalent bond 2. A nonmetal and a representative group metal react (ionic compound): The valence orbitals of the metal are emptied to achieve NGEC. The valence electron configuration of the nonmetal achieves NGEC. Predicting ormulas of Ionic Compounds 1. Neutrality 2. Noble Gas Electron configuration Ca: [Ar]4s 2 O: [e]2s 2 2p 4 Ca 2+ : [Ar] O 2- : [Ne] CaO(s) Al, O Al 2 O 3 Mg, O MgO Cs, Cl CsCl solid gas
Ionic ond Ions: Electronic Configurations and Sizes Size of Ions ow to define? unit: pm Metals Nonmetals r x > r x+ r x < r x- d = r + +r - r Isoelectronic ions: ions containing the the same number of electrons Z, r
Ionic ond Energy Effects in inary Ionic Compounds Li: [e]2s 1 : [e]2s 2 2p 5 E dissociation (x 0.5) Li + : [e] - : [Ne] ionization electron affinity (electron gain) Li(s) sublimation NGEC lattice energy What is the deriving force to form ionic compunds? D f Lattice energy: The change in energy when separated gaseous ions are packed together to form an ionic solid. (exothermic) M + (g) + X - (g) MX(s)
Ionic ond Energy Effects in inary Ionic Compounds Lattice Energy Calculations C. attraction C. repulsion Interactions between ions Coulombic Intearction 1/r (attraction and repulsion) Van der Waals Interaction 1/r 6 (working at only short range) Repulsion by electron overlap exp(-d/d * ) (repulsion) approx. Q1Q 2 Lattice Energy k( ) r
Ionic ond Energy Effects in inary Ionic Compounds Lattice Energy Calculations Q1Q 2 Lattice Energy k( ) r big effect of charge dissociation (x 0.5) NGEC dissociation (x 0.5) sublimation NGEC D f
Covalent ond The Covalent ond: A Model Models are attempts to explain how nature operates on the microscopic level based on experiences in the macroscopic world undamental Properties of Models A model does not equal reality. Models are oversimplifications, and are therefore often wrong. strong and weak points Models become more complicated as they age. modification We must understand the underlying assumptions in a model so that we don t misuse it. In covalent bonding, electrons are shared between bonding partners. Think of the covalent bond as the electron density existing between the C and atoms.
Covalent ond The Covalent ond: A Model C(s) + 2 2 (g) C 4 (g) D fo = -75 kj/mol (1) (1/2) 2 (g) (g) D fo = 217 kj/mol (2) C(s) C(g) D fo = 709 kj/mol (3) C 4 (g) C(g) + 4(g) D o = 1652 kj/mol 4 x (2) + (3) - (1) ond Energy: energy required to break a bond. strength of a bonding interaction D(C-) = 1652/4 = 413 kj/mol Cl C 3 Cl(g) C(g) + 3(g) + Cl(g) D o = 1578 kj/mol D(C-Cl) = (1578-3 x 413) kj/mol = 339 kj/mol a model : looking at a molecule as a collection of individual bonds.
Covalent ond Covalent ond Energies and Chemical Reactions Energy required (kj/mol) C 4 (g) C 3 (g) + (g) 435 C 3 (g) C 2 (g) + (g) 453 C 2 (g) C(g) + (g) 425 average = 413 C(g) C(g) + (g) 339 Molecule Measured C- bond energy (kj/mol) Cr 3 453 CCl 3 380 C 3 430 C 2 6 410 1652 D is sensitive to environments, but the idea of D is still hold. single bond : sharing one pair of electrons double bond : sharing two pairs of electrons triple bond : sharing three pairs of electrons
Covalent ond Covalent ond Energies and Chemical Reactions ond Energy and Enthalpy D r = SD(bonds broken) - SD(bonds formed) energy required energy released Ex) D r of C 4 (g)+ 2Cl 2 (g) + 2 2 (g) --> C 2 Cl 2 (g) + 2(g) + 2Cl(g)? Reactants atoms products D r = 4 x 413 + 2 x 239 + 2 x 154-2 x 485-2 x 339-2 x 565-2 x 427 = -1194 (kj/mol) well fit to experimental data
Covalent ond Covalent ond Energies and Chemical Reactions ond Energy and Enthalpy or purely covalent bond Expected D(-) = D(-) + D(-) 2 = 293 kj/mol D(-) = 565 kj/mol Expected D(-Cl) = D(-) + D(Cl-Cl) 2 = 336 kj/mol D(-Cl) = 427 kj/mol What happens?
Electronegativity D = D(-X) actual - D(-X) expected or, D = 272 kj/mol Linus Pauling (1901-1994) (Nobel, Chemistry 1954) Nobel, Peace 1962) ionic interaction d + d - polar covalent bond (polarity in bond) covalent interaction "the 'excess' bond energy is caused by electrostatic attraction between the partially charged atoms in the heternuclear species A" Electronegativity (c): The ability of an atom in a molecule to attract shared electrons to itself. D(A) = (1/2)[D(AA)+D()] + 96.48(c A - c ) 2 NaCl 2 Ionic Polar covalent bond Covalent
Electronegativity
ond Polarity and Dipole Moments A molecule, such as, that has a center of positive charge and a center of negative charge is said to be polar (dipolar), or to have a dipole moment. polar covalent bond (polarity in bond) d + d - dipole moment of molecule polar molecule (poarity in molecule) polar covalent bond non-polar molecule
Partial Ionic Character of Covalent onds 100% covalent bond polar covalent bond 100% ionic bond Percent ionic character of bond measured dipole moment of X - Y calculated dipole moment of X Y - 100% No 100 % ionic bond. Compounds with more than 50 % ionic character are normally considered to be ionic solids. N 4 Cl, Na 2 SO 4 : Any compound that conducts an electric current when melted will be classified as ionic.
Covalent ond Models to explain the structures and/or energies of the covalent molecules Localized Electron (LE) onding Model A molecule is composed of atoms that are bound together by sharing pairs of electrons using the atomic orbitals of the bound atoms. Electron pairs in the molecule are assumed to be localized on a particular atom or in the space between two atoms Electron pairs localized on an atom : lone pairs Electron pairs found in the space between the atoms : bonding pairs Lewis Structure Valence Shell Electron Pair Repulsion (VSEPR) Model Valence ond Theory (ybridization) Delocalized Electron onding Model Molecular Orbital (MO) Theory Description of valence electron arrangement Prediction of geometry Description of atomic orbital types used to share electrons or hold lone pairs. Analagous to atomic orbitals for atoms, MOs are the quantum mechanical solutions to the organization of valence electrons in molecules.
Covalent ond Lewis Structure The most stable arrangement of electrons is one in which all atoms have a noble gas configuration. [Octet rule, Duet Rule ()] Ex) NaCl vs Na + Cl - Na: [Ne]3s 1 Cl: [Ne]3s 2 3p 5 Noble gas configuration for molecules Na + : [Ne] Cl - : [Ne]3s 2 3p 6 = [Ar] Atoms are represented by atomic symbols surrounded by valence electrons Ex) NaCl vs Na + Cl - Na: Na Cl: Cl Na + : Na Cl - : Cl + - 2 2 Electron pairs between atoms indicate bond formation. bonding pair - 2 2 lone pair (x6)
Covalent ond Lewis Structure N A = S N: Total number of valence electrons when considered as noble gases A: Total number of actual valence electrons S: Number of valence electrons shared 2 O -O- (2 x 2 + 8) - (2 x 1 + 6 ) = 4 - CN - C=N (2 x 8) - (4 + 5 +1 ) = 6 N 2 N=N (2 x 8) - (2 x 5) = 6 2 2 x 8-14 = 2 CO 2 3 x 8 - (4 + 2 x 6 ) = 8 N 3 N (8 + 3 x 2) - (5 + 3) = 6 O=C=O Cl 2 O Cl O Cl (3 x 8) - (2 x 7 + 6) = 4
Covalent ond Lewis Structure Exception to the Octet Rule 3 (4 x 8) - (3 + 3 x 7) = 8 Some atoms (e and in particular) undergo bonding, but will form stable molecules that do not fulfill the octet rule Expanded Shell in 3rd and higher period S 6 PCl 5 old : using empty d orbitals new : not necessarily (MO theory) I 3 - N 3 + 3 3 N 3 N Cl 3 Cl
Covalent ond Lewis Structure Exception to the Octet Rule Odd -Electron Molecules : odd electron systems where full pairs will not exist. O Cl O Unpaired electron Comments About the Octet Rule 2nd row elements C, N, O, observe the octet rule. 2nd row elements and e often have fewer than 8 electrons around themselves - they are very reactive. 3rd row and heavier elements can exceed the octet rule using empty valence d orbitals. When writing Lewis structures, satisfy octets first, then place electrons around elements having available d orbitals.
Covalent ond Lewis Structure Resonance O 3 O O O O O O (3 x 8) - (3 x 6) = 6 Resonance : occurs when more than one valid Lewis structure can be written for a particular molecule. The actual structure is an average of the resonance structures. (O 3 : two 1.5 bonds) NO 3 - C 6 6
Covalent ond Lewis Structure ormal Charge Sum of formal charges = net charge of the molecule or ion ormal charge : the net charge of an atom when a bond is considered as a completely covalent bond. 0 A halve the shared electrons formal charge = net charge of an atom Oxidation number : the net charge of an atom when a bond is considered as a completely ionic bond. +1 C (for c A > c ) +1-4 +1 +1 an atom having bigger electronegativity (c A > c ) has the shared electrons A 0 0 C 0 0 oxidation number = net charge of an atom
Covalent ond Lewis Structure ormal Charge -1 0 0-1 0 +1 0 0 0 0 0 elp in assigning bonding when there are several possibilities of Lewis structure. 1. Structures with small C (-2,+2 or less) are more likely. 2. Nonzero Cs on adjacent atoms are usually of opposite sign. 3. More electronegative atoms should have negative C. 4. Cs of opposite signs separated by large distance are unlikely. SO 4 2- -1 0-1 +2-1 -1 0-1 and resonance structure -1 0
Covalent ond Valence Shell Electron Pair Repulsion (VSEPR) Model VSEPR: The structure around a given atom is determined principally by minimizing electron pair (bonding pair and lone pair) repulsions. C 4 C Lewis Structure VSEPR Structure
Covalent ond Valence Shell Electron Pair Repulsion (VSEPR) Model Refinement of VSEPR (1): Repulsion (Lp-Lp) > Repulsion (Lp-p) > Repulsion (p-p) N 3 N # of e - pairs = 4 tetrahedral trigonal pyramid 2 O -O- Lone pairs require more room than bonding pairs and tend to compress the angles between the bonding pairs. tetrahedral bent (V shape)
Covalent ond Valence Shell Electron Pair Repulsion (VSEPR) Model Refinement of VSEPR (1): Repulsion (Lp-Lp) > Repulsion (Lp-p) > Repulsion (p-p) PCl 5 PCl 4 - A A A A A trigonal bipyramid lp-bp : 90 o (x2) lp-bp : 90 o (x3) see-saw Cl 3 I 3 - A lp-lp : 90 o (x1) lp-bp : 90 o (x3) A lp-lp : 180 o (x1) lp-bp : 90 o (x6) A lp-lp : 120 o (x1) lp-bp : 90 o (x4) A A A lp-lp : 120 o (x3) lp-lp : 90 o (x2) lp-lp : 90 o (x2) T-structure A A linear # of e - pairs = 5
Covalent ond Valence Shell Electron Pair Repulsion (VSEPR) Model Refinement of VSEPR (1): Repulsion (Lp-Lp) > Repulsion (Lp-p) > Repulsion (p-p) # of e - pairs = 6 Xe 4 A A octahedral no dipole moment A lp-lp : 180 o (x1) lp-bp : 90 o (x8) A lp-1p : 90 o (x1) lp-bp : 90 o (x6) sqaure plananr
Covalent ond Valence Shell Electron Pair Repulsion (VSEPR) Model Refinement of VSEPR (2): Multiple bonds count as one effective electron pair. Multiple bonds have slightly greater repulsive effects than single bonds. (caution) 120 o Molecules Containing No Single Central Atom : Approach step by step C 3 O -C-O-
Covalent ond Valence Shell Electron Pair Repulsion (VSEPR) Model ow Well Does VSEPR Work? VSEPR is such a simple model. It can predict corrrectly (and amazingly) so many molecules. ecause it is very simple, there are some cases that the structres are predicted by more extended rules. N 3 P 3 N P 107 o 94 o