Chapter 8 outline Chemical Bonding Ionic Bonding Lattice energy Exceptions to octet rule Covalent Bonding Bond Polarity Electronegativity Lewis Structures drawing Lewis structures deciding between alternate Lewis structures: formal charge resonance structures exceptions to the octet rule Bond properties Bond length, bond energy Mary J. Bojan Fall 2010 1
3 forms of bonding IONIC electrons traded to form separate ions COVALENT electrons shared between a few nuclei distinct molecules METALLIC electrons shared among all nuclei Mary J. Bojan Fall 2010 2
For ionic and covalent bonding: 1. Valence electrons are involved in bonding 2. Octet rule: elements tend to gain, lose or share electrons so as to gain an inert gas configuration. (Duet for H and He) Mary J. Bojan Fall 2010 3
IONIC BONDING Ionic bond is due to electrostatic attraction. Coulomb's Law: Electrons are exchanged to form separate ions with complete octets. Mary J. Bojan Fall 2010 4
IONIC COMPOUNDS Characteristics Mary J. Bojan Fall 2010 5
Ionic vs. Molecular Ionic compounds: a compound that consists of positive and negative ions. Metal + nonmetal (usually) Ionic compounds have high melting points and are brittle. Molecular compounds: compounds consisting of individual molecules All nonmetals or nonmetals and metalloids. Molecular compounds usually have low melting points Mary J. Bojan Fall 2010 6
Naming Inorganic Compounds and Ions Name potassium ion See Section 2.8 Tables 2.4, 2.5 Formula ammonium ion CO 3 2 sodium bicarbonate Combine Ca +2 and PO 4 3 H 2 SO 4 " Cu(II) and sulfate ion Mary J. Bojan Fall 2010 7
Strength of Ionic Bond Lattice energy is the change in energy when an ionic solid is separated into isolated ions in the gas phase. Lattice energy cannot be determined experimentally Mary J. Bojan Fall 2010 8
Born-Haber cycle Mary J. Bojan Fall 2010 9
NaCl E = 788 kj MgO E =3795 kj Why the big difference??? Lattice Energy Q Q d 1 2 E d Mary J. Bojan Fall 2010 10
Salts and Ionic Lattices ION SIZES: Ion sizes are important in ionic bonds Which has the greatest lattice energy? (How do you know?) NaF NaCl NaBr NaI lattice energy can t be measured: what can we measure that is related to lattice energy? Mary J. Bojan Fall 2010 11
Salts and Ionic Lattices mp NaF 993 o C NaCl 801 o C NaBr 747 o C NaI 661 o C MgO 2800 o C Melting point increases as lattice energy increases. Mary J. Bojan Fall 2010 12
TRANSITION METAL IONS (exception to octet rule) s electrons are part of valence electrons transition metals can have variable charges. When forming an ion: s electrons are lost first then maybe d electrons Mary J. Bojan Fall 2010 13
COVALENT BONDING When neither atom is "willing" to give up electrons (completely) atoms share electrons: each atom has a noble gas configuration. COVALENT BOND: Bond strengths Lewis Structures Shared electron pairs bonds Lone pairs Mary J. Bojan Fall 2010 14
Multiple Bonds Only single, double, and triple bonds are allowed. N-N N=N N N l 1.47Å 1.24Å 1.10 Å E 163 418 941 kj/mole Mary J. Bojan Fall 2010 15
Bond Polarity Nonpolar covalent: electrons are shared equally (H 2, Br 2, Cl 2 ) Polar covalent: unequal sharing of electrons (HF, ICl) Ionic: no sharing of electrons: (NaCl, LiF) Mary J. Bojan Fall 2010 16
Bond polarity Electron sharing in covalent bonds depends on the electronegativity of atoms. ELECTRONEGATIVITY: not the same as electron affinity not directly measurable: Most electronegative: Least electronegative: Mary J. Bojan Fall 2010 17
Periodic Trends: Electronegativity nonmetals are most electronegative metals are least electronegative (especially active metals, Groups I and II) Mary J. Bojan Fall 2010 18
Electronegativity and Bonding ionic: large difference in electronegativity polar covalent: some difference in electronegativity covalent: no difference in electronegativity. Mary J. Bojan Fall 2010 19
RULES FOR WRITING LEWIS STRUCTURES 1. Count up the number of valence electrons. 3. Write the atom symbols and connect bonded atoms with single bonds. 4. Distribute electrons (in pairs) to complete octets of atoms. 5. Not enough electrons? Make multiple bonds to complete octets if necessary. 6. Extra electrons? Put them on the central atom. Mary J. Bojan Fall 2010 20
Example Lewis structure of sulfite ion SO 3 2- valence electrons draw structure distribute electrons Check to make sure all atoms have a complete octet! Mary J. Bojan Fall 2010 21
Example: HCN 1. valence electrons 2 connect atoms 3. distribute pairs not enough electrons! Mary J. Bojan Fall 2010 22
Example Carbonyl chloride: COCl 2 # of valence electrons = Connect atoms distribute electrons Mary J. Bojan Fall 2010 23
Deciding between alternate Lewis Structures Use FORMAL CHARGE (FC): FC = VE LSE VE =number of valence electrons in an isolated atom LSE = number of electrons on the atom in the Lewis structure LSE = lone pair electrons + ½ shared electrons The most stable structure is the one in which the atoms bear the smallest formal charge. Mary J. Bojan Fall 2010 24
Valence electrons: NITRATE ION: NO 3 Connect atoms, distribute electron pairs What does the Lewis structure indicate about bond lengths and strengths in NO 3? Mary J. Bojan Fall 2010 25
RESONANCE STRUCTURES Some molecules can not be represented by a single Lewis structure. NO 3 - O O O O N O O N O O N O 3 resonance structures Mary J. Bojan Fall 2010 26
RESONANCE Molecule has RESONANCE when more than 1 Lewis structure can be drawn for a fixed nuclear arrangement. Different Lewis structures are equally good (usually). Molecules with resonance exhibit extra stability Properties (e.g. bond length, bond strength) are averaged over resonance structures Mary J. Bojan Fall 2010 27
Aromatic Hydrocarbons Contain alternating single and double C-C Bonds H C H C C H H C H C C H Benzene is prototypical molecule H C C C H H H C C C H H Usually written: Aromatic hydrocarbons are less reactive than alkenes: Special kind of bonding Have delocalized π electrons: results in added stability Mary J. Bojan Fall 2010 28
Review Lewis structures What is the Lewis structure for C 2 H 4 SiF 4 XeF 4 TeF 4 Mary J. Bojan Fall 2010 29
EXCEPTIONS TO OCTET RULE 1. Odd # of electrons (rare) E.g. NO, NO 2, ClO 2 # of valence e 2. Incomplete octet (# e - < 8) Rare: Be, B F F Example: BF 3 F B F F B F BF 3 reacts readily with electron pair donors (like NH 3 ). Mary J. Bojan Fall 2010 30
EXCEPTIONS TO OCTET RULE 3. Molecule with atom having more than 8 electrons fairly common never occurs with period 2 atoms occurs with atoms in period 3 and below WHY?? Conditions for expanded octet Examples NF 5 PF 5 PF 5 PCl 5 AsF 5 PBr 5 PI 5 Mary J. Bojan Fall 2010 31
Bond properties COVALENT BOND LENGTHS and ENERGIES Bond length: distance between nuclei Mary J. Bojan Fall 2010 32
BOND STRENGTH Mary J. Bojan Fall 2010 33
BOND DISSOCIATION ENERGY bond (dissociation) energy: D enthalpy of bond breaking reaction in the gas phase. D > 0 (ΔH > 0) for diatomics, D is ΔH of one reaction: H H(g) 2H(g) D H-H = ΔH rxn = 436kJ/mol for polyatomics, D is an averaged quantity H O H(g) HO(g) + H(g)+494kJ/mol H O(g) H(g) + O(g) +424kJ/mol D O H = 463 kj/mol * * value obtained from averaging over many molecules Mary J. Bojan Fall 2010 34
λ Energy Interaction with matter??? gamma 10 12 m 1.20 10 8 kj/mol X-Ray 10 10 m 1.20 10 6 kj/mol ultraviolet 10 7 m =100nm 1.20 10 3 kj/mol = 1200 kj/mol visible 400nm - 750nm infrared 10 6 m µm 299 kj/mol 160 kj/mol 120 kj/mol vibration microwave 10 2 m 12.0 J/mol rotation electronic transitions heat (translation) radio 1m 0.12 J/mol flip nuclear spin Mary J. Bojan Fall 2010 35