Chemical Bonding I: Basic Concepts Chapter 8

Bonding in Solids In crystalline solids atoms are arranged in a very regular pattern. Amorphous solids are characterized by a distinct lack of order in the arrangement of atoms.

9.1 Valence electrons are the outer shell electrons of an atom. The valence electrons are the electrons that particpate in chemical bonding. 1A 1ns 1 2A 2ns 2 3A 3ns 2 np 1 4A 4ns 2 np 2 5A 5ns 2 np 3 6A 6ns 2 np 4 7A 7ns 2 np 5 Group# of valence e - e - configuration

9.1

9.2 Li + F Li + F - The Ionic Bond 1s 2 2s 1 1s 2 2s 2 2p 5 1s 2 1s 2 2s 2 2p 6 [He][Ne] Li Li + + e - e - + FF - F - Li + + Li + F -

9.3 Lattice energy (E) increases as Q increases and/or as r decreases. cmpd lattice energy MgF 2 MgO LiF LiCl Q= +2,-1 Q= +2,-2 r F < r Cl Electrostatic (Lattice) Energy E = k Q+Q-Q+Q- r Q + is the charge on the cation Q - is the charge on the anion r is the distance between the ions k= 2.31 X Jnm Lattice energy (E) is the energy required to completely separate one mole of a solid ionic compound into gaseous ions.

9.3 Born-Haber Cycle for Determining Lattice Energy ΔH overall = ΔH 1 + ΔH 2 + ΔH 3 + ΔH 4 + ΔH 5 oooooo

Estimate ΔH f for Sodium Chloride Na(s) + ½ Cl 2 (g)  NaCl(s) Lattice Energy-786 kJ/mol Ionization Energy for Na495 kJ/mol Electron Affinity for Cl-349 kJ/mol Bond energy of Cl kJ/mol Enthalpy of sublimation for Na109 kJ/mol Na(s)  Na(g) kJ Na(g)  Na + (g) + e kJ ½ Cl 2 (g)  Cl(g) + ½(239 kJ) Cl(g) + e -  Cl - (g) kJ Na + (g) + Cl - (g)  NaCl(s) -786 kJ Na(s) + ½ Cl 2 (g)  NaCl(s) -412 kJ/mol

9.3

Chemistry In Action: Sodium Chloride Mining SaltSolar Evaporation for Salt

A covalent bond is a chemical bond in which two or more electrons are shared by two atoms. Why should two atoms share electrons? FF + 7e - FF 8e - F F F F Lewis structure of F 2 lone pairs single covalent bond 9.4

8e - H H O ++ O HH O HHor 2e - Lewis structure of water Double bond – two atoms share two pairs of electrons single covalent bonds O C O or O C O 8e - double bonds Triple bond – two atoms share three pairs of electrons N N 8e - N N triple bond or 9.4

Bond Type Bond Length (pm) C-CC-C 154 C=C133 C≅CC≅C 120 C-NC-N 143 C=N138 C≅NC≅N 116 Lengths of Covalent Bonds Bond Lengths Triple bond < Double Bond < Single Bond 9.4

H F F H Polar covalent bond or polar bond is a covalent bond with greater electron density around one of the two atoms electron rich region electron poor region e - riche - poor α+α+ α-α- 9.5

Electronegativity is the ability of an atom to attract toward itself the electrons in a chemical bond. Electron Affinity - measurable, Cl is highest (most neg) Energy associated with the addition of a e- to a gaseous atom Electronegativity - relative, F is highest X (g) + e - X - (g) 9.5

Covalent share e - Polar Covalent partial transfer of e - Ionic transfer e - Increasing difference in electronegativity Classification of bonds by difference in electronegativity DifferenceBond Type 0Covalent > 2 Ionic 0 < and <2 Polar Covalent 9.5

C. Johannesson C. Bond Polarity Most bonds are a blend of ionic and covalent characteristics. Difference in electronegativity determines bond type.

Bonding Triangle You will NOT see this on the AP Exam.

Classify the following bonds as ionic, polar covalent, or covalent: The bond in CsCl; the bond in H 2 S; and the NN bond in H 2 NNH 2. Cs – 0.7Cl – – 0.7 = 2.3Ionic H – 2.1S – – 2.1 = 0.4Polar Covalent N – – 3.0 = 0Covalent 9.5

1.Draw skeletal structure of compound showing what atoms are bonded to each other. Put least electronegative element in the center. 2.Count total number of valence e -. Add 1 for each negative charge. Subtract 1 for each positive charge. 3.Complete an octet for all atoms except hydrogen 4.If structure contains too few electrons, form double and triple bonds on central atom as needed. Writing Lewis Structures 9.6

Write the Lewis structure of nitrogen trifluoride (NF 3 ). Step 1 – N is less electronegative than F, put N in center FNF F Step 2 – Count valence electrons N - 5 (2s 2 2p 3 ) and F - 7 (2s 2 2p 5 ) 5 + (3 x 7) = 26 valence electrons Step 3 – Draw single bonds between N and F atoms and complete octets on N and F atoms. Step 4 - Check, are # of e - in structure equal to number of valence e - ? 3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons 9.6

Write the Lewis structure of the carbonate ion (CO 3 2- ). Step 1 – C is less electronegative than O, put C in center OCO O Step 2 – Count valence electrons C - 4 (2s 2 2p 2 ) and O - 6 (2s 2 2p 4 ) -2 charge – 2e (3 x 6) + 2 = 24 valence electrons Step 3 – Draw single bonds between C and O atoms and complete octet on C and O atoms. Step 4 - Check, are # of e - in structure equal to number of valence e - ? 3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons 9.6 Step 5 - Too many electrons, form double bond and re-check # of e - 2 single bonds (2x2) = 4 1 double bond = 4 8 lone pairs (8x2) = 16 Total = 24

9.7 Two possible skeletal structures of formaldehyde (CH 2 O) HCOH H CO H An atom’s formal charge is the difference between the number of valence electrons in an isolated atom and the number of electrons assigned to that atom in a Lewis structure. formal charge on an atom in a Lewis structure = 1 2 total number of bonding electrons () total number of valence electrons in the free atom - total number of nonbonding electrons - The sum of the formal charges of the atoms in a molecule or ion must equal the charge on the molecule or ion.

HCOH C – 4 e - O – 6 e - 2H – 2x1 e - 12 e - 2 single bonds (2x2) = 4 1 double bond = 4 2 lone pairs (2x2) = 4 Total = 12 formal charge on C = ½ x 6 = -1 formal charge on O = ½ x 6 = +1 formal charge on an atom in a Lewis structure = 1 2 total number of bonding electrons () total number of valence electrons in the free atom - total number of nonbonding electrons

C – 4 e - O – 6 e - 2H – 2x1 e - 12 e - 2 single bonds (2x2) = 4 1 double bond = 4 2 lone pairs (2x2) = 4 Total = 12 H CO H formal charge on C = ½ x 8 = 0 formal charge on O = ½ x 4 = 0 formal charge on an atom in a Lewis structure = 1 2 total number of bonding electrons () total number of valence electrons in the free atom - total number of nonbonding electrons

Formal Charge and Lewis Structures For neutral molecules, a Lewis structure in which there are no formal charges is preferable to one in which formal charges are present. 2.Lewis structures with large formal charges are less plausible than those with small formal charges. 3.Among Lewis structures having similar distributions of formal charges, the most plausible structure is the one in which negative formal charges are placed on the more electronegative atoms. Which is the most likely Lewis structure for CH 2 O? HCOH +1 H CO H 00

A resonance structure is one of two or more Lewis structures for a single molecule that cannot be represented accurately by only one Lewis structure (after formal charge has been determined!). More possible structures gives the overall structure more validity. 9.8 The true structure is an AVERAGE of all the possible structures.

Ozone OOO + - OOO + - OCO O -- OCO O - - OCO O What are the resonance structures of the carbonate (CO 3 2 -) ion?

Benzene

Violations of the Octet Rule Usually occurs with B and elements of higher periods and most nonmetals. Common exceptions are: Be, B, P, S, Xe, Cl, Br, and As. How do you know if it’s an EXPANDED octet? –More than 4 bonds –Formal Charge doesn’t work out with just 8 BF 3 SF 4 Be: 4 B: 6 P: 8 OR 10 S: 8, 10, OR 12 Xe: 8, 10, OR 12

Exceptions to the Octet Rule The Incomplete Octet HHBe Be – 2e - 2H – 2x1e - 4e - BeH 2 BF 3 B – 3e - 3F – 3x7e - 24e - FBF F 3 single bonds (3x2) = 6 9 lone pairs (9x2) = 18 Total =

Exceptions to the Octet Rule Odd-Electron Molecules N – 5e - O – 6e - 11e - NO N O The Expanded Octet (central atom with principal quantum number n > 2) SF 6 S – 6e - 6F – 42e - 48e - S F F F F F F 6 single bonds (6x2) = lone pairs (18x2) = 36 Total =

The enthalpy change required to break a particular bond in one mole of gaseous molecules is the bond energy. H 2 (g) H (g) + ΔH 0 = kJ Cl 2 (g) Cl (g) + ΔH 0 = kJ HCl (g) H (g) +Cl (g) ΔH 0 = kJ O 2 (g) O (g) + ΔH 0 = kJ OO N 2 (g) N (g) + ΔH 0 = kJ N N Bond Energy Bond Energies Single bond < Double bond < Triple bond 9.10

Bond Energies (BE) and Enthalpy changes in reactions ΔH 0 = total energy input – total energy released = ΣBE broken(reactants) – ΣBE formed(products) Imagine reaction proceeding by breaking all bonds in the reactants and then using the gaseous atoms to form all the bonds in the products endothermic exothermic

9.10 H 2 (g) + Cl 2 (g) 2HCl (g)2H 2 (g) + O 2 (g) 2H 2 O (g)

Metallic Structure The structures of many metals conform to one of the cubic unit cells.

Use bond energies to calculate the enthalpy change for: H 2 (g) + F 2 (g) 2HF (g) ΔH 0 = ΣΔBE(reactants) – ΣΔBE(products) bonds broken bonds formed Δ is always positive, represents the bond energy/mole of bonds Type of bonds broken Number of bonds broken Bond energy (kJ/mol) Energy change (kJ) HH FF Type of bonds formed Number of bonds formed Bond energy (kJ/mol) Energy change (kJ) HF ΔH 0 = – 2 x = kJ 9.10

Alloys Alloys are combinations of two or more elements, the majority of which are metals. Adding a second (or third) element changes the properties of the mixture to suit different purposes.

Alloys In substitutional alloys, a second element takes the place of a metal atom. In interstitial alloys, a second element fills a space in the lattice of metal atoms.

Metallic Bonding In elemental samples of nonmetals and metalloids, atoms generally bond to each other covalently. Metals, however, have a dearth of valence electrons; instead, they form large groups of atoms that share electrons among them.

Metallic Bonding One can think of a metal, therefore, as a group of cations suspended in a sea of electrons. The electrical and thermal conductivity, ductility, and malleability of metals is explained by this model.