2 Covalent Bondscovalent combination of prefix co- (Latin - “together”),valere, - “to be strong”2 e-’s shared have strength to hold 2 atoms togetherparticle called a “molecule”
3 Molecules Some elements in nature are molecules: neutral group of atoms covalently bondedEx. - air contains O molecules, 2 O atoms joined covalentlyCalled “diatomic molecule” (O2)
4 (diatomic hydrogen molecule) The nuclei repel each other, (both have + charge)How does H2 form?(diatomic hydrogen molecule)++++
5 How does H2 form?nuclei attraction to e-’s stronger than repulsion of nucleie-’s sharedcovalent bondOnly NONMETALS!++
6 Covalent bonding w/ Fluorine atoms Covalent bondsNonmetals hold valence e-’sdon’t give away e-’sstill want NGCshare valence e-’s with each other = covalent bondingboth atoms count e-’s for NGCCovalent bonding w/ Fluorine atoms
7 F F Covalent bonding Fluorine has seven valence electrons A second atom also has sevenBy sharing electrons……both end with full orbitalsFF8 Valence electrons
8 single covalent bond between 2 H atoms Covalent bondingFluorine has seven valence electronsA second atom also has sevenBy sharing electrons……both end with full orbitalsFF8 Valence electronssingle covalent bond between 2 H atoms
9 Molecular Compounds gases or liquids at room temperature Compounds bonded covalently called molecular compoundsMolecular compounds havelower melting and boiling pointsWeaker bond than ionicgases or liquids at room temperaturea molecular formula:Shows how many atoms of each element a molecule contains
10 Reminder from Ch. 7 No “molecule” of sodium chloride Ionic cmpds exist as collection of + & - charged ions arranged in repeating 3D patterns.
11 Molecular Compounds The formula for water is written as H2O The subscript “2” behind hydrogen means 2 atoms of hydrogensubscript 1 omittedMolecular formulas do not tell any information about structure (arrangement of various atoms).
12 - Page 215These are some of the different ways to represent ammonia:3. The ball and stick model is BEST, because it shows 3D arrangement.1. The molecular formula shows how many atoms of each element are present2. The structural formula ALSO shows the arrangement of these atoms!
14 A Single Covalent Bond is... sharing 2 valence e-’sOnly nonmetals and hydrogen.Different from ionic bond b/c they actually form molecules.Two specific atoms joinedIn an ionic solid, you can’t tell which atom e-’s moved from or to
15 How to show the formation… It’s like a jigsaw puzzle.You put the pieces together to end up with the right formula.Carbon is a special example - can it really share 4 electrons: 1s22s22p2?2p1s sCYes, due to electron promotion!
16 How to show the formation… It’s like a jigsaw puzzle.You put the pieces together to end up with the right formula.Carbon is a special example - can it really share 4 electrons: 1s22s22p2?2p1s s
17 WaterAnother example: water is formed with covalent bonds, by using an electron dot diagramHEach hydrogen has 1 valence electron- Each hydrogen wants 1 moreThe oxygen has 6 valence electrons- The oxygen wants 2 moreThey share to make each other completeO
18 H O Water Put the pieces together The first hydrogen is happy The oxygen still needs one moreHO
19 H O H Water So, a second hydrogen attaches Every atom has full energy levelsNote the two “unshared” pairs of electronsHOH
21 Examples:Conceptual Problem 8.1 on page 220We’ll do #7 & 8
22 Double and Triple Covalent Bonds Sometimes atoms share more than one pair of valence e-’sdouble bond: atoms share 2 pairs of e-’s (4 total)triple bond: atoms share 3 pairs of e-’s (6 total)Table 8.1, p Know these 7 elements as diatomic:Br2 I2 N2 Cl2 H2 O2 F2
23 Dot diagram for Carbon dioxide CO2 - Carbon is central atom ( more metallic )Carbon has 4 valence e-’sWants 4 moreOxygen has 6 valence e-’sWants 2 moreCOThe chemistry of CO2 6:44
24 Carbon dioxideAttaching 1 oxygen leaves the oxygen 1 short, and the carbon 3 shortCO
25 Carbon dioxideAttaching the second oxygen leaves both of the oxygen 1 short, and the carbon 2 shortOCO
27 Carbon dioxideThe only solution is to share moreOCO
28 Carbon dioxideThe only solution is to share moreOCO
29 Carbon dioxideThe only solution is to share moreOCO
30 Carbon dioxideThe only solution is to share moreOCO
31 Carbon dioxideThe only solution is to share moreOCO
32 O C O Carbon dioxide The only solution is to share more Requires 2 double bondsEach atom can count all the electrons in the bondOCO
33 O C O Carbon dioxide The only solution is to share more Requires two double bondsEach atom can count all the electrons in the bond8 valence electronsOCO
34 O C O Carbon dioxide The only solution is to share more Requires two double bondsEach atom can count all the electrons in the bond8 valence electronsOCO
35 How covalent bonds form - Mark Rosengarden Carbon dioxideThe only solution is to share moreRequires two double bondsEach atom can count all the electrons in the bond8 valence electronsOCOHow covalent bonds form - Mark Rosengarden
36 How to draw them? Use the handout guidelines: Add up all valence e-’s Count total e-’s needed to make all atoms happy (stable)Subtract; Divide by 2 (tells you how many bonds to draw)Choose central atom (least electronegative)Start w/ most electronegative atom, fill in remaining valence e-’s to fill atoms up
37 N H Examples NH3, which is ammonia N – central atom; has 5 valence electrons, wants 8H - has 1 (x3) valence electrons, wants 2 (x3)NH3 has 5+3 = 8NH3 wants 8+6 = 14(14-8)/2= 3 bonds4 atoms with 3 bondsNH
38 H H N H Examples Draw in the bonds; start with singles All 8 e- accounted forEverything full – DONE!HHNH
39 Example: HCN HCN: C is central atom N - has 5 valence electrons, wants 8C - has 4 valence electrons, wants 8H - has 1 valence electron, wants 2HCN has = 10HCN wants = 18(18-10)/2= 4 bonds3 atoms with 4 bonds – this will require multiple bonds - not to H however
40 H C N HCN Put single bond between each atom Need to add 2 more bonds Must go between C and N (Hydrogen is full)HCN
41 H C N HCN Put in single bonds Needs 2 more bonds Must go between C and N, not the HUses 8 electrons – need 2 more to equal the 10 it hasHCN
42 H C N HCN Put in single bonds Need 2 more bonds Must go between C and NUses 8 electrons - 2 more to addMust go on the N to fill its octetHCN
43 Another way of indicating bonds Often use a line to indicate a bondCalled a structural formulaEach line = 2 valence e-’sHOHHOH=
45 A Coordinate Covalent Bond... When one atom donates both electrons in a covalent bond.Carbon monoxide (CO) is a good example:Both the carbon and oxygen give another single electron to shareOC
46 Coordinate Covalent Bond When one atom donates both electrons in a covalent bond.Carbon monoxide (CO) is a good example:Oxygen gives both of these electrons, since it has no more singles to share.This carbon electron moves to make a pair with the other single.CO
47 Coordinate Covalent Bond When one atom donates both electrons in a covalent bond.Carbon monoxide (CO)The coordinate covalent bond is shown with an arrow as:COC O
48 Coordinate covalent bond Most polyatomic cations and anions contain covalent & coordinate covalent bondsTable 8.2, p.224Sample Problem 8.2, p.225The ammonium ion (NH4+) can be shown as another example
49 Bond Dissociation Energies... Total energy required to break bond btwn 2 covalently bonded atomsHigh dissociation energy usually means compound relatively unreactive, b/c it takes hi energy to break bond
50 Resonance is... When more than one valid dot diagram is possible. Consider the two ways to draw ozone (O3)Which one is it? Does it go back and forth?It’s hybrid of both, shown by double-headed arrowfound in double-bond structures!
51 Resonance in OzoneNote the different location of the double bondNeither single structure is correct, actually a hybrid of the two. To show it, draw all possible structures, and join them with a double-headed arrow.
52 ResonanceOccurs when more than one valid Lewis structure can be written for particular molecule (due to position of double bond)resonance structures of carbonate ion (used in production of carbonated beverages).The actual structure is an avg (or hybrid) of these structures.
53 Polyatomic ions – note the different positions of the double bond. Resonance in a carbonate ion (CO32-):Resonance in an acetate ion (C2H3O21-):
54 The 3 Exceptions to Octet rule For some molecules, it is impossible to satisfy the octet rule#1. usually when there is an odd number of valence electronsNO2 has 17 valence electrons, because the N has 5, and each O contributes 6. Note “N” page 228It is impossible to satisfy octet rule, yet the stable molecule does exist
55 Exceptions to Octet rule Another exception: BoronPage 228 shows boron trifluoride, and note that one of the fluorides might be able to make a coordinate covalent bond to fulfill the boron#2 -But fluorine has a high electronegativity (it is greedy), so this coordinate bond does not form#3 -Top page 229 examples exist because they are in period 3 or below
56 Section 8.3 Bonding Theories OBJECTIVES:Describe the relationship between atomic and molecular orbitals.
57 Section 8.3 Bonding Theories OBJECTIVES:Describe how VSEPR theory helps predict the shapes of molecules.
58 Molecular Orbitals are... The model for covalent bonding assumes orbitals are those of individual atoms = atomic orbitalOrbitals that apply to overall molecule, due to atomic orbital overlap are molecular orbitalsbonding orbital is molecular orbital occupied by 2 e-’s of covalent bond
59 Molecular Orbitals - definitions Sigma bond- 2 atomic orbitals combine to form molecular orbital symmetrical along axis connecting nucleiPi bond- bonding e-’s likely above and below bond axis (weaker than sigma)Note pictures - next slideHybridization video – 1:36
60 - Pages 230 and 231Sigma bond is symmetrical along axis between 2 nuclei.Pi bond is above and below bond axis - weaker than sigma
61 Attractive & repulsive forces in H2 bond + nuclei repele-’s repelNuclei and e-’s attractAttractive forces stronger than repulsionAs nuclei distances decrease, PE decreasesIf distance decreases more, PE increases b/c increased repulsionBond forms w/ bond length = interatomic distance (PE minimum)
62 VSEPR: stands for... Valence Shell Electron Pair Repulsion Predicts 3D shape of moleculesThe name tells you the theory:Valence shell = outside e-’sElectron Pair repulsion = e- pairs try to get as far away as possible from each other.determines angles of bonds.
63 VSEPR Based on # of pairs of ve-’s, bonded & unbonded. Unbonded pair called lone pair.CH4 - draw structural formulaHas 4 + 4(1) = 8wants 8 + 4(2) = 16(16-8)/2 = 4 bonds
64 VSEPR for methane (a gas): Single bonds fill all atoms.4 pairs of e-’s pushing awayThe furthest they can get away is 109.5°
65 H C H H H 4 atoms bonded 109.5º Basic shape -tetrahedral pyramid w/ triangular baseSame shape for everything with 4 pairsH109.5ºCHHH
66 Other angles, pages 232 - 233 Ammonia (NH3) = 107o Water (H2O) = 105o Carbon dioxide (CO2) = 180o
67 VSEPR modelsVSEPR theory video 4:52- Page 232Methane has an angle of 109.5o, called tetrahedralAmmonia has an angle of 107o, called pyramidalNote the unshared pair that is repulsion for other electrons.
69 Hybrid OrbitalsProvides info for molecular bonding & shape
70 Hybridization w/ single bonds Orbitals combineC outer e- configuration 2s2 2p2 but one 2s e- promoted to 2pOne 2s e- and 3 2p e-’sAllows bond in methane (CH4)All bonds same due to orbital hybridizationMix to form 4 sp3 hybrid orbitals
71 Hybridization w/ double bonds Ethene C2H4 – 1 C-C double bond and 4 C-H single bondssp2 hybrid orbitals form from one 2s and two 2p atomic orbitals of C
72 Section 8.4 Polar Bonds and Molecules OBJECTIVES:Describe how electronegativity values determine the distribution of charge in a polar molecule.
73 Section 8.4 Polar Bonds and Molecules OBJECTIVES:Describe what happens to polar molecules when they are placed between oppositely charged metal plates.
74 Section 8.4 Polar Bonds and Molecules OBJECTIVES:Evaluate the strength of intermolecular attractions compared with the strength of ionic and covalent bonds.
75 Section 8.4 Polar Bonds and Molecules OBJECTIVES:Identify the reason why network solids have high melting points.
76 Bond Polarity do covalent bonds always share equally? e-’s pulled - tug-of-war, btwn nucleiIn equal sharing (such as diatomic molecules), the bond is called nonpolar covalent bond
77 polar bond Bond Polarity When 2 different atoms bond covalently, unequal sharingmore electronegative atom stronger attractionslightly negative chargepolar bondLower case delta
78 Electronegativity? Linus Pauling 1901 - 1994 The ability of an atom in a molecule to attract shared electrons to itself.Linus Pauling
79 Bond Polarity Refer to periodic table w/ EN Consider HCl H = electronegativity of 2.1Cl = electronegativity of 3.0Polar bondCl: slight - chargeH: slight + charge
80 Partial charges Bond Polarity d+ d- d+ and d- Partial charges, much less than 1+ or 1- in ionic bondH ClPartial chargesd d-d+ and d-
81 Bond Polarity H Cl Can also be: arrow points to more EN atom KNOW Table 8.3, p.238 shows how electronegativity indicates bond typeH Cl
82 Polar molecules Sample Problem 8.3, p.239 polar bond tends to make entire molecule “polar”areas of “difference”HCl has polar bonds, thus polar moleculemolecule w/ 2 poles called dipole, like HCl
83 Polar moleculeseffect of polar bonds on polarity of entire molecule depends on molecule shapeCO2 has 2 polar bondslinearnonpolar molecule
84 Polar moleculeseffect of polar bonds on molecule polarity depends on shapewater has 2 polar bonds - bent shapehighly electronegative O pulls e- away from Hvery polar!polar bond of water moleculeChemistry of water 4:46
86 Polar and non-polar covalent bonds - Mark Rosengarten
87 Attractions between molecules p. 240 makes solid & liquid molecular cmpds possibleweakest called van der Waal’s forces - there are two kinds:#1. Dispersion forces: weakest of all, caused by motion of e- - increases as # e- increases (momentarily more on side of molecule closest to neighboring molecule, neighboring molecule’s e-’s move to opp side)halogens start as gases; bromine (l); iodine (s) – all in Group 7A
88 #2. Dipole interactionsOccurs when polar molecules attracted to each other2. Dipole interaction happens in water Figure 8.25, page 240+region of one molecule attracts -region of another molecule
89 #2. Dipole interactions H F d+ d- H F d+ d- Occur when polar molecules attracted to each otherSlightly stronger than dispersion forcesOpposites attract, but not completely hooked like ionic solidsH Fd+ d-H Fd+ d-
91 #3. Hydrogen bonding…is the attractive force caused by hydrogen bonded to N, O, F, or ClN, O, F, and Cl very electronegative, so this is very strong dipoleAnd, the hydrogen shares with lone pair in molecule next to itThis is strongest of the intermolecular forces
92 Order of Intermolecular attraction strengths Dispersion forces are weakestLittle stronger are dipole interactionsStrongest is H bondingAll are weaker than ionic bonds
93 #3. Hydrogen bonding defined: When H atom is: a) covalently bonded to a highly EN atom, AND b) is also weakly bonded to unshared e- pair of nearby highly EN atomThe H is left very e- deficient (only had 1 to start with!) - it shares with something nearbyH is ONLY element with no shielding for its nucleus when involved in covalent bond!
94 Hydrogen Bonding (Shown in water) This H is bonded covalently to: 1) the highly negative O, and 2) a nearby unshared pair.
95 H bonding allows H2O to be a liquid at room temp
96 Attractions and properties Why are some chemicals gases, some liquids, some solids?Depends on type of bonding!Table 8.4, page 244Network solids – solids where all atoms covalently bonded to each other
97 Attractions and properties Figure 8.28, page 243Network solids melt at very high temps, or not at all (decomposes)Diamond does not really melt, but vaporizes to a gas at 3500 oCSiC, used in grinding, has melting pt of 2700 oC
98 Covalent Network Compounds Some covalently bonded substances DO NOT form discrete molecules.Diamond, a network of covalently bonded carbon atomsGraphite, a network of covalently bonded carbon atoms