11 Section 8.1 Molecular Compounds OBJECTIVES:Distinguish between the melting points and boiling points of molecular compounds and ionic compounds.
12 Section 8.1 Molecular Compounds OBJECTIVES:Describe the information provided by a molecular formula.
13 Bonds are…Forces that hold groups of atoms together and make them function as a unit. Two types:Ionic bonds – transfer of electrons (gained or lost; makes formula unit)Covalent bonds – sharing of electrons. The resulting particle is called a “molecule”
14 Covalent BondsThe word covalent is a combination of the prefix co- (from Latin com, meaning “with” or “together”), and the verb valere, meaning “to be strong”.Two electrons shared together have the strength to hold two atoms together in a bond.
15 Molecules Many elements found in nature are in the form of molecules: a neutral group of atoms joined together by covalent bonds.For example, air contains oxygen molecules, consisting of two oxygen atoms joined covalentlyCalled a “diatomic molecule” (O2)
16 (diatomic hydrogen molecule) The nuclei repel each other, since they both have a positive charge (like charges repel).How does H2 form?(diatomic hydrogen molecule)++++
17 How does H2 form? But, the nuclei are attracted to the electrons They share the electrons, and this is called a “covalent bond”, and involves only NONMETALS!++
18 Covalent bonds Nonmetals hold on to their valence electrons. They can’t give away electrons to bond.But still want noble gas configuration.Get it by sharing valence electrons with each other = covalent bondingBy sharing, both atoms get to count the electrons toward a noble gas configuration.
19 Covalent bondingFluorine has seven valence electrons (but would like to have 8)F
20 F F Covalent bonding Fluorine has seven valence electrons A second atom also has sevenFF
21 F F Covalent bonding Fluorine has seven valence electrons A second atom also has sevenBy sharing electrons…FF
22 F F Covalent bonding Fluorine has seven valence electrons A second atom also has sevenBy sharing electrons…FF
23 F F Covalent bonding Fluorine has seven valence electrons A second atom also has sevenBy sharing electrons…FF
24 F F Covalent bonding Fluorine has seven valence electrons A second atom also has sevenBy sharing electrons…FF
25 F F Covalent bonding Fluorine has seven valence electrons A second atom also has sevenBy sharing electrons…FF
26 F F Covalent bonding …both end with full orbitals Fluorine has seven valence electronsA second atom also has sevenBy sharing electrons……both end with full orbitalsFF
27 F F Covalent bonding Fluorine has seven valence electrons A second atom also has sevenBy sharing electrons……both end with full orbitalsFF8 Valence electrons
28 F F Covalent bonding Fluorine has seven valence electrons A second atom also has sevenBy sharing electrons……both end with full orbitalsFF8 Valence electrons
31 Molecular CompoundsCompounds that are bonded covalently (like in water, or carbon dioxide) are called molecular compoundsMolecular compounds tend to have relatively lower melting and boiling points than ionic compounds – this is not as strong a bond as ionic
32 Molecular CompoundsThus, molecular compounds tend to be gases or liquids at room temperatureIonic compounds were solidsA molecular compound has a molecular formula:Shows how many atoms of each element a molecule contains
33 Molecular Compounds The formula for water is written as H2O The subscript “2” behind hydrogen means there are 2 atoms of hydrogen; if there is only one atom, the subscript 1 is omittedMolecular formulas do not tell any information about the structure (the arrangement of the various atoms).
34 - Page 215These are some of the different ways to represent ammonia:3. The ball and stick model is the BEST, because it shows a 3-dimensional arrangement.1. The molecular formula shows how many atoms of each element are present2. The structural formula ALSO shows the arrangement of these atoms!
36 BellringerWrite down THREE differences between ionic compounds and covalent compounds, and give an example of each.
37 Table 8. Ionic bondingCovalent bondingRepresentative UnitFormula unitMoleculeBond formationTransferSharingType of elementsMetals with nonmetalsNonmetalsPhysical stateSolidSolid, liquid, or gasMelting pointHighLowSolubilityUsually highHigh to lowElectrical conductivityGood conductorPoor conductor
38 Section 8.2 The Nature of Covalent Bonding OBJECTIVES:Describe how electrons are shared to form covalent bonds, and identify exceptions to the octet rule.
39 Section 8.2 The Nature of Covalent Bonding OBJECTIVES:Demonstrate how electron dot structures represent shared electrons.
40 Section 8.2 The Nature of Covalent Bonding OBJECTIVES:Describe how atoms form double or triple covalent bonds.
41 Section 8.2 The Nature of Covalent Bonding OBJECTIVES:Distinguish between a covalent bond and a coordinate covalent bond, and describe how the strength of a covalent bond is related to its bond dissociation energy.
42 Section 8.2 The Nature of Covalent Bonding OBJECTIVES:Describe how oxygen atoms are bonded in ozone.
43 A Single Covalent Bond is... A sharing of two valence electrons.Only nonmetals and hydrogen.Different from an ionic bond because they actually form molecules.Two specific atoms are joined.In an ionic solid, you can’t tell which atom the electrons moved from or to
44 Sodium Chloride Crystal Lattice Ionic compounds organize in a characteristic crystal lattice of alternating positive and negative ions, repeated over and over, can covalent compounds do this…? We’ll answer this later.
45 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?Yes, due to electron promotion!Another example: lets show how water is formed with covalent bonds, by using an electron dot diagram
46 H O Water Each 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 completeHO
47 H O Water Put the pieces together The first hydrogen is happy The oxygen still needs one moreHO
48 H O H Water So, a second hydrogen attaches Every atom has full energy levelsNote the two “unshared” pairs of electronsHOH
49 Examples:Conceptual Problem 8.1 on page 220Do PCl3
50 Multiple BondsSometimes atoms share more than one pair of valence electrons.A double bond is when atoms share two pairs of electrons (4 total)A triple bond is when atoms share three pairs of electrons (6 total)Table 8.1, p Know these 7 elements as diatomic:Br2 I2 N2 Cl2 H2 O2 F2What’s the deal with the oxygen dot diagram?
51 Dot diagram for Carbon dioxide CO2 - Carbon is central atom ( more metallic )Carbon has 4 valence electronsWants 4 moreOxygen has 6 valence electronsWants 2 moreCO
52 Carbon dioxideAttaching 1 oxygen leaves the oxygen 1 short, and the carbon 3 shortCO
53 Carbon dioxideAttaching the second oxygen leaves both of the oxygen 1 short, and the carbon 2 shortOCO
54 Carbon dioxideThe only solution is to share moreOCO
55 Carbon dioxideThe only solution is to share moreOCO
56 Carbon dioxideThe only solution is to share moreOCO
57 Carbon dioxideThe only solution is to share moreOCO
58 Carbon dioxideThe only solution is to share moreOCO
59 Carbon dioxideThe only solution is to share moreOCO
60 O C O Carbon dioxide The only solution is to share more Requires two double bondsEach atom can count all the electrons in the bondOCO
61 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
62 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
63 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
64 How to draw them? Use the handout guidelines: Add up all the valence electrons.Count up the total number of electrons to make all atoms happy.Subtract; then Divide by 2Tells you how many bonds to drawFill in the rest of the valence electrons to fill atoms up.
65 N H Example 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
66 H H N H Examples Draw in the bonds; start with singles All 8 electrons are accounted forEverything is full – done with this one.HHNH
67 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
68 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
69 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
70 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
71 Another way of indicating bonds Often use a line to indicate a bondCalled a structural formulaEach line is 2 valence electronsHOHHOH=
73 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
74 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
75 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
76 Coordinate covalent bond Most polyatomic cations and anions contain covalent and coordinate covalent bondsTable 8.2, p.224Sample Problem 8.2, p.225The ammonium ion (NH41+) can be shown as another example
77 Bond Dissociation Energies... The total energy required to break the bond between 2 covalently bonded atomsHigh dissociation energy usually means the chemical is relatively unreactive, because it takes a lot of energy to break it down.
78 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 is a hybrid of both, like a mule; and shown by a double-headed arrowfound in double-bond structures!
79 Resonance in OzoneNote the different location of the double bondNeither structure is correct, it is actually a hybrid of the two. To show it, draw all varieties possible, and join them with a double-headed arrow.
80 ResonanceOccurs when more than one valid Lewis structure can be written for a particular molecule (due to position of double bond)These are resonance structures of benzene.The actual structure is an average (or hybrid) of these structures.
81 Polyatomic ions – note the different positions of the double bond. Resonance in a carbonate ion (CO32-):Resonance in an acetate ion (C2H3O21-):
82 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
83 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 beyond
84 Section 8.3 Bonding Theories OBJECTIVES:Describe the relationship between atomic and molecular orbitals.
85 Section 8.3 Bonding Theories OBJECTIVES:Describe how VSEPR theory helps predict the shapes of molecules.
86 Molecular Orbitals are... The model for covalent bonding assumes the orbitals are those of the individual atoms = atomic orbitalOrbitals that apply to the overall molecule, due to atomic orbital overlap are the molecular orbitalsA bonding orbital is a molecular orbital that can be occupied by two electrons of a covalent bond
87 Molecular Orbitals - definitions Sigma bond- when two atomic orbitals combine to form the molecular orbital that is symmetrical along the axis connecting the nucleiPi bond- the bonding electrons are likely to be found above and below the bond axis (weaker than sigma)Note pictures on the next slide
88 - Pages 230 and 231Sigma bond is symmetrical along the axis between the two nuclei.Pi bond is above and below the bond axis, and is weaker than sigma
89 VSEPR: stands for... Valence Shell Electron Pair Repulsion Predicts the three dimensional shape of molecules.The name tells you the theory:Valence shell = outside electrons.Electron Pair repulsion = electron pairs try to get as far away as possible from each other.Can determine the angles of bonds.
90 VSEPRBased on the number of pairs of valence electrons, both bonded and unbonded.Unbonded pair also called lone pair.CH4 - draw the structural formulaHas 4 + 4(1) = 8wants 8 + 4(2) = 16(16-8)/2 = 4 bonds
91 VSEPR for methane (a gas): Single bonds fill all atoms.There are 4 pairs of electrons pushing away.The furthest they can get away is 109.5ºHHCHHThis 2-dimensional drawing does not show a true representation of the chemical arrangement.
92 H C H H H 4 atoms bonded Basic shape is tetrahedral. A pyramid with a triangular base.Same shape for everything with 4 pairs.H109.5ºCHHH
93 Other angles, pages 232 - 233 Ammonia (NH3) = 107o Water (H2O) = 105o Carbon dioxide (CO2) = 180oNote the shapes of these that are pictured on the next slide
94 - 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.
95 WS and naming rules for ionic vs covalent Check-in on project AgendaWS and naming rules for ionic vs covalentCheck-in on project8.4 lecture (late BR)Electronegativitybond polaritypolar vs nonpolar moleculesIntermolecular attractions
96 8.4 BellringerWhat is electronegativity, and why would it matter when different atoms bond together?
97 Section 8.4 Polar Bonds and Molecules OBJECTIVES:Describe how electronegativity values determine the distribution of charge in a polar molecule.
98 Section 8.4 Polar Bonds and Molecules OBJECTIVES:Describe what happens to polar molecules when they are placed between oppositely charged metal plates.
99 Section 8.4 Polar Bonds and Molecules OBJECTIVES:Evaluate the strength of intermolecular attractions compared with the strength of ionic and covalent bonds.
100 Section 8.4 Polar Bonds and Molecules OBJECTIVES:Identify the reason why network solids have high melting points.
101 Bond Polarity Covalent bonding means shared e- but, do they share equally?E- are pulled, (like a tug-of-war) between the atoms’ nucleiIn equal sharing (such as diatomic molecules), the bond is a nonpolar covalent bond
102 Bond Polarity When 2 different atoms bond covalently unequal sharing the more electronegative atom will have a stronger attraction, and will have a slightly negative chargeThis is a polar covalent bond, or polar bond.
103 Electronegativity?The ability of an atom in a molecule to attract shared electrons to itself.Linus Pauling
104 Table of Electronegativities Higher electronegativity
105 Table 8.3 P. 238 Electronegativity difference range Most probably type of bondExampleNonpolar covalentH—HModerately polar covalentH—ClVery polar covalentH—F>2.0IonicNa+ Cl-
106 Bond Polarity Refer to Table 6.2, p. 177 Consider HCl H = electronegativity of 2.1Cl = electronegativity of 3.0the bond is polarthe chlorine acquires a slight negative charge, and the hydrogen a slight positive charge
107 Bond Polarity d+ d- d+ and d- Only partial charges, much less than a true 1+ or 1- as in ionic bondWritten as:H Clthe positive and minus signs (with the lower case delta: ) denote partial charges.d+ d-d+ and d-
108 Bond Polarity H Cl Can also be shown: the arrow points to the more electronegative atom.Shows how e- are pulledTable 8.3, p.238 shows how the electronegativity can also indicate the type of bond that tends to formH Cl
109 Polar molecules Sample Problem 8.3, p.239 A polar bond tends to make the entire molecule “polar”areas of “difference”HCl has polar bonds, thus is a polar molecule.A molecule that has two poles is called dipole, like HCl
110 Polar moleculesThe effect of polar bonds on the polarity of the entire molecule depends on the molecule shapecarbon dioxide has two polar bonds, and is linear = nonpolar molecule!
111 Polar moleculesThe effect of polar bonds on the polarity of the entire molecule depends on the molecule shapewater has two polar bonds and a bent shape; the highly electronegative oxygen pulls the e- away from H = very polar!
112 Polar moleculesWhen polar molecules are placed between oppositely charged plates, they tend to become oriented with respect to the positive and negative plates.Figure 8.24, page 239
113 Attractions between molecules They are what make solid and liquid molecular compounds possible.The weakest are called van der Waal’s forces - there are two kinds:#1. Dispersion forcesweakest of all, caused by motion of e-increases as # e- increaseshalogens start as gases; bromine is liquid; iodine is solid – all in Group 7A
114 #2. Dipole interactionsOccurs when polar molecules are attracted to each other.2. Dipole interaction happens in waterFigure 8.25, page 240positive region of one molecule attracts the negative region of another molecule.
115 #2. Dipole interactions H F d+ d- H F d+ d- Occur when polar molecules are attracted to each other.Slightly stronger than dispersion forces.Opposites attract, but not completely hooked like in ionic solids.H Fd+ d-H Fd+ d-
117 #3. Hydrogen bonding…is the attractive force caused by hydrogen bonded to N, O, F, or ClN, O, F, and Cl are very electronegative, so this is a very strong dipole.And, the hydrogen shares with the lone pair in the molecule next to it.This is the strongest of the intermolecular forces.
118 Order of Intermolecular attraction strengths Dispersion forces are the weakestA little stronger are the dipole interactionsThe strongest is the hydrogen bondingAll of these are weaker than ionic bonds
119 #3. Hydrogen bonding defined: When a hydrogen atom is: a) covalently bonded to a highly electronegative atom, AND b) is also weakly bonded to an unshared electron pair of a nearby highly electronegative atom.The hydrogen is left very electron deficient (it only had 1 to start with!) thus it shares with something nearbyHydrogen is also the ONLY element with no shielding for its nucleus when involved in a covalent bond!
120 Hydrogen Bonding (Shown in water) This hydrogen is bonded covalently to: 1) the highly negative oxygen, and 2) a nearby unshared pair.
121 Hydrogen bonding allows H2O to be a liquid at room conditions.
122 Attractions and properties Why are some chemicals gases, some liquids, some solids?Depends on the type of bonding!Table 8.4, page 244Network solids – solids in which all the atoms are covalently bonded to each other
123 Attractions and properties Figure 8.28, page 243Network solids melt at very high temperatures, or not at all (decomposes)Diamond does not really melt, but vaporizes to a gas at 3500 oC and beyondSiC, used in grinding, has a melting point of about 2700 oC
124 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