Lecture 7: Chemistry 2024 PDF

Mid-Semester Feedback GOT Feedback?  Lecture #7, p. 1 Lecture 7: Announcements Today: Brown 8.1 Lewis Symbols and the Octet Rule 8.2 Ionic Bonding...

Mid-Semester Feedback GOT Feedback?  Lecture #7, p. 1 Lecture 7: Announcements Today: Brown 8.1 Lewis Symbols and the Octet Rule 8.2 Ionic Bonding 8.3 Covalent Bonding 8.4 Bond Polarity and Electronegativity 8.5 Drawing Lewis Structures 8.6 Resonance Structures 8.7 Exceptions to the Octet Rule 8.8 Strengths and Lengths of Covalent Bonds 5.8 Bond Enthalpies 9.1 Molecular Shapes 9.2 VSEPR Model Chemistry  Lecture #7, p. 2 Lecture 7: Announcements Problem Set 6: Due before Exercise #7 tomorrow; upload on Moodle link Problem Set 7: Posted on Moodle; due before Exercise #8 next week Study Center: Wednesdays 18:00–20:00 in ETA F 5 Office Hours: Prof. Norris and Brisby, Thursdays 17:00–18:00 in LEE P 210 Next Week: Brown 11.1 Molecular Comparison of Gases, Liquids, and Solids 11.2 Intermolecular Forces 11.3 Select Properties of Liquids 11.4 Phase Changes 11.5 Vapor Pressure 11.6 Phase Diagrams Chemistry  Lecture #7, p. 3 Review In Lecture 6, we discussed electronic structure and periodic table Light waves, quanta of energy, photons Line spectra, atomic lines, Bohr model of hydrogen Beyond Bohr model: quantum mechanics Electron clouds, orbitals, quantum numbers: n, ℓ, mℓ, ms Orbital shapes: s, p, d, f Spin, Pauli Exclusion Principle, Hund’s rule Multielectron atoms, shape of periodic table Electron configurations Screening, effective nuclear charge Atomic radii, ionic radii Ionization energy, electron affinity Chemistry  Lecture #7, p. 4 Today: Basics of Chemical Bonding we have mentioned Atoms or ions are held together in molecules and solids by bonds everal times How do we describe? Bonds involve valence electrons (v.e.’s) Typically, electrons in outermost shell Types of bonding? Ionic Ions held together by electrostatic attraction Ex: Na+Cl− ionic solid Covalent Atoms held together by shared v.e.’s Ex: H2O molecules Metallic V.e.’s shared with entire solid Ex: Ag (s) metallic solids Chemistry  Lecture #7, p. 5 Todiscussbonding wefirstintroduce a simpletool Lewis Symbols For each atom: draw atomic symbol plus v.e.’s (as “dots”) Ex: sulfur [Ne] 3s23p4 ⇒ 6 valence electrons S Rules: Place 2 “dots” maximum on each of 4 sides Spread out the valence electrons available Top, bottom, left, and right sides of symbol are all equivalent Other ex’s: [He] Li Be B C N O F Ne Chemistry  Lecture #7, p. 6 Octet Rule Atoms tend to gain, lose, or share electrons until surrounded by 8 v.e.’s Why 8? Because then they have the stable “noble gas” configuration Ne Note: Exceptions exist to the octet rule Still very useful idea to understand bonding! Most useful for atoms with v.e.’s in s and p subshells Rather than d and f subshells Chemistry  Metals nonmetals Lecture #7, p. 7 ecall Ionic Bonding Metal atoms lose valence electrons To satisfy the octet rule Non-metal atoms gain valence electrons Ex: NaCl Na + Cl Na+ + [ Cl ]− Na+Cl− (s) Ionic bonding is strong! Due to electrostatic attraction of ions Opposite charges attract! This lowers energy as they approach iii Strength quantified by lattice energy Energy required to separate ions to infinite distance Eel 4 e ∆"!"# Ex: NaCl Na+Cl− (s) Na+ (g) + Cl− (g) ∆"!"# = +788 kJ/mol Chemistry How can we calculate the lattice energy? Iii ThereversereationformingNaclis fromNatand Ct is highlyexothermic Heat  Lecture #7, p. 8 Wecancalculate thatbyputtingmanyideasthatwehavelearnedtogether Hess'slaw HI ionizationenergyandelectron affinity EA Calculating the Lattice Energy ∆"!"# = +788 kJ/mol Chemistry seep 376ofBrownfordetails  Lecture #7, p. 9 Butmostsubstances are notionic e.g moleculesarenotionic Theyexploit Covalent Bonding Two atoms share their valence electrons ⇒ covalent bond The shared pair of valence electrons ⇒ “glue” that holds atoms together Ex: H2 H + H HH it Each atom now has noble-gas configuration Cl2 Cl + Cl Cl Cl ⇒ He and Ar, respectively Bonding pair H H Unshared or non-bonding electrons drawn as line: kept as dots ⇒ “lone pairs” Cl Cl The atoms can also share multiple pairs... Chemistry the bond  Lecture #7, p. 10 Multiple Covalent Bonds Single pair of shared electrons represents one covalent bond ⇒ single bond Two atoms that share two pairs of electrons ⇒ double bond Ex: CO2 O + C + O O C O or O C O Two atoms that share three pairs of electrons ⇒ triple bond Ex: N2 N + N N N or N N In general, bond lengths decrease with number of shared electron pairs Ex: Bond N–N N=N N≡N Length 147 124 110 pm Chemistry  Lecture #7, p. 11 ButLewis structuresimplythatelectronpair is shared equally betweenthebondedatoms How Are Valence Electrons Shared in Covalent Bond? Bond polarity How equally is bonding pair distributed? If Equally ⇒ Nonpolar covalent bond Ex: Cl2 Cl Cl N2 N N sharedequa Unequally ⇒ Polar covalent bond Ex: HF H F To estimate Ability of atom in molecule polarity of bond, Electronegativity to attract electrons to itself we can use... Electronegative atoms Large negative electron affinity Large ionization energy Chemistry aiis.tttatgtigtefelfin'ssafat5ct.nos 1994 iii E  Lecture #7, p. 12 Bond Polarity Chemistry  Lecture #7, p. 13 Electronegativity Table Chemistry LinusPauling 19011994 Electronegativity scale based onthermodynamicdata nounits  Lecture #7, p. 14 Calculate Bond Polarity? Use electronegativity difference Table! Ex: HF 4.0 − 2.1 = 1.9 ⇒ Polar covalent bond I is Ic Can represent with partial charges "! H F "" " ! partial positive charge " " partial negative charge Rule of thumb: if electronegativity difference ≥ 2 ⇒ ionic bond Electronegativity increases as we move right and to the top of periodic table Chemistry Ex Lif difference is 4.0 1.0 3.0 ionic  Lecture #7, p. 15 Dipole Moments Partial charges lead to dipole moment, "⃗ u mu #! H F #" Separation of charge quantified by vector pointing from "⃗ negative to positive in "⃗ = &' ±& ≡ charges separated by distance ' FogHE Very polar bonds ⇒ large "⃗ Units? Debye (D) D = 3.34 × 10−30 C⋅m Co er A proton and electron separated by 0.1 nm ⇒ "⃗ = 4.8 D "⃗#$ = 1.8 D Chemistry Note Weareusingthe physicists conventionforthedirection ofthedipole negativetopositive Chemistsuse oppositeconvention  Lecture #7, p. 16 Ionic versus Covalent Not a strict line between ⇒ continuum of behavior I ite More covalent ⇒ molecular behavior ⇒ low melting/boiling temperature More ionic ⇒ ionic solids ⇒ brittle, high-melting temperature Electronegativity difference rule of thumb ⇒ useful, but not perfect EastEe As oxidation number of metal increases ⇒ bonding more covalent Ex: MnO IONIC Mn2O7 COVALENT +2 −2 +7 −2 Chemistry iiiiiiiiiiiiiiiii.i.i.i.iiiiiiiiiiiiitiiii  Lecture #7, p. 17 Asmoleculesbecome morecomplicated weneedrules to drawLewis structures Drawing Lewis Structures Rules: Ex: HCN 1. Sum valence electrons of all atoms 1+4+5 = 10 Factor in overall charge 2. Write symbols and connect with single bonds Central atom often written first in formula (but not always) H C N I 3. Complete octets around non-central atoms H C N H only needs 2 electrons 4. Place remaining valence electrons around central atom H C N Even if it then has more than 8 electrons 5. Try multiple bonds if central atom does not have octet H C N Chemistry in iii d  Lecture #7, p. 18 How It.inies'ErusitEItetsaie'emiiethuie Thenwhichoneiscorrect Alternative Lewis Structures Sometimes octet rule satisfied with more than one Lewis structure Which one is correct? Calculate formal charge of each atom in each Lewis structure: Formal charge Atom’s 1 Atom’s Atom’s non- on specific atom = valence e−’s − 2 bonding e−’s − bonding e−’s Nostioffor ! ! Ex: CN− [C N C: 4 − N: 5 − −[ 6 − 2 = −1 6 −2 =0 " " yanideion Dominant Lewis structure for molecule has formal charges closest to zero and any negative formal charges are on more electronegative atoms Chemistry i thf.imTcnantgensegative  Lecture #7, p. 19 Alternative Lewis Structures: thiocyanate ion Chemistry electronegative Nismore than s  Lecture #7, p. 20 Wenowhavethreewaysto describetheelectronicchargeon a specificatomin a molecule oxidationstateformalchargeandpartialcharge What'sthedifference Oxidation Number vs. Formal Charge vs. Partial Charge: Oxidation Number: Charges atoms would have if bonds completely ionic Shared e’s given completely to more electronegative atom Overestimates role of electronegativity Formal Charge: Charges atoms would have if bonding e’s shared equally Ignores electronegativity Partial Charge: Somewhere in between !+ !− Ex: H—Cl H—Cl H—Cl +1 −1 0 0 0.18+ 0.18− oxidation formal actual partial numbers charges charges Chemistry I É tneserepresentactualchar g.es  Lecture #7, p. 21 Resonance Structures Two alternative, but completely equivalent Lewis structures Ex 1: ozone, O3 18 v.e.’s Central O needs 1 single bond and 1 double bond Which one is correct? Actual structure techie ben seebelow Blend of both resonance structures Bond lengths same Chemistry  Lecture #7, p. 22 Resonance Structures Two alternative, but completely equivalent Lewis structures Ex 2: benzene, C6H6 alternating single bond and double bonds But bond lengths in the ring are same Intermediate between C–C single and double bond Electrons from double bonds delocalized over the ring Chemistry  Lecture #7, p. 23 Exceptions to the Octet Rule Some molecules fail the octet rule Exceptions involve: Odd number of electrons: 11 v.e.’s Less than octet of v.e.’s: 0 +1 0 0 B has 6 electons 0 −1 −1 −1 +1 0 0 +1 0 0 0 0 More than octet of v.e.’s: P has 10 electons Called hypervalent Period 3 or beyond Chemistry No nitricoxide or nitrogenmonoxide  Lecture #7, p. 24 Strengths of Covalent Bonds Bond enthalpies Enthalpy change for breaking particular bond Positive number: energy required to break bond Values available in tables Give average bond enthalpies for specific bonds Can use to estimate ∆""#$ : enthalpies enthalpies ∆"!"# = % for bonds − % for bonds broken formed Chemistry  Lecture #7, p. 25 Lengths of Covalent Bonds Bonds get shorter between two atoms as number of bonds increases Chemistry  In Chapter 8 we used Lewis structures to account for the formulas of covalent com- pounds. (Section 8.5) Lewis structures, Cl however, do not indicate the shapes of molecules; they simply show the number and types of bonds. For example, the Lewis Lecture #7, p. 26 9.1 | MOLECULAR SHAPES The Lewis structure of CCl structure is drawn with the atoms all in the same plane. As shown in ! FIGURE 4 tells us only that four Cl atoms are bonded to a central C atom: 9.1, however, the actual arrangement is the Cl atoms at the corners of a tetrahedron, a geo- metric object with four corners and fourCl faces, each an equilateral triangle. The shape of a molecule is determined by its bond angles, the angles made by the In Chapter 8 we used Lewis structures to account for the formulas lines joining the nuclei of the atoms Cl in the C molecule. Cl The bond angles of a molecule, to- gether with the bond lengths (Section 8.8), define the shape and size of the pounds. (Section 8.5) Lewis structures, however, do not indi Lewis structure is drawn withall thehave atoms Cl molecule. In Figure 9.1, you should be able to see that there are six Cl ¬ C ¬ Cl bond The angles ! FIGURE in CCl4 and that they theall in the same same value of plane. 109.5°As shown , the angleinsize characteris- molecules; they simply show the number and types of bonds. For e 9.1, tic however, metric the actual arrangement of a tetrahedron. object In addition,is all thefour Cl atoms C ¬ at Clthe corners bonds are of thea same tetrahedron, lengtha(1.78 geo- Å). Thus, thewith shapefour andcorners size of and CClfour faces, each an equilateral triangle. 4 are completely described by stating that the molecule is structure of CCl tells us only that four Cl atoms are bonded to a cen The shape of tetrahedral lines joining 4thewith a molecule nuclei is determined C ¬ Cl bonds of the atoms of lengthby of in 1.78 the molecule. bond angles, the angles made by the its Å. The bond angles of(and a molecule, to- like What About Molecular Shape? We begin our discussion molecular shapes with molecules ions) that, gether CClwith, have thea bond single lengths central (Section atom bonded 8.8), to two define or more the shape atoms of and the size same of type.the Such 4 molecule. In Figure 9.1, general you should be able ABntoinseewhich that there Cl ¬AC is are sixatom ¬bonded Cl bondto n Cl molecules have the formula the central 3D versus 2D angles in CCl4 and that they all have the same value of 109.5°, the angle size characteris- tic of a tetrahedron. In addition, all four C ¬ Cl bonds are the same length (1.78 Å). Thus, the shape and size of CCl4 are completely described by stating that the molecule is Lewis structures useful, but two dimensional InWethebegin tetrahedral G with O F ICG¬ UCl space-filling R Ebonds of length 1.78 Å. our discussionmodel, what determines of molecular shapes with Cl themolecules C relative sizes Cl (and of thethat, ions) spheres? like CCl4, have a single central atom bonded to two or more atoms of the same type. Such Four equivalent C— Clinbond All AB molecules Ex: tetrachloromethane (carbon tetrachloride), CClfaces 4 have the general formula n lengths 1.78 Å CCl4 Cl which the central atom A is bonded to n CCl4 Show connectivity, bonding, but not Theshape Lewis structure is drawn with the atoms all in the same plane. As sh GO FIGURE In the space-filling model, what determines the relative sizes of the spheres? 9.1, however, the actual arrangement is the Cl atoms at the corners of a Actual shape: four Cl atoms at corners of tetrahedron Four equivalent All C— Cl bond CCl All Cl— C— Cl CCl metric object with faces four Tetrahedron corners lengths angles and 1.78 Å 109.5° four faces, Ball and stick model each 4 an equilateral triang Space-filling model 4 Shape = connectivity + bond angles The shapeFIGUREof a9.1molecule is determined by its bond angles, the a " Tetrahedral shape of CCl4. lines joining the nuclei of the atoms in the molecule. The bond angles Size = shape + bond lengths gether with the bondAllanglesCl— lengths C— Cl 109.5° (Section 8.8), define the shap Tetrahedron Ball and stick model Space-filling model molecule. InFIGURE Figure 9.1, you should be able to see that there are six How can we predict anglesthe shapeand in CCl of simple 9.1 that they " molecules? Tetrahedral shape of CCl4. all have the same value of 109.5°, the ang 4 tic of a tetrahedron. In addition, all four C ¬ Cl bonds are the sam Chemistry Thus, the shape and size of CCl4 are completely described by stating t tetrahedral with C ¬ Cl bonds of length 1.78 Å. We begin our discussion of molecular shapes with molecules (a CCl4, have a single central atom bonded to two or more atoms of th molecules have the general formula ABn in which the central atom  Lecture #7, p. 27 SEC Predicting Shapes? n=2 Simple Molecules: ABn CO2 AB2 linear SECTION SO 9.1 2 AB2 bent Molecular Shapes SO333 3 AB3 trigonal planar NF3 AB3 trigonal pyramidal Limited number of possible shapes ! FIGURE 9.2 Shapes of AB2 and AB3 molecules. n=3 Depends on value of n B atoms. Both CO2 and H2O are AB2 molecules, for example, whereas SO3 and NH3 are AB3 molecules, and so on. The number of shapes possible for ABn molecules depends on the value of n. Those CO2 SO2 SO NF3and AB molecules areClFshown Most molecules derived from 5 basic shapes: 3 commonly found for AB 2 3 3 in ! FIGURE 9.2. An AB2 mol- AB2 linear AB2 bent AB3 trigonal planar ecule must be either linear (bond angle =AB180 AB3 trigonal 3 T-shaped °) or bent (bond angle Z 180°). For AB3 pyramidal molecules, the two most common shapes place the B atoms at the corners of an equilat- ! FIGURE 9.2 Shapes of AB2 and AB3 molecules. eral triangle. If the A atom lies in the same plane as the B atoms, the shape is trigonal planar. If the A atom lies above the plane of the B atoms, the shape is trigonal pyramidal By removing atoms we obtain other possible shapes: B atoms. Both CO2 and H2O are AB2 molecules, for example, whereas (a pyramid with an equilateral triangle as its base). Some AB3 molecules, such as ClF3, SO3 andthe are T-shaped, NH 3 are relatively unusual shape shown in Figure 9.2. The atoms lie in one AB3 molecules, and so on. plane, but the angles between them vary as shown. The number of shapes possible for ABn molecules depends on the value ofFigures Compare n. Those 9.1 and 9.2 to notice the difference between NF3 and CCl4. The commonly found for AB2 and AB3 molecules are shown in ! FIGURE 9.2. An AB CCl4 molecule 2 mol- is tetrahedral because the four atoms bonded to the carbon are disposed ecule must be either linear (bond angle = 180°) or bent (bondatangle the Zfour °). For of 180apexes ABa3 tetrahedron around the central atom. The NF3 molecule is molecules, the two most common shapes place the B atoms at the corners of pyramidal an equilat- because the three atoms bonded to nitrogen lie at the base of a trigonal eral triangle. If the A atom lies in the same plane as the B atoms, the shape is trigonal pyramid. planar. If the A atom lies above the plane of the B atoms, the shape isThetrigonal shapespyramidal that maximize the separation of outer atoms are shown in " FIGURE (a pyramid with an equilateral triangle as its base). Some AB3 9.3molecules, such to. In addition as the ClFshapes 3, we have already seen, this figure shows those encountered are T-shaped, the relatively unusual shape shown in Figure 9.2. whenThethere atomsarelie fiveinorone six atoms surrounding a central atom. The trigonal bipyramid can plane, but the angles between them vary as shown. be thought of as two face-to-face trigonal pyramids; the octahedron is like two face-to- Compare Figures 9.1 and 9.2 to notice the difference between NF3 and face square CCl4. The pyramids. CCl4 molecule is tetrahedral because the four atoms bonded to the carbon are disposed at the four apexes of a tetrahedron around the central atom. The NF3 molecule is Chemistry pyramidal because the three atoms bonded to nitrogen lie at the G base O FofI GaU trigonal RE pyramid. Which of these molecular shapes do you expect for the SF6 molecule? The shapes that maximize the separation of outer atoms are shown in " FIGURE 9.3. In addition to the shapes we have already seen, this figure shows those encountered when there are five or six atoms surrounding a central atom. The trigonal bipyramid can be thought of as two face-to-face trigonal pyramids; the octahedron is like two face-to- 109.5! face square pyramids. 180! 120! GO FIGURE Which of these molecular shapes do you expect for the SF6 molecule? AB2 linear AB3 trigonal planar AB4 tetrahedral  Lecture #7, p. 28 Why Remove Atoms? Valence-shell electron-pair repulsion (VSEPR) model Electrons repel each other Electron pairs move as far apart as possible Explains 5 basic shapes and bond angles In addition to bonds, we also count lone pairs Electron domains: bonds or “lone pairs” around central atom Shape determined by number of electron domains around central atom Electron-domain geometry versus molecular geometry Includes atoms Includes just atoms and “lone pairs” Chemistry  B A B Does this structure follow the octet rule? How many electron domains are there Lecture #7, p. 29 around the A atom? The VSEPR model is based on the idea that electron domains are negatively charged and therefore repel one another. Like the balloons in Figure 9.5, electron domains try to stay out of one another’s way. The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them. In fact, the analogy between electron domains and balloons is so close that the same preferred geometries are found in both cases. Like the balloons in Figure 9.5, two electron domains orient linearly, three domains orient in a trigonal-planar fashion, and four orient tetrahedrally. These arrangements, together with those for five- and six-electron domains, are summarized in ! TABLE 9.1. If you compare the geometries in Table 9.1 with those in Figure 9.3, TABLE 9.1 Electron-Domain Geometries as a Function of Number of Electron Domains Repulsion of Electron Domains Number of Arrangement of Electron-Domain Predicted Electron Domains Electron Domains Geometry Bond Angles 180! Balloon analogy 2 Linear 180! 120! 3 Trigonal 120! planar 109.5! 4 Tetrahedral 109.5! Possible Electron-Domain 90! Geometries 5 Trigonal 120! 120! bipyramidal 90! 90! 90! 6 Octahedral 90! Chemistry with ne 9.1)vertex one withvertex one occupied vertex occupied byoccupied a by a by a ctron-domain ahedral al electron-domain electron-domain geometry geometry Sn Sn geometry Sn SECTION 9.2 The VSEPR Model 337 nding ins domains means domains means the molecular means the molecular Cl Cl Cl Cl Cl Cl the molecular As one more example, let’s determine the shape of the CO2 molecule. Its Lewis struc- Lecture #7, p. 30 ture reveals two electron domains (each one a double bond) around the central carbon: ble ). 9.2). Cl Cl Cl O C O Two electron domains orient in a linear electron-domain geometry (Table 9.1). Because neither domain is a nonbonding pair of electrons, the molecular geometry is also linear, and the O ¬ C ¬ O bond angle is 180°. ! TABLE 9.2 summarizes the possible molecular geometries when an ABn molecule 2- 2- has four or fewer electron domains about A. These geometries are important because olecular ar metries for (a)for geometries geometries (a)for SeCl (a)CO (b) 2,SeCl (b) 2,SeCl., (b) 3 2CO 3 CO. 32-. they include all the shapes usually seen in molecules or ions that obey the octet rule. nal trigonal nar,planar, trigonal planar, trigonal planar trigonal planarplanar TABLE 9.2 Electron-Domain and Molecular Geometries for Two, Three, and Four Electron Domains around a Central Atom ofPossible Molecular Number of Electron- Effect Effect of Effect Nonbonding of Nonbonding Nonbonding Electrons Electrons Electrons andandand Electron Domain Bonding Nonbonding Molecular Domains Geometry Domains Domains Geometry Example The VSEPR Model 339 Geometries SECTION 9.2 Multiple Multiple Multiple Bonds Bonds Bonds on Bond on Bond onAngles Bond Angles Angles 2 2 0 O C O Linear Linear We canWerefine can Werefine Becausecan the VSEPR refine the VSEPR multiple the model VSEPR bondsmodel to contain explain model to explain to slight explain slight a higherdistortions slight distortions distortions from from electronic-charge the ideal from the ideal densitygeometries thethan ideal geometriesgeometries single summarized summarized bonds, summarized in Table multiple in TableRefinements: 9.2. in bonds For Table 9.2. example, alsoFor 9.2.example, Forconsider represent example, consider enlargedmethane considermethane (CH electron methane 4(CH ), ammonia domains.4(CH ), ammonia 4), (NH ammonia Consider3 3(NH ),the and (NH ), and 3Lewis 3), and 3 GO FIGURE 0 F waterstructure water (H2O). water (HAll O). 2of (H three All 2O). three have phosgene: All three ahave tetrahedral have a tetrahedral a tetrahedral electron-domain electron-domain electron-domain geometry, geometry, geometry, but their but bond their but their bond an- bond an- an- Why is the volume occupied by F B the F nonbonding electron Trigonal pair planar domain Trigonal planar gles differ gles differ gles slightly: differ slightly:Lone pairs take up more space slightly: Cl larger than the volume occupied ! H H H 2 by the 1bonding domain? O N O C O C C C N N N O O O Bonding electron Bent pair H H H H H HH Cl H H H H H H H H H 109.5" H 109.5"H 109.5" H 107"H 107"H 107" H 104.5" H 104.5" H 104.5" H 4 4 0 C Because three electron domains surround the central atom, we might expect a trigonal- H H H NoticeNotice thatNotice planar the thatbond geometry the thatbond angles the bond with angles 120decrease angles decrease ° bond asdecrease theasnumber angles. theasdouble The number theofnumber nonbonding of nonbonding bond, of nonbonding however, electronelectron seems pairs electron to actpairs in-much pairs in- in- Tetrahedral Tetrahedral creases. creases. A bonding creases. A bonding Apair bonding of pair electrons of pair electrons of is electrons attracted is attracted isby attracted both by nuclei both by both nuclei of the nuclei of bonded theofbonded the atoms, bonded atoms, but atoms, but but like a nonbonding pair of Multiple electrons, bonds reducingtaketheupClmore ¬C¬ space Cl bond angle to 111.4 : ° a nonbonding a nonbonding a nonbonding pair ispair attracted is pairattracted is primarily attracted primarily by primarily only by one only by nucleus. only one nucleus. oneBecause nucleus. Because a Because nonbonding a nonbonding a nonbonding 3 1 N pair experiences pair experiences pair experiences less nuclear less nuclear less attraction, nuclear attraction, Cl attraction, its electron its electron itsdomain 124.3 "electron domain is domain spread is spread out is spread more out morein outspace more in space in space H H H than is than thethan iselectron theiselectron thedomain electron domain fordomain a bonding for a for bonding a111.4 pair "C bonding! FIGURE (pair ! FIGURE O(pair (!9.7FIGURE 9.7). 9.7 ). Nonbonding Nonbonding ). Nonbonding electron electron electron Nuclei Trigonal pyramidal pairs pairs therefore pairs therefore take therefore up takemore up takemore space up more space thanspace bonding than than bonding pairs. bonding 124.3 " pairs. As a pairs. result, As a result, Aselectron a result, electron domains electron domains for domainsfor for Cl phosgene nonbonding nonbonding nonbonding electron electron pairs electron pairs exert pairs exert greaterexert greater repulsive greater repulsive forces repulsive forces on adjacent forces on adjacent onelectron adjacent electron domains electron domains anddomains and and 2 2 O H tend to tend compress to tendcompress tobond compress bond angles. In general,Chemistry bond angles. angles. electron domains for multiple bonds exert a greater repulsive force on adja- H Nonbonding Bent pair cent electron domains than do electron domains for single bonds. Phosgette usedas achemicalweaponduringWorldWarI GIVE IT SOME THOUGHT Responsiblefor 85 ooodeaths One resonance structure of the nitrate ion is ! O  we have four electron domains. We know from Table 9.1 that " FIGURE the repulsions 9.6 shows how these stepsamong four are applied to predict the geometry of the fourth balloon, theyNH adopt a tetrahedral molecule. The three bondsshape. and one We see that nonbonding pair an optimum Lecture in the geomet #7, p.tell Lewis structure 31us electron domains are minimized when the domains 3 point toward theWevertices ofTable a tetra- for each number of balloons. we have four electron domains. know from 9.1 that the repulsions among four hedron, so the electron-domain geometry ofelectron NH3domains is tetrahedral. are minimized We whenknow frompoint the domains thetoward the vertices of a tetra- In some ways, hedron, Lewis structure that one electron domain holds theaelectrons nonbonding in molecules pair ofgeometry so the electron-domain behave electrons, like these balloons. We ha of NHwhich 3 is tetrahedral. We know from the occupies one of the four that a single of thecovalent bond is formed between two atoms whenpair a pair of electron Lewis structure that one electron domain holds a nonbonding of electrons, which vertices tetrahedron. The of bonding arrangement is there- Three balloons occupies one the four vertices of the tetrahedron. The bonding arrangement is there- bonding fore three atoms bondedpies to a the space central between atom, with thecentral the fore three atoms. atoms atom bonded (Section to anot in atom, central 8.3)theA the same with plane central atom notpair of electro in the same plane trigonal-planar orientation defines a region in which the as the three electrons others. are This is just themost likely situation we findto in be the found. We middle molecule will refer of Figure as the three others. This is just the situation we find in the middle molecule of Figure 9.4. Hence, the molecular geometry of NH3 is trigonal pyramidal. Notice that the tetra-