Covalent_Bonding_&_Molecular_Compounds_Presentation_v_1.1.notebookOctober 26, 2012

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1 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, 2012 oter book 5 ovalent onding and Molecular ompounds hemical onds Recall that there are three basic types of bonds: Ionic The electrostatic attraction between ions ovalent The sharing of electrons between s Metallic Each metal bonds to other metals s within a mobile "sea" of electrons (not covered until P hemistry) hemical onds Ionic onding Ionic bonds occur when the difference in electronegativity between two s is more than 1.7. ovalent onding If the difference of electronegativity is less than 1.7, neither takes electrons from the other; they share electrons. This type of bonding typically takes place between two non metals. ovalent onding In the case of ionic bonding, a 3 lattice of ionic ions is the result...not individual s. The chemical formula for an ionic compound is just the ratio of each type of ion in the lattice, not a particular number of ions in a. In contrast, covalent bonding results in individual s; each with its own unique shape. These shapes help determine the physical and chemical properties of everything around us! 1 Which pair of s will form a covalent bond? 2 Which pair of s will form a covalent bond? Li and e Li and I K and r a and l and K and l a and l and 1

2 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, 2012 Molecular ompounds ovalent compounds are formed between two nonmetals. When s are bonded covalently, the s are held together by sharing electrons. Such a compound is called a molecular compound (). In covalent bonds, electron sharing usually occurs so that s attain the electron configurations of noble gases. oth s used the shared electrons to reach that goal. aming inary Molecular ompounds Use prefixes to indicate the number the s ll end in "ide" Examples 2 = nitrogen dioxide P 2 5 = diphosphorous pentoxide ( penta oxide > pentoxide) aming inary Molecular ompounds 3 hlorine monoxide is Look on your reference sheets for the prefixes. The with the lower electronegativity is usually written first. If there is only one of the first, the mono is left off. Examples l2 l l 2l = carbon monoxide 2 = carbon dioxide 4 initrogen tetroxide is 5 2 is 2 ydrogen monoxide 24 ihydrogen monoxide 3 ydrogen oxide 42 ydrogen dioxide 2

3 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, S 3 is 7 Mg is sulfate monomagnesium monoxide sulfur oxide magnesium monoxide sulfur trioxide monomagnesium oxide sulfite magnesium oxide 8 P 4 10 is iphosphorous pentoxide Tetraphosphorous decoxide Lewis Structures Lewis structures are diagrams that show valence electrons as dots. Lewis structures are also known as Lewis dot or electron dot diagrams. Phosphorous oxide Phosphate ote that no electrons are paired until after the fourth one. 9 ow many valence electrons does nitrogen have? 10 The Lewis structure for nitrogen is E 7 True alse 3

4 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, 2012 The ctet Rule Recall that s tend towards having the electron configuration of a noble gas. or most s, that means having 8 valence electrons. The ctet Rule also applies to molecular compounds. So, in covalent bonding, an will share electrons in an effort to obtain eight electrons around it (except hydrogen which will attempt to obtain 2 valence electrons). ow do electron dot structures represent shared electrons? Two s held together by sharing a pair of electrons are joined by a single covalent bond. + ydrogen 1s ydrogen ydrogen Shared pair of electrons pair of valence electrons that is not shared between s is called an unshared pair, also known as a lone pair or a nonbonding pair. 1s ydrogen ow do electron dot structures represent shared electrons? n electron dot structure such as : represents the shared pair of electrons of the covalent bond by two dots. + Shared pair of electrons Structural ormulas structural formula represents the covalent bonds by dashes and shows the arrangement of covalently bonded s. s in the example below, one shared pair of electrons is represented by one dash. Shared pair of electrons ydrogen 1s 1s ydrogen ydrogen ydrogen move me ydrogen 11 ow many electrons are shared by two s to create a single covalent bond? 2 1 Single ovalent onds The halogens form single covalent bonds in their diic s. luorine is one example. luorine + > R luorine luorine 1s 2s 2p luorine 1s 2s 2p 4

5 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, 2012 Lewis Structures In a water, each hydrogen and oxygen attains a noble gas configuration by sharing electrons. 2 + > or ydrogen s 2s xygen 1s 2p Water Lewis Structures In the ammonia, 3, each attains a noble gas configuration by sharing electrons. This has one unshared pair of electrons. 3 + > or ydrogen itrogen mmonia 2s 1s 2p 1s 1s Water The water has two unshared, or lone, pairs of electrons. 1s 1s 1s mmonia rawing Lewis Structures rawing Lewis Structures irst, find the total number of valence electrons in the polyic ion or. If it is an anion, add an electron for each negative charge. If it is a cation, subtract an electron for each positive charge. The P has 5 valence electrons. l has 7, and there are three of them. The total number of valence electrons is: The central is the least electronegative element (excluding hydrogen). onnect the other s to it by single bonds. P has an electronegativity of 2.1 and l has an electronegativity of 3.0, P will be the central. The l s will surround the P. The single bonds are shown as single lines. 1. ount each single bond as a pair (two) of electrons. 2. dd electons to the outer s to give each one 8 (a full shell), or just 2 electrons for hydrogen. 3. o the same for the central. 4. heck: oes each have a full outer shell (8 except, 2 for hydrogen)? ave you used up all the valence electrons? ave you used too many electrons? rawing Lewis Structures rawing Lewis Structures irst, find the total number of valence electrons in the polyic ion or. If it is an anion, add an electron for each negative charge. If it is a cation, subtract an electron for each positive charge. 3 The has 5 valence electrons and each of the three s has 1 so the total number of valence electrons is, 5 + 3(1) = 8 5

6 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, 2012 rawing Lewis Structures The central is the least electronegative element (excluding hydrogen because it can only have one bond). onnect the other s to it by single bonds. can never be the central so must be The s will surround the. The single bonds are shown as single lines. 3 rawing Lewis Structures ount each single bond as a pair (two) electrons. ow add electons to the outer s to give each one a full shell (2 in the case of ). ext, do the same for the central. heck: oes each have a full outer shell? ave you used up all the valence electrons you started with? ave you used too many electrons? Each already has two electrons, so that's done. ut we have to add electrons to to make 8. VSEPR Theory Valence Shell Electron Pair Repulsion ccording to VSEPR theory, the s will adopt a shape/geometry so as to reduce the repulsion between the bonded electrons. VSEPR umbers The VSEPR number of a is a three digit number that can be used to determine a 's shape. ere's how you find it. 1. raw the Lewis structure for the. Locate the central, if applicable. 2. The first digit of the VSEPR number is the total number of electron domains around the central. Electron domains are either shared pairs of electrons or lone pairs of electrons Multiple bonds (i.e. double or triple bonds) count as only E electron domain. VSEPR umbers 3. The second digit of the VSEPR number is the total number of bonding domains around the central. onding domains are single, double or triple bonds. 4. The third digit of the VSEPR number is the total number of lone pairs around the central. Each pair of electrons that are not involved in bonds counts as one lone pair. 5. heck your work the first digit is equal to the sum of the second and third. l P Si S Se Xe I 4 raw a Lewis Structure and use that to determine the VSEPR number Slide for nswer heck to make sure that each has a full outer shell. ow calculate the VSEPR #. Electron domains = 4 onding domains = 4 Lone pairs of electrons = 0 Its VSEPR number is

7 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, 2012 l P Si S Se Xe I 3 raw a Lewis Structure and use that to determine the VSEPR number Slide for nswer heck to make sure that each has a full outer shell. ow calculate the VSEPR #. Electron domains = 4 onding domains = 3 Lone pairs of electrons = 1 l P Si S Se Xe I Si 4 raw a Lewis Structure and use that to determine the VSEPR number Slide for nswer Si heck to make sure that each has a full outer shell. ow calculate the VSEPR #. Electron domains = 4 onding domains = 4 Lone pairs of electrons = 0 Its VSEPR number is Its VSEPR number is l P Si S Se Xe I P 4 3 raw a Lewis Structure and use that to determine the VSEPR number Slide for nswer P heck to make sure that each has a full outer shell. ow calculate the VSEPR #. Electron domains = 4 onding domains = 4 Lone pairs of electrons = 0 Lewis Structures If you are drawing the Lewis Structure for an I... negative ion has extra electrons, add the charge of the ion to your valence electron count. l 2 has 1(7) + 2(6) + 1 = 20 electrons positive ion is missing electrons, subtract the charge of the ion to your valence electron count. + 4 has 1(5) + 4(1) 1 = 8 electrons Its VSEPR number is Lewis Structures raw the Lewis dot structure for the sulfate ion, S 4 2 Lewis Structures raw the Lewis dot structure for the hydronium ion, 3 + 7

8 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, 2012 l P Si S Se Xe I 2 raw a Lewis Structure and use that to determine the VSEPR number We ran out of electrons, but carbon does not have an octet yet! ow What? Slide for nswer ouble and Triple ovalent onds toms form double or triple covalent bonds if they can attain a noble gas structure by sharing two pairs or three pairs of electrons. bond that involves two shared pairs of electrons is a double covalent bond. bond formed by sharing three pairs of electrons is a triple covalent bond. ouble and Triple ovalent onds arbon ioxide, 2 arbon ioxide, 2 1. etermine the # of valence electrons. 1 (4) + 2 (6) = 16 e 2. orm single bonds. This leaves 12 electrons, 6 pairs 3. Place lone pairs on oxygen s to give each heck: We had 16 electrons to work with; how many have we used? 5. There are too many electrons in our drawing. We must form ULE S between and. Instead of sharing only 1 pair, a double bond shares 2 pairs. So one pair is taken away from each and replaced with another bond. ouble and Triple ovalent onds ouble and Triple ovalent onds ond Type ond Energy ond Type ond Energy 348 kj 163 kj 614 kj 418 kj 839 kj 941 kj It requires more energy to break double and triple bonds compared to single bonds. Triple bonds are the strongest of the three. 8

9 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, 2012 ouble and Triple ovalent onds ouble and Triple ovalent onds This chart compares bond strength and bond length for single, double, and triple bonds. Type of ond Electrons Shared ond Strength ond Length Single 2 weakest longest ouble 4 intermediate intermediate Triple 6 strongest shortest 12 s the number of bonds between a pair of s increases, the distance between the s: increases 13 s the number of bonds between a pair of s increases, the strength of the bond between the s: decreases remains unchanged varies, depending on the s increases decreases remains unchanged varies, depending on the s 14 s the number of bonds between a pair of s increases, the energy of the bond between the s: 15 ow many electrons are shared by two s to create a single bond? increases decreases remains unchanged varies, depending on the s 9

10 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, ow many electrons are shared by two s to create a double bond? 17 ow many electrons are shared by two s to create a triple bond? Writing Lewis Structures If you run out of electrons before the central has an octet form multiple bonds until it does. ouble and Triple ovalent onds xygen + > or xygen xygen xygen 1s 2s 2p xygen 1s 2s 2p l P Si S Se Xe I raw a Lewis Structure and use that to determine the VSEPR number Slide for nswer arbon has the lower electronegativity, so we will consider it the "central"... Electron domains = 2 onding domains = 1 Unpaired electrons = 1 Its VSEPR number is oordinate ovalent onds 10

11 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, 2012 oordinate ovalent onds In carbon monoxide, oxygen has a stable configuration but the carbon does not. oordinate ovalent onds coordinate covalent bond is a covalent bond in which one contributes both bonding electrons. + > arbon xygen 1s 2s 2p arbon monoxide In a structural formula, you can show coordinate covalent bonds as arrows that point from the donating the pair of electrons to the receiving them. In a coordinate covalent bond, the shared electron pair comes from one of the bonding s. 1s 2p 2s arbon monoxide arbon has 4 valence electrons, oxygen has 6. l P Si S Se Xe I 2 raw a Lewis Structure and use that to determine the VSEPR number Slide for nswer Electron domains = 4 onding domains = 1 Unpaired electrons = 3 Its VSEPR number is iic Molecules is a neutral group of s joined together by covalent bonds. ir contains oxygen s. diic is a consisting of two s. ertain elements do not exist as single s; they always appear as pairs. When s turn into ions, this LGER PPES! Remember: lri ydrogen itrogen xygen luorine hlorine romine Iodine l P Si S Se Xe I 3 raw a Lewis Structure and use that to determine the VSEPR number Slide for nswer or the central oxygen: Electron domains = 3 onding domains = 2 Unpaired electrons = 1 Its VSEPR number is onsider the Lewis structure we would draw for ozone, 3: Resonance We would expect the double bond to have a shorter bond length than the single bond. owever, the true, observed structure of ozone shows that both bonds are the same length. ow can this be? 11

12 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, 2012 Resonance ne Lewis structure cannot accurately depict a like ozone. Therefore, we use multiple structures, called resonance structures, to describe the. zone has two resonance structures. Resonance The actual ozone is a synthesis of these two resonance structures. The bond length for both outer oxygen s falls somewhere between the single and double bond length. Resonance structure Resonance structure zone Resonance The nitrate ion, 3 1 also requires resonance structures to explain its covalent bonding. Lewis Structures raw the Lewis dot structure for S 3: There are three resonance structures for the nitrate ion: 18 ow many resonance structures can be drawn for the carbonate ion, 3 2? 1 Resonance The benzene is a regular hexagon of carbon s with a hydrogen bonded to each one. There are two resonance structures for benzene E 5 enzene, 6 6, is obtained from the distillation of fossil fuels. More than 4 billion pounds of benzene is produced annually in the United States. ecause benzene is a carcinogen, its use is closely regulated. 12

13 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, 2012 Resonance In truth, the shared pairs of electrons do not always remain between adjacent s. They are not localized. Instead, the electrons are said to be delocalized, meaning that they they can move around the 6 carbon ring. Exceptions to the ctet Rule There are three types of ions or s that do not follow the octet rule: Ions or s with an odd number of electrons Ions or s with less than an octet < > or Ions or s with more than eight valence electrons (an expanded octet) enzene is commonly depicted as a hexagon with a circle inside to signify the delocalized electrons in the ring... we will talk more about this at the end of the year when we study organic chemistry. dd umber of Electrons ewer Than Eight Electrons Though relatively rare and usually quite unstable and reactive, there are ions and s with an odd number of electrons. is an example: eryllium (e) this metal is shown to form molecular compounds, rather than ionic compounds as expected; only needs 4 electrons to be stable oron () only needs 6 electrons to be stable Memorize these exceptions Exceptions to the ctet Rule raw the Lewis dot structure for boron trifluoride, 3: Exceptions to the ctet Rule Expanded ctet raw the Lewis dot structure for phosphorous pentachloride, Pl 5: Slide for nswer 13

14 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, 2012 Exceptions to the ctet Rule Expanded ctet The only way Pl 5 exists is if phosphorus has 10 electrons around it. Exceptions to the ctet Rule ow many electrons do these central s have around them? This is called an expanded octet. toms on the third energy level or higher are allowed to expand their octet to 10 or 12 electrons. The d orbitals in these s participate in bonding, allowing the expanded octet. Lewis Structures Lewis Structures raw the Lewis dot structure for the xenon hexafluoride, Xe 6. raw the Lewis dot structure for the iodine tricholoride, Il 3. VSEPR Model ow does VSEPR theory help predict the shapes of s? VSEPR stands for Valence Shell Electron Pair Repulsion The VSEPR model uses electron dot diagrams to help predict molecular shapes based on this fact: In a, each electron pair will position itself as far away as possible from other electron pairs in order to minimize repulsion. ccording to VSEPR theory, the repulsion between electron pairs causes molecular shapes to adjust so that the valence electron pairs stay as far apart as possible. The shape of a plays an important role in determining its chemical and physical properties. To determine a 's shape, i.e. its molecular geometry, we must first determine its electron domain geometry. 14

15 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, 2012 ow does VSEPR theory help predict the shapes of s? Recall: ow does VSEPR theory help predict the shapes of s? Electron domains are either shared pairs of electrons or lone pairs of electrons onding domains are single, double or triple bonds. Each pair of electrons that are not involved in bonds counts as one lone pair. To determine the electron domain geometry, look at the first number and use the following chart... Electron omain Geometry (EG) The EG (2,3,4,5,or 6) gives us the general shape of the, as shown here. owever, these domains do not have to be bonds. The molecular geometry tells us if there is a bond or lone pair of electrons present, thereby specializing the general shape. Linear Electron omain Geometry Two s around a central one will form a linear shape with bond angles of 180 o Linear Let's take a closer look... Linear Molecular Geometry Trigonal Planar Electron omain Geometry Three s around a central one will form a trigonal planar shape with bond angles of 120 o There is only one molecular geometry for linear electron domain: linear molecular geometry (220). trigonal planar 15

16 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, 2012 Trigonal Planar Molecular Geometry Trigonal Planar Molecular Geometry trigonal planar (330) bent (321) There are two molecular geometries: Trigonal planar, if all the electron domains are bonding (330) ent, if one of the domains is a nonbonding pair (321) Tetrahedral Electron omain Geometry Tetrahedral Molecular Geometry our s around a central one will form a tetrahedral shape with bond angles of o tetrahedral There are three molecular geometries: Tetrahedral, if all are bonding pairs (440) Trigonal pyramidal, if one is a nonbonding pair (431) ent, if there are two nonbonding pairs (422) Tetrahedral Molecular Geometry Trigonal ipyramidal Electron omain Geometry ive s around a central one will form a trigonal bipyramidal shape with bond angles of 120 o and 90 o tetrahedral (440) trigonal pyramidal (431) bent (422) trigonal bipyramidal 16

17 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, 2012 Trigonal ipyramidal Molecular Geometry Trigonal bipyramidal Trigonal ipyramidal Molecular Geometry There are four molecular geometries for the trigonal bipyramidal electron domain geometry: Seesaw T shaped Linear Trigonal ipyramidal (550) See Saw (541) T Shape (532) Linear (523) ctahedral Electron omain Geometry ctahedral Molecular Geometry Six s around a central one will form an octahedral shape with bond angles of 90 ο ctahedral Square Pyramidal octahedral Square Planar ctahedral Molecular Geometry There are only three molecular geometries for the octahedral electron domain geometry: ow does VSEPR theory help predict the shapes of s? Using VSEPR numbers, you can determine molecular geometry. VSEPR numbers are a set of 3 numbers. ctahedral (660) Square Pyramidal (651) Square Planar (642) 1) the total number of electron domains 2) the number of bonding domains* 3) the number of unshared pairs of electrons (* Remember that multiple bonds count as E domain) Electron domain geometry has the same name as the first shape. 17

18 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, 2012 VSEPR umbers and Molecular Geometries VSEPR umbers and Molecular Geometries raw the Lewis structure for ammonia, 4. raw the Lewis structure for l 3. ote that s containing LY halogens will usually violate the octet rule. What are the VSEPR numbers for 4? Slide for nswer Slide for nswer 4,3,1 What are the VSEPR numbers for l 3? What is the electron domain geometry of 4? What is the molecular shape of 4? tetrahedral trigonal pyramidal What is the electron domain geometry of l 3? What is the molecular shape of l 3? 5,3,2 trigonal bipyramidal T shape 19 The methane ( 4 ) has which geometry? linear 20 Give the VSEPR number for this. trigonal bipyramidal trigonal planar tetrahedral and this is (polar/nonpolar) 21 Give the VSEPR number for this. 22 Give the VSEPR number for this. Xe 18

19 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, Which compound below contains an that is surrounded by more than an octet of electrons? P5 4 Polarity of onds Though s often form compounds by sharing electrons, the electrons are not always shared equally. In a covalent bond, one has a greater ability to pull the shared pair toward it. r3 2 Polarity of onds Polarity of onds Identical s will have an electronegativity difference of ZER. s a result, the bond is PLR. TYPE on Polar ovalent ELETREGTIVITY IEREE zero, or very small Polar ovalent about 0.2 to 1.6 Ionic above 1.7 (between metal & non metal) Electronegativities Polarity of onds Therefore, the fluorine end of the has more electron density than the hydrogen end. We use the symbol to designate a dipole (2 poles). The "+" end is on the more positive end of the and the arrow points towards the more negative end. When two s share electrons unequally, a bond dipole results. 19

20 ovalent_onding_&_molecular_ompounds_presentation_v_1.1.notebookctober 26, 2012 Polarity of onds ond lengths, Electronegativity, ifferences and ipole Moments of the ydrogen alides Polarity of Molecules ut just because a possesses polar bonds does not mean the as a whole will be polar. or instance, in the case of 2: ompound ond Electronegativity ipole length (0) ifferenece Moment () l r I The polar bond is shown as a dipole, the arrow points to the more negative. ipoles add as vectors. Polarity of Molecules y adding the individual bond dipoles, one can determine the overall dipole moment for the. Polarity of Molecules These are some examples of polar & nonpolar s. What are their VSEPR numbers? or a to be polar, it must a) contain one or more dipoles b) have these polar bonds arranged asymmetrically Slide 110(?), for nswer polar Slide 431, polar for nswer In other words, if all the dipoles are symmetrical, they will cancel each other out and the will be PLR. Slide 440, for nonpolar nswer Many s with lone pairs of electrons will be PLR. Slide 330, nonpolar for nswer Slide 440, for polar nswer 24 Which of these are polar s? E a & b a, b & c a & c a, c & d c & e 20