chemical bonding presentation notebook August 17, 2012 Chemical Bonds Ionic Compounds and Ionic Bonding

Transcription

1 hemical onds Ionic ompounds and Ionic onding There are three basic types of bonds: Ionic The electrostatic attraction between ions ovalent The sharing of electrons between atoms Metallic Each metal atom bonds to other metals atoms within a mobile "sea" of electrons (not covered until P hemistry) Ionic onding Electronegativity is how strongly an atom attracts electrons. toms with a high electronegativity will be able to attract electrons away from atoms with a much lower electronegativity. Ionic onding n electronegativity difference of approximately 1.7 can only occur between a metal and a nonmetal. This removal of electrons can occur when the difference in electronegativity between the two atoms is approximately 1.7 or higher. nce a positive and negative ion are formed, they will be attracted to each other via the electrostatic force: F = k q 1 q 2 r 2 ote: The heavier nonmetals from 4,6,5 th groups( In, Tl, Sn, Pb, Sb i ) may act like metals 1 Which pair of atoms will form an ionic bond? 2 Which pair of atoms will form an ionic bond? Li and e Li and e K and r a and Mg K and s K and a S and l a and l 1

2 The ctet Rule In forming compounds, atoms tend towards the electron configuration of a noble gas (s 2 p 6 configuration with 8 electons in the outer shell) toms in the first three groups (1 3) will lose electrons and become cations (think: metals). Ions formed by Representative Elements The charges of the ions of representative elements (s block and p block elements) can be determined by the position on the periodic table. Recall that elements shown here gain or lose electrons to establish a noble gas electron configuration. toms in groups 5 7 (or 15 17) will gain electrons and become anions (think: nonmetals). toms in group 8 (or 18) are the noble gases, which do not generally gain or lose electrons. ote: The heavier nonmetals from 4,6,5 th groups( In, Tl, Sn, Pb, Sb i ) may act like metals Valence Electrons Valence electrons are the electrons in the highest occupied energy level of an element s atoms. The number of valence electrons largely determines the chemical properties of an element. To find the number of valence electrons in an atom of a representative element, simply look at its group number. 3 ow many valence electrons does luminum have? toms in group 3 have 3 valence electrons, atoms in group 7 have 7 valence electrons, etc. 4 ow many valence electrons does arium have? 1 2 The Formation of ations Metals usually give up valence electrons This results in a noble gas (8 electron) outer shell. a : 1s 2 2s 2 2p 6 3s 1 a +1 : 1s 2 2s 2 2p 6 e atom 52 3 Loss of valence electrons The configuration of the Sodium ion is the same as eon 2

3 The Formation of ations The Formation of ations ations of Group 1 elements always have a charge of 1+. loses e ations of Group 2 elements always have a charge of 2+. a atom 11p 11e a+ ion 11p 10e The Formation of nions The gain of negatively charged electrons by a neutral atom produces an anion. n anion is an atom or a group of atoms with a negative charge. The Formation of nions gain of one electron gives chlorine an octet and converts a chlorine atom into a chloride ion. l: 1s2 2s2 2p6 3s2 3p5 l 1s2, 2s2, 2p6, 3s2, 3p6 r atom The name of a monoatomic anion typically ends in ide. onmetals from Group 5 and above gain electrons to form anion. Group 4 elements do not form cations or anions chloride ion has the same electron configuration as argon. The Formation of nions The Formation of nions nions of Group 5 elements have a charge of 3 l atom 17P 17e Gains an e l ion 17p 18e nions of Group 6 elements always have a charge of 2 nions of Group 7 elements have a charge of 1 3

4 5 The ion formed from a calcium atom 6 onmetals tend to lose electrons forming ions a + a 2+ True False a a 2 7 Metals lose electrons to form cations 8 The ion formed from nitrogen True False nions are formed from nonmetals Formation of Ionic ompounds True False ompounds composed of cations and anions are called ionic compounds. lthough they are composed of ions, ionic compounds are electrically neutral. 4

5 Ionic onds The electrostatic forces that hold ions together in ionic compounds are called ionic bonds. Ionic bonds are formed by the "harpoon method". Ionic onds For instance, when sodium and chlorine are close together, sodium's valance electron flies off and "harpoons" the chlorine atom. The result is a sodium cation (+) next to a chloride anion ( ) These oppositely charged two ions attract: they reel one another together to form an ionic bond. a l a+ l 1s 2 2s 2 2s 2 2p 6 3s 1 1s 2 2s 2 2p 6 3s 2 3p 5 1s 2 2s 2 2p 6 1s 2 2s 2 2p 6 3s 2 3p 6 1s 2 2s 2 2p 6 1s 2 2s 2 2p 6 3s 2 3p 6 e r Ionic onds Ionic onds The electron transfer process in creating an ionic bond: a l The dots represents the valence electrons in an atom. y/flash_viewer.php?oid=1349&mid= 55 try/chemistry ii/chemical bonding/ionicbonding animation.php Formula Units Properties of Ionic ompounds chemical formula shows the kinds and numbers of atoms in the smallest representative unit of a substance. They are crystalline solids at room temperature They have high melting points They conduct electricity when melted (molten) or dissolved in water (aqueous) formula unit is the lowest whole number ratio of ions in an ionic compound. Every ionic compound has a 3 array of positive and negative ions. 5

6 Predicting an Ionic ompound Formula Potassium (K) with an electronegativity of 0.8 and oxygen () with an electrogegativity of 3.5 will form an ionic compound. What is the formula for an ionic compound of potassium and oxygen? ow many additional valence electrons does oxygen want? 2 ow many valence electrons does potassium have? 1 Predicting an Ionic ompound Formula What is the formula for an ionic compound of Mg and? ow many additional valence electrons does want? 3 ow many valence electrons does Mg have to offer? 2 ow many Mg atoms will it take to give how many the electrons it needs? (Find the lowest common multiplier first.) ow many potassium atoms will it take to give oxygen the electrons it needs? 2 Mg 3 Mg : 2 K K The formula unit is K 2 lways Metal First( low electonegativity) Mg Mg The formula unit is Mg 3 2 Predicting an Ionic ompound Formula If you don't like finding least common multipliers, you can use this alternative method: 1. Write down the ions side by side along with their charge. lways write the metal first. 2. "riss cross" the numerical values of the charges. 3. Reduce subscripts to lowest ratio. Predicting an Ionic ompound Formula Example: Write the formula for calcium sulfide. Step 1: Identify the cation & write its common ion alcium is in group 2 a 2+ Step 2: Identify the anion & write its formula Mg Mg Mg Sulfur is in group 6 S 2 Step 3: riss cross; reduce subscripts if necessary a 2+ S 2 a 2S 2 as Predicting an Ionic ompound Formula 10 The formula for the ionic between s and What is the compound formed between Mg and S? Mg +2 S 2 Mg 2S 2 lways use the lowest ratio of the ions! = MgS s2 s2 s2 s2 6

7 11 The ionic compound between a and 12 The ionic compound between l and a a22 a32 a23 l32 l23 l l22 13 What is the ionic compound formed between a and l? 14 What is the ionic compound formed between P and r? al a3l2 l2a3 o compound P 3r rp no ionic compound (rp) 2 15 What is the formula for sodium phosphide? SP 3 ap a 3P ap 3 16 What is the formula for strontium bromide? Srr Srr 2 Sr 2r rsr 2 7

8 17 The formula for barium sulfide is a 2S 2. True False aming inary Ionic ompounds ations Many cations have the same name as the original, neutral atom. harge formula name ydrogen ion Li + Lithium ion K + Potassium ion s + esium ion g + Silver ion 2 + Mg 2+ Magnesium ion a 2+ alcium ion a 2+ arium ion Zn 2+ Zinc ion admium ion d l 3+ luminum ion aming inary Ionic ompounds nions ll monoatomic anions all end in " ide". The ions that are produced from Group 7 (or 17) elements are called halide ions. Group 15 Group 16 Group 17 itride 3 xide 2 Fluoride F Phosphide P 3 Sulfide S 2 hloride l romide r Iodide I aming inary Ionic ompounds inary (two element) compounds are named by writing the name of the cation followed by the name of the anion. The name of the cation is the same as the metal name. The name of the anion is the name of the non metal with the suffix changed to ide. inary compounds end in " ide." Examples: al = sodium chloride KI = potassium iodide Li 2S = lithium sulfide 18 a 2S is Sodium sulfate Sodium sulfide i sodium sulfide Sulfur nitride 19 The correct name for Sr is. E strontium oxide strontium hydroxide strontium peroxide strontium monoxide strontium dioxide 8

9 20 The correct name for l 2 3 is. ations formed by Transition Elements E aluminum trioxide dialuminum oxide dialuminum trioxide aluminum oxide aluminum hydroxide Recall that s block metals have only one possible ionic charge, based on the ctet Rule. owever, most transition metals can have more than one ionic charge. For this reason, there is a system for designating each ion. Sn, Pb from the p block will form more than one type of ions and behave like transition metals. ations Formed by Transition Elements ations formed by Transition Elements We will use the Stock naming system (Roman numerals) to name transition metals. Formula ame u +1 opper (I) ion obalt (II) ion o +1 nly common transition metals are shown. Silver, cadmium and zinc only form one cation, g +, d 2+ and Zn 2+ ote the mercury cations. Tin and Lead act like transition metals Fe +2 Mn +2 Pb +2 r +3 Fe +3 Iron (II) ion Manganese (II) ion lead (II) ion hromium (III) ion Iron (III) ion Writing Formulas with Transition Metals The charge on the cation is indicated by the Roman numeral, as shown in these examples. Iron (III) oxide: Fe 3+ 2 Write ion formulas. Fe 3+ 2 riss cross charges. 21 The name of Fel 3 is iron chloride iron (II) chloride iron (III) chloride Fe 2 3 Reduce if necessary. Tin (IV) oxide Sn 4+ 2 Write ion formulas. Sn 4+ 2 riss cross charges. Sn 2 4 Reduce if necessary. Sn 2 9

10 Formulas with Transition Metals Formulas with Transition Metals In order to correctly name a formula containing a transition metal, it is necessary to first determine the charge on the cation. Since all compounds are neutral, then the total positive cation charge must equal the total negative anion charge. In other words, Total cation charge + Total anion charge = 0 (charge of cation) (# of cations) + (charge of anion) (# of anions) = 0 In the case of Fel 3, we make the following substitutions: (charge of cation) (# of cations) +(charge of anion) (# of anions) = 0 (x) (1) + ( 1) (3) = 0 Thus x = 3 and the cation is Fe 3+ or iron(iii). short cut method is to "uncriss cross" the ions, but you must always double check your ions (or you'll get in trouble!). Fel3: Fe1 l3 Uncriss cross. Fe 3+ l 1 heck the ions l does form a 1 ion and Fe 3+ is Iron (III) Iron (III) loride r: r1 1 Uncriss cross. r + heck the ions forms a 2 ion and r+ does not exist! (this formula had to be reduced from r22) romium (II) xide 22 Which metal is capable of forming more than one cation? E K s a l Sn 23 What is the charge on zirconium ion in Zr The formula for tin (IV) oxide is Sn Sn 2 Sn 2 Sn 25 The formula for copper (II) sulfide is us2 us u2 S2 (us)2 10

11 26 Which one of the following compounds is copper(i) chloride? E ul ul2 u2l u2l2 u3l2 27 The charge on the cation in the salt Fe 2 3 is E 6 polyatomic ion is a group of atoms bonded together that have a charge and acts like a single unit or ion They are not free compounds and are not found free in nature. They are found combined with other ions. Eg: Sulfate = (S 4 ) 2 itrate = ( 3) arbonate = ( 3) 2 Polyatomic Ions Use " ( ) " to keep the atoms together. o not change the subscripts inside the "( )" Most of the polyatomic ions contain oxygen atoms Many anions names end with ite or ate In ite/ate pairs, the ion with fewer oxygen atoms will have the ite ending Examples: sulfite /sulfate nitrite /nitrate Polyatomic Ions (con't) ote that the suffix does not indicate the actual number of atoms. Polyatomic Ions (con't) Familiarize yourself with the polyatomic ions on your reference sheet e careful of ide, ite, and ate! + = proton or hydrogen ion or bicarbonate ] aming Ternary Ionic ompounds Ternary ionic compounds contain three or more different elements due to the presence of polyatomic ion (s). Just as in binary ionic compounds, the name of the cation is given first, followed by the name of the anion. ames of ternary compounds often end in ite or ate. Examples a 3 = calcium carbonate (in eggshells) Zn( 2 3 2) 2 = zinc acetate g 3 = silver nitrate a 2S 3 = sodium sulfite 11

12 Writing Formulas for Ternary Ionic ompounds Ternary ionic compounds are neutral, just like binary ionic compounds. Therefore, the goal is to find the lowest ratio of cations to anions that will yield a neutral compound. This ratio is represented in a formula unit. Writing Formulas for Ternary Ionic ompounds (con't) To write a formula, the criss cross method can again be used. Example: Write the formula for lithium phosphate. Step 1: Identify the cation & write its formula Lithium is in group 1 > Li + Examples of formula units a( 3 ) Zn( 2 3 2) 2 g( 3) a 2(S 3) Step 2: Identify the anion & write its formula Phosphate is a polyatomic ion > P 3 4 Step 3: riss cross; reduce subscripts if necessary Li 1+ P 3 4 Li 3(P 4) 1 or simply Li 3(P 4) Writing Formulas for Ternary Ionic ompounds 28 The formula for sodium hydroxide is Example: Write the formula for calcium nitrite. a 2+ ( 3) a( 3) 2 When writing formulas with polyatomic ions, there are two important things to remember: 1) It is helpful to use " ( ) " to keep the atoms together, keeping the charge UTSIE the ( ) For example nitrate = ( 3) 1 carbonate = ( 3) 2 2) EVER alter any symbols or subscripts ISIE inside the "( )" a ()2 a() a(2) a() 29 The formula for aluminum phosphate is 30 The formula for magnesium carbonate is l(p4 ) l3(p4) l2(p4)3 l3(p4)3 Mg2(3) Mg(3) Mg2(3)2 Mg(3)2 12

13 31 The formula for calcium sulfate is a(s4) a2(s4)2 a(s3) a2(s3)2 32 al is sodium chlorate sodium chloride sodium chlorite sodium hypochlorite 33 Mg( 3) 2 is 34 mmonium carbonate is Magnesium carbonate Magnesium hydrogen carbonate Magnesium hydroxide Magnesium carboxide ( 4)( 3) ( 4) 2( 3) ( 4)( 3) 2 ( 4) 2( 2) PRTIE Recognizing and Writing Symbols PRTIE Writing Formulas for Ionic ompounds omplete the table by filling in the gaps. omplete the table by filling in the formula for the ionic compound formed by each pair of cations and anions, as shown for the first pair. 13

14 PRTIE Writing Formulas for Ionic ompounds Write the formula for the following compounds: 1. Magnesium iodide 2. alcium sulfite 3. arium hydrogen carbonate 4. Iron (III) phosphate hemical onds otebook ovalent onding and Molecular ompounds Recall that there are three basic types of bonds: Ionic The electrostatic attraction between ions ovalent The sharing of electrons between atoms Metallic Each metal atom bonds to other metals atoms 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 atoms is more than 1.7. ovalent onding If the difference of electronegativity is less than 1.7, neither atom 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 ions is the result...not individual molecules. 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 molecule. In contrast, covalent bonding results in individual molecules; each with its own unique shape. These shapes help determine the physical and chemical properties of everything around us! 14

15 1 Which pair of atoms will form a covalent bond? 2 Which pair of atoms will form a covalent bond? Li and e Li and I K and r a and l and K and Fl a and l and Molecular ompounds ovalent compounds are formed between two nonmetals. When atoms are bonded covalently, the atoms are held together by sharing electrons. Such a compound is called a molecular compound (molecule). In covalent bonds, electron sharing usually occurs so that atoms attain the electron configurations of noble gases. oth atoms used the shared electrons to reach that goal. aming inary Molecular ompounds Use prefixes to indicate the number the atoms ll end in "ide" Examples 2 = nitrogen dioxide P2 5 = diphosphorous pentoxide ( penta oxide > pentoxide) aming inary Molecular ompounds 3 hlorine monoxide is Look on your reference sheets for the prefixes. The atom with the lower electronegativity is usually written first. If there is only one of the first atom, the mono is left off. Examples l2 l l 2l = carbon monoxide 2 = carbon dioxide 15

16 4 initrogen tetroxide is 5 2 is 2 ydrogen monoxide 24 ihydrogen monoxide 3 ydrogen oxide 42 ydrogen dioxide 6 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 Phosphorous oxide Phosphate Lewis Structures Lewis structures are diagrams that show valence electrons as dots. Lewis structures are also known as Lewis dot or electron dot diagrams. ote that no electrons are paired until after the fourth one. 16

17 9 ow many valence electrons does nitrogen have? 10 The Lewis structure for nitrogen is E 7 True False The ctet Rule Recall that atoms tend towards having the electron configuration of a noble gas. For most atoms, that means having 8 valence electrons. The ctet Rule also applies to molecular compounds. So, in covalent bonding, an atom 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 atoms held together by sharing a pair of electrons are joined by a single covalent bond. + ydrogen atom 1s ydrogen atom ydrogen molecule Shared pair of electrons pair of valence electrons that is not shared between atoms is called an unshared pair, also known as a lone pair or a nonbonding pair. 1s ydrogen molecule 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 Formulas structural formula represents the covalent bonds by dashes and shows the arrangement of covalently bonded atoms. s in the example below, one shared pair of electrons is represented by one dash. Shared pair of electrons ydrogen atom 1s 1s ydrogen atom ydrogen molecule ydrogen molecule move me ydrogen molecule 17

18 11 ow many electrons are shared by two atoms to create a single covalent bond? 2 1 Single ovalent onds The halogens form single covalent bonds in their diatomic molecules. Fluorine is one example. F F F F F F Fluorine atom + > R Fluorine atom Fluorine molecule 1s 2s 2p Fluorine molecule 1s 2s 2p Lewis Structures In a water molecule, each hydrogen and oxygen atom attains a noble gas configuration by sharing electrons. 2 + > or ydrogen atoms 2s xygen atom 1s 2p Water molecule Lewis Structures In the ammonia molecule, 3, each atom attains a noble gas configuration by sharing electrons. This molecule has one unshared pair of electrons. 3 + > or ydrogen atom itrogen atom mmonia molecule 2s 1s 2p 1s 1s Water molecule The water molecule has two unshared, or lone, pairs of electrons. 1s 1s 1s mmonia molecule rawing Lewis Structures rawing Lewis Structures First, find the total number of valence electrons in the polyatomic ion or molecule. 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 atom has 5 valence electrons. l atom has 7, and there are three of them. The total number of valence electrons is: The central atom is the least electronegative element (excluding hydrogen). onnect the other atoms 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 atom. The l atoms will surround the P atom. The single bonds are shown as single lines. 18

19 1. ount each single bond as a pair (two) of electrons. 2. dd electons to the outer atoms to give each one 8 (a full shell), or just 2 electrons for hydrogen. 3. o the same for the central atom. 4. heck: oes each atom 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 First, find the total number of valence electrons in the polyatomic ion or molecule. 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 atom has 5 valence electrons and each of the three atoms has 1 so the total number of valence electrons is, 5 + 3(1) = 8 rawing Lewis Structures The central atom is the least electronegative element (excluding hydrogen because it can only have one bond). onnect the other atoms to it by single bonds. can never be the central atom so must be The atoms will surround the atom. 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 atoms to give each one a full shell (2 in the case of ). ext, do the same for the central atom. heck: oes each atom 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 molecules will adopt a shape/geometry so as to reduce the repulsion between the bonded electrons. VSEPR umbers The VSEPR number of a molecule is a three digit number that can be used to determine a molecule's shape. ere's how you find it. 1. raw the Lewis structure for the molecule. Locate the central atom, if applicable. 2. The first digit of the VSEPR number is the total number of electron domains around the central atom. 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. 19

20 VSEPR umbers 3. The second digit of the VSEPR number is the total number of bonding domains around the central atom. 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 atom. 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. F 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 atom has a full outer shell. ow calculate the VSEPR #. Electron domains = 4 onding domains = 4 Lone pairs of electrons = 0 Its VSEPR number is F l P Si S Se Xe I F 3 raw a Lewis Structure and use that to determine the VSEPR number F Slide for nswer F heck to make sure that each atom has a full outer shell. ow calculate the VSEPR #. F Electron domains = 4 onding domains = 3 Lone pairs of electrons = 1 Its VSEPR number is F l P Si S Se Xe I SiF 4 raw a Lewis Structure and use that to determine the VSEPR number F F Slide for nswer Si F heck to make sure that each atom has a full outer shell. ow calculate the VSEPR #. F Electron domains = 4 onding domains = 4 Lone pairs of electrons = 0 Its VSEPR number is 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 F 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 atom has a full outer shell. ow calculate the VSEPR #. Electron domains = 4 onding domains = 4 Lone pairs of electrons = 0 Its VSEPR number is

21 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 + F 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. Form single bonds. This leaves 12 electrons, 6 pairs 3. Place lone pairs on oxygen atoms 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 atom and replaced with another bond. 21

22 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. 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 atoms increases, the distance between the atoms: increases 13 s the number of bonds between a pair of atoms increases, the strength of the bond between the atoms: decreases remains unchanged varies, depending on the atoms increases decreases remains unchanged varies, depending on the atoms 22

23 14 s the number of bonds between a pair of atoms increases, the energy of the bond between the atoms: 15 ow many electrons are shared by two atoms to create a single bond? increases decreases remains unchanged varies, depending on the atoms 16 ow many electrons are shared by two atoms to create a double bond? 17 ow many electrons are shared by two atoms to create a triple bond? Writing Lewis Structures ouble and Triple ovalent onds If you run out of electrons before the central atom has an octet form multiple bonds until it does. xygen molecule + > or xygen xygen atom xygen atom molecule 1s 2s 2p xygen molecule 1s 2s 2p 23

24 F 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" atom... Electron domains = 2 onding domains = 1 Unpaired electrons = 1 Its VSEPR number is oordinate ovalent onds 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 atom contributes both bonding electrons. + > arbon atom 2s xygen atom 1s 2p arbon monoxide In a structural formula, you can show coordinate covalent bonds as arrows that point from the atom donating the pair of electrons to the atom receiving them. In a coordinate covalent bond, the shared electron pair comes from one of the bonding atoms. 1s 2p 2s arbon monoxide molecule arbon has 4 valence electrons, oxygen has 6. F l P Si S Se Xe I F 2 raw a Lewis Structure and use that to determine the VSEPR number Slide for nswer F F Electron domains = 4 onding domains = 1 Unpaired electrons = 3 Its VSEPR number is iatomic Molecules molecule is a neutral group of atoms joined together by covalent bonds. ir contains oxygen molecules. diatomic molecule is a molecule consisting of two atoms. ertain elements do not exist as single atoms; they always appear as pairs. When atoms turn into ions, this LGER PPES! Remember: lrif ydrogen itrogen xygen Fluorine hlorine romine Iodine 24

25 F l P Si S Se Xe I 3 raw a Lewis Structure and use that to determine the VSEPR number Slide for nswer For the central oxygen: Electron domains = 3 onding domains = 2 Unpaired electrons = 1 Its VSEPR number is Resonance onsider the Lewis structure we would draw for ozone, 3: 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? Resonance ne Lewis structure cannot accurately depict a molecule like ozone. Therefore, we use multiple structures, called resonance structures, to describe the molecule. zone has two resonance structures. Resonance The actual ozone molecule is a synthesis of these two resonance structures. The bond length for both outer oxygen atoms falls somewhere between the single and double bond length. Resonance structure Resonance structure zone molecule 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: 25

26 18 ow many resonance structures can be drawn for the carbonate ion, 2 3? 1 Resonance The benzene molecule is a regular hexagon of carbon atoms with a hydrogen atom 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. Resonance Exceptions to the ctet Rule In truth, the shared pairs of electrons do not always remain between adjacent atoms. They are not localized. Instead, the electrons are said to be delocalized, meaning that they they can move around the 6 carbon ring. There are three types of ions or molecules that do not follow the octet rule: Ions or molecules with an odd number of electrons Ions or molecules with less than an octet < > or Ions or molecules 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 Fewer Than Eight Electrons Though relatively rare and usually quite unstable and reactive, there are ions and molecules 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 26

27 Exceptions to the ctet Rule raw the Lewis dot structure for boron trifluoride, F 3: Exceptions to the ctet Rule Expanded ctet raw the Lewis dot structure for phosphorous pentachloride, Pl 5: Slide for nswer 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 atoms 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 atoms participate in bonding, allowing the expanded octet. Lewis Structures Lewis Structures raw the Lewis dot structure for the xenon tetrafluoride, XeF 4. raw the Lewis dot structure for the iodine tricholoride, Il 3. 27

28 VSEPR Model ow does VSEPR theory help predict the shapes of molecules? 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 molecule, 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 molecule plays an important role in determining its chemical and physical properties. To determine a molecule's shape, i.e. its molecular geometry, we must first determine its electron domain geometry. ow does VSEPR theory help predict the shapes of molecules? Recall: ow does VSEPR theory help predict the shapes of molecules? 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) Linear Electron omain Geometry The EG (2,3,4,5,or 6) gives us the general shape of the molecule, as shown here. Two atoms around a central one will form a linear shape with bond angles of 180 o 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 Let's take a closer look... 28

29 Linear Molecular Geometry Trigonal Planar Electron omain Geometry Three atoms 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 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 Four atoms 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) 29

30 Tetrahedral Molecular Geometry Trigonal ipyramidal Electron omain Geometry Five atoms 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 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 atoms around a central one will form an octahedral shape with bond angles of 90 ο ctahedral Square Pyramidal octahedral Square Planar 30

31 ctahedral Molecular Geometry There are only three molecular geometries for the octahedral electron domain geometry: ow does VSEPR theory help predict the shapes of molecules? 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. VSEPR umbers and Molecular Geometries VSEPR umbers and Molecular Geometries raw the Lewis structure for ammonia, 3. What are the VSEPR numbers for 3? Slide for nswer 4,3,1 raw the Lewis structure for lf 3. ote that molecules containing LY halogens will usually violate the octet rule. What are the VSEPR numbers for lf 3? Slide for nswer What is the electron domain geometry of 3? What is the molecular shape of 3? tetrahedral trigonal pyramidal What is the electron domain geometry of lf 3? What is the molecular shape of lf 3? 5,3,2 trigonal bipyramidal T shape 19 The methane molecule ( 4) has which geometry? 20 Give the VSEPR number for this molecule. linear trigonal bipyramidal trigonal planar tetrahedral 31

32 21 Give the VSEPR number for this molecule. 22 Give the VSEPR number for this molecule. F Xe F 23 Which compound below contains an atom that is surrounded by more than an octet of electrons? PF5 Polarity of onds Though atoms often form compounds by sharing electrons, the electrons are not always shared equally. In a covalent bond, one atom has a greater ability to pull the shared pair toward it. 4 r3 F2 Polarity of onds Polarity of onds Identical atoms will have an electronegativity difference of ZER. s a result, the bond is PLR. TYPE on Polar ovalent ELETREGTIVITY IFFEREE zero, or very small Polar ovalent about 0.2 to 1.6 Ionic above 1.7 (between metal & non metal) 32

33 Electronegativities Polarity of onds Therefore, the fluorine end of the molecule has more electron density than the hydrogen end. F We use the symbol to designate a dipole (2 poles). The "+" end is on the more positive end of the molecule and the arrow points towards the more negative end. When two atoms share electrons unequally, a bond dipole results. Polarity of onds ond lengths, Electronegativity, ifferences and ipole Moments of the ydrogen alides Polarity of Molecules ut just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar. For instance, in the case of 2: ompound ond Electronegativity ipole length (0) ifferenece Moment () F l r I The polar bond is shown as a dipole, the arrow points to the more negative atom. ipoles add as vectors. Polarity of Molecules y adding the individual bond dipoles, one can determine the overall dipole moment for the molecule. Polarity of Molecules These are some examples of polar & nonpolar molecules. What are their VSEPR numbers? For a molecule 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 molecule will be PLR. Slide 440, for nonpolar nswer Many molecules with lone pairs of electrons will be PLR. Slide 330, nonpolar for nswer Slide 440, for polar nswer 33

34 24 Which of these are polar molecules? E a & b a, b & c a & c a, c & d c & e ybridization of tomic rbitals ybridization is the process of mixing up of different orbitals to produce a hybrid orbital. The valence shell electron pair repulsion model (VESPR) was devised to account for the molecular shapes. In this model, atoms and pairs of electrons will be arranged to minimize the repulsion of these atoms and pairs of electrons. The process of hybridization will tell you where the electron pairs are located, which orbitals are participating and how the molecule gets the shape. The number of these new hybrid orbitals must be equal to the numbers of atoms and non bonded electron pairs surrounding the central atom. ybridization ybridization Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals. These sp hybrid orbitals have two lobes like a p orbital. ne of the lobes is larger and more rounded as is the s orbital. These two degenerate orbitals would align themselves 180 from each other. This is consistent with the observed geometry of beryllium compounds: linear. overlap region + s orbital p orbital F 2p orbital sp hybrid orbital F 2p orbital sp hybrid orbitals the two sp hybrid orbitals together The sp orbitals are higher in energy than the 1s orbital but lower than the 2p. Sigma (σ) onds Pi bonds (π) 1s 1s 1s 3p Pi bonds are characterized by Side to side overlap. Electron density above and below the internuclear axis. π bond Internuclear axis 3p 3p p p Sigma bonds are characterized by ead to head overlap. ylindrical symmetry of electron density about the internuclear axis. 34

35 ybridization 4 we envision that the s electron in arbon is promoted to the p level in order to make room for the 4 ydrogen electons to share. 25 what is the hybridization in ef 2 4 sp 3 hybrid orbitals of same energy and shape are created. sp3 sp2 sp spd 26 what type of hybridization is there in Sil 4 ybridization sp3 sp2 sp2d sp ybridization 27 ybridization in water molecule is sp2 sp3 sp sp3d 35

36 28 what is the hybridization in ichloro methane ybridization Involving Multiple onds The sigma bond is defined as the linear overlap of ( axial or along the axis) atomic orbitals (hybrids except for hydrogen) in which two electrons are directly between the two bonded nuclei. sp3 sp2 sp sp3d Pi bonds are defined as the parallel overlap of p orbitals. double bond has one sigma bond and one pi bond. triple bond thus consists of a sigmabond and two pi bonds with the pibonds in different planes. Ethylene molecule, 2=2 ybridization Involving Multiple onds ybridization Involving Multiple onds cetylene, ybridization Involving Multiple onds enzene 29 each carbon atom in the above molecule is sp3 hybridized. True False ount the total number of electron domains around the atom. That many hybrid orbitals are needed via combination of s, p or d remember double bonds are counted only once. 36

37 30 ame the hybridization in each carbon 31 3 = 2 what is the hybridization of the middle carbon atom? sp3 sp3 sp sp2 sp2 sp4 32 What is the hybridization of the Xe atom in XeF4? SP Sp3d2 sp3d sp3 33 What is the geometry of a molecule with sp3d hybridization? tetrahedron octahedron trigonal pyramid trigonal bipyramid 37