Covalent Bonds and Molecular Structure

Bonding Patterns Chapter 7 z Place the following formulas into two groups.  NaCl  P4O10 Covalent Bonds and Molecular Structure  SF6  MgCl2  CoF3  NH3  OF2 z How would you label or classify your groups? 1 Trends in Electronegativity Differences in Covalent Bonds z Covalent bond: a bond that results from the sharing of electrons between atoms. z Electronegativity: the ability of an atom in a molecule to attract e- to itself. What other atomic property does this remind you of? electronegativity trends  Atoms don’t always share electrons equally Electronegativity Diff. 3 Polar or Nonpolar? z 4 Bonding Nonpolar covalent bonds have no e-neg difference (F2, CH); polar covalent bonds (HF) have small differences; ionic bonds have large differences(LiF). The greater the difference in electronegativities, the more ionic a bond is. z z z z z z z z z Red = high electron density; Blue = low electron density; Green = in between 5 z Identify the type of bond (ionic, polar covalent, nonpolar covalent) holding atoms together in the following compounds. NaCl Electronegativities H2 H 2.1 N 3.0 PCl3 Na 0.9 O 3.5 H2O Ca 1.0 Cl 3.0 CaBr2 P 2.1 Br 2.8 CH4 C 2.5 F 4.0 NaNO3 Problems 7.1, 7.2, 7.3 1 Group Work z z z z z Identify the type(s) of bonds (ionic, polar covalent, nonpolar covalent) holding ALL atoms together in the following compounds: MgCl2 NCl3 MgCO3 Cl2 Lewis Dot Symbols z Group Work Answers z z z z Lewis_symbols Can use Lewis Dot Symbols to represent the numbers of valence electrons for each atom (based on electron configurations) z ionic polar covalent ionic AND polar covalent nonpolar l covalent l Electron-Dot Structures z Electron-Dot Structures MgCl2 NCl3 MgCO3 Cl2 Used to show how electrons are shared between nonmetals in a covalent bond. Use valence electrons to give each atom an octet, with a few e e exceptions cep o s (e (e.g., g , H). ) Single Bonds Lewis Symbol.exe Procedure for drawing structures: 1) Draw an atomic skeleton (first atom is usually in the center) 2) Count total valence electrons z Single bond represented as H:H or H-H z F2: 14 valence electrons total .. .. .. .. : .F. : F :.F. - .F. : . .: • Add or subtract electrons for polyatomic ions 3) Place electron pairs between bonded atoms 4) Place remaining electrons on the outside q atoms, then the central atom r 5) Shift electrons, as necessary, to make r multiple bonds p q z Each atom is trying to get an octet. Once you count valence electrons, you don’t care how many electrons each atom brings…. Satisfy the octet rule and # of valence electrons z 11 12 2 Electron-Dot Structures Multiple Bonds z 1) How many total valence electrons? 2) Does every atom have an octet? z z H only l gets one b bond; d halogens h l usually ll only l form single bonds. z Draw structures for the following: What do you do if there are too many electrons in the Lewis Dot Structure? Draw Lewis Structures for O2 and N2.  CH4, NH3, H2O, CCl2F2, SOCl2, NH4+, SO42z Worked Ex. 7.1, 7.2; Problems 7.4, 7.5 14 Covalent Bond z Strengths of Covalent Bonds z There is an optimum distance between atoms in a covalent bond. This is the bond length: calculated by adding the radii of two atoms. z Comparison of Bonds Closer nuclei result in a stronger bond. Shorter bond = stronger bond. Energy is required (absorbed) to break a bond. Energy is given off (released) when a bond is formed. Comparison of Bonds •What type of bond (single, double, or triple) do you predict to be the strongest? The weakest? Why? •What type of bond do you predict to be the shortest? The longest? Why? zBond Bond strength energy (bond strength): Single bond < double bond < triple bond zBond length (distance between atom nuclei) Bond length Single bond > double bond > triple bond zBond strength and bond length are inversely related 17 3 Group Work z z z z z z Exceptions to the Octet Rule Draw Lewis Structures for the following substances: CO2 CO HCN SO32Worked Ex. 7.3, 7.4; Problems 7.6, 7.7 z Odd-Electron Molecules  Draw a Lewis structure for NO and NO2  Why does NO2 combine with itself to form N2 O 4 ? z Incomplete Octets  Draw structures for BeCl2, BH3, BF3, AlCl3 19 Exceptions to the Octet Rule z z z z 20 Exceptions to the Octet Rule What do you do if there are too many electrons? Draw Lewis Structures for the following: SF4 BrF3 XeF5+ XeF4 XeF2 PCl5 ClF3 BrF5 When do we find expanded octets? (Hint: what shells (rows) are the central atoms in?) Worked Ex. 7.6 – 7.8; Problems 7.9 – 7.11 Cl 21 Electron-Dot Structures: Oxyacids and Carbon Chains z Draw structures for: 22 The Concept of Resonance z  H2SO4  H3PO4  H2CO3 z  Examples: O3, SO3, NO3-, CO32- Carbon always gets 4 electron pairs!  C2 H6  CH3CH2CH2CH3  CH2CHCN Sometimes a single Lewis structure doesn’t accurately represent bonds. Sometimes multiple structures are needed to adequately show how electrons are shared. z Example: nectarines. They are a combination of a peach and plum but you have to use both fruits to describe it accurately. Same with resonance structures. No one structure is accurate – we have to draw multiple structures. 24 4 The Concept of Resonance Resonance z Draw the structure for O3. z Expect O – O bond (148 pm) to be longer than O = O bond (121 pm) Experimental evidence shows both bonds are 128 pm long Resonance structures: two or more Lewis structures used for a single molecule that cannot be represented accurately by only one Lewis structure (a blend of two structures) z z z How many different valid Lewis formulas can you draw for the following molecules or ions? NO3SO2 SO3 CO32- z NCS- Worked Ex. 7.9, Problems 7.12 – 7.14 26 Resonance Molecular Shapes: The VSEPR Model z z z z z z z z z In your groups, draw (or find in your notes) Lewis Structures for the following molecules: CH4 NH3 H2 O BF3 SO2 CO2 PCl5 SF6 27 Molecular Shapes: The VSEPR Model z z z Valence-Shell Electron-Pair Repulsion Electron pairs (or groups of pairs) try to avoid one another because of repulsions between like-charged particles Regions where electrons are likely to be found are called electron domains:  Lone electron pairs  Single, double, and triple bonds z z Five Fundamental Geometries candicevsepr.exe.lnk candiceVSEPR.exe Electron domains occur as far apart as possible Electronic geometry vs. molecular geometry 29 30 5 VSEPR Theory z Molecular Geometries z Use A, B, E notation: A = central atom; B = # outer atoms; E = # lone e- pairs Electronic Geometry Molecular Geom. Molecular Geom. AB3 Trigonal Planar AB4 Tetrahedral AB2E Bent AB3E Trigonal Pyramidal AB2E2 Bent AB5 Trig. Bipyramidal AB4E See-Saw AB3E2 T-Shaped AB6 Octahedral AB5E Square Pyramid AB4E2 Square Planar  CH4 = AB4, NH3 = AB3E, H2O = AB2E2 z z z z z z z Can predict the angles between electron domains (charge clouds): 2 domains - linear (180o) 3 domains - trigonal planar (120o) VSEPR 4 domains - tetrahedral (109.5o) 5 domains - trigonal bipyramidal (90o & 120o) 6 domains - octahedral (90o) Worked Ex. 7.11; Problems 7.17, 7.19 AB2 Linear 31 What are the molecular geometries of the following molecules? z z z z z z z z z z CO2 SO2 BF3 CH4 NH4+ CCl4 NH3 H2O SF2 COCl2 z z z z z rwin32b2.exe.lnk z z z z A = central atom; B = # outer atoms; E = # lone e- pairs Molecular Shapes PCl5 OF2 ClO4SF6 CS2 BiF5 AlCl4ClO2AsH3 z rwin32b2.exe.lnk Why don’t CO2 and SO2 have the same geometry? 33 Valence Bond Theory z z z z 34 Creating Bonds z Lewis structures and VSEPR give information about the shapes of molecules and the distributions of electrons. They don’t explain why a bond forms. Valence-bond theory considers both bond formation and molecular shape Looks at how electrons are shared in a covalent bond VB theory considers the atomic orbitals occupied by the valence electrons Sigma.exe Half-filled orbitals overlap so that 2 electrons can share space and form a covalent bond.  We can combine two s orbitals (H2)  We can combine an s and a p orbital (HCl)  We can combine two p orbitals (F2) 35 6 Examples to Use z z z z z z z z Valence Bond Theory: Hybridization Before we discuss this process, draw Lewis Structures and determine molecular geometries for the following: CH4 BF3 BeCl2 PCl5 SF6 CH2CH2 C2H2 Bond Formation z Combine these concepts: Electron configurations z Orbitals z  Valence electrons (short-hand notation)  s: spherical  p: dumb-bell shaped  Reminder: An orbital only holds two electrons z z Hybridization in Methane Liger: lion + tiger geep z z Zeedonk: zebra + donkey z z Hybrids In chemistry, covalent bonds are created by making orbitals equivalent in energy. This is done by combining or mixing atomic orbitals to create new, or hybrid, orbitals. This is the basis of Valence Bond Theory: using atoms’ valence electrons to explain how bonds form. Hybridization in Methane z z Lewis Dot Structures VSEPR z z Methane: CH4 Lewis Dot Structure? Carbon is central atom El Electron configuration fi i off carbon? b ?  [He] 2s2 2p2 z VSEPR shape?  Tetrahedral - indicates that all bonds are equal Rasmol CH4 Carbon Hybridization Need 4 equivalent orbitals to form the 4 single (σ) bonds (based on VSEPR and experiments) Ground state configuration: 2p E 2s sp3 formation 7 sp3 Hybridization z Hybridization in Boron Trifluoride Combine one s orbital and three p orbitals Æ four hybridized orbitals (sp3) z z z z Boron trifluoride: BF3 Lewis Dot Structure? Boron is central atom El Electron configuration fi i off b boron? ?  [He] 2s2 2p1 z VSEPR shape?  Trigonal planar sp2 hybridization Sigma – C + H Hybridization in Beryllium Chloride z z z z Beryllium chloride: BeCl2 Lewis Dot Structure? Beryllium is central atom El Electron configuration fi i off beryllium? b lli ? Hybridization in Phosphorus Pentachloride z z z z  [He] 2s2 z VSEPR shape? z VSEPR shape?  Trigonal bipyramidal sp hybridization Rasmol BeCl2 Hybridization in Phosphorus Pentachloride z Phosphorus pentachloride: PCl5 Lewis Dot Structure? Phosphorus is central atom El Electron configuration fi i off phosphorus? h h ?  [Ne] 3s2 3p3  Linear z Rasmol BF3 How many single bonds in PCl5? Where does the 5th orbital come from? Rasmol PCl5 Hybridization in Sulfur Hexafluoride z z  Expanded octet z  sp3d z Sulfur Hexafluoride: SF6 Lewis Dot Structure? Sulfur is central atom El Electron configuration fi i off sulfur? lf ?  [Ne] 3s2 3p4 z VSEPR shape? z Problems 7.20 – 7.22 3p E 3s  Octahedral Rasmol SF6 8 Hybridization in Sulfur Hexafluoride z z Need 6 equivalent orbitals to form the 6 single bonds. Where does the 6th orbital come from?  Expanded octet Hybridization in Multiple Bonds: Ethylene z z z z  sp3d2 Ethylene: CH2CH2 Lewis Dot Structure? Carbon is central atom El Electron configuration fi i off carbon? b ?  [He] 2s2 2p2 z  Trigonal planar 3p E VSEPR shape? 3s Hybridization in Ethylene z z Hybridization in Ethylene How many single bonds in CH2CH2? Need 3 equivalent orbitals to form the 3 single bonds around one carbon atom. z z sp2 hybridization accounts for single (σ) bonds, but what about the unhybridized 2p orbital? That orbital contributes to the double (π) bond. Sigma bond 2p E 2s Pi bond sp2 hybridization Hybridization in Acetylene z z z z Hybridization in Acetylene Acetylene: HCCH Lewis Dot Structure? Carbon is central atom El Electron configuration fi i off carbon? b ? z z How many single bonds in HCCH? Need 2 equivalent orbitals to form the 2 single bonds around 1 carbon atom.  [He] 2s2 2p2 z 2p VSEPR shape? E  Linear 2s Rasmol HCN Aorbs.exe sp hybridization 9 Hybridization in Acetylene z z Hybrid Orbitals in a Triple Bond sp hybridization accounts for single (σ) bonds, but what about the unhybridized 2p orbitals? Those 2 unhybridized 2p orbitals help make the two π bonds. bonds z z What does the hybridization for N2 look like? Worked Ex. 7.12; Problems 7.23 – 7.25 56 Group work z z z z z z Determine the hybridization of the central atoms in your group quiz molecules: POCl3 BF3 NO2PCl5 CO32- Summary z According to Valence Bond Theory, covalent bonds form when:  there are two electrons in each orbital; one electron from each atom  these two orbitals “overlap” z The number of hybridized orbitals equals the number of atomic orbitals that are combined.  sp Æ 2 orbitals combined (BeCl2 and HCN)  sp2 Æ 3 orbitals combined (BF3 and CH2O)  sp3 Æ 4 orbitals combined (CH4)  sp3d Æ 5 orbitals combined (PCl5)  sp3d2 Æ 6 orbitals combined (SF6) Summary z z z z z The number of hybridized orbitals equals the number of electron domains (including lone epairs) around a central atom (starting with s)  sp, sp2, sp3, sp3d, sp3d2 A single bond has 1 σ bond (the same goes for a lone pair of electrons) A double bond has 1 σ bond and 1 π bond A triple bond has 1 σ bond and 2 π bonds Unhybridized p orbitals participate in π bonding (to make double and triple bonds) 10