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Chapter 9 2010 Announcements Chapter 9 Chapter 8 Chapter 9 (Read 279 to top 283) Skip mid-283- end of Section 9.2. Read Section 9.3 Skip 9.4 Read 9.5 Chapter 10 - 11.2 Read all of it Exam III (Chapter 7-11.2) Wednesday, September 29, 2010 Models of Chemical Bonding Time: 6:00PM - 7:30PM Rooms to be announced Chem 7 Comprehensive Final Exam Wednesday, Oct 13 1:30-3:30AM Chapter 1-11.2 + 12 60 multiple choice questions only Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Metal-Metal bonding or metallic bonding “pools Figure 9.2 The three models of chemical bonding. valence electrons” into an evenly distributed “sea” of delocalized electrons that can move freely (conduct electricity). 9- Atoms tend to form three types of chemical bonds: An ionic compound (salt) results from the chemical ionic, covalent and metallic. reaction between a metal and non-metal. Ionic Bonding: Electrons from one atom are Covalent compounds result from chemical reactions Covalent Bonding between non-metals. results from sharing transferred to another element (atom) that has a one or more electrons Cadmium pairs between atoms tendency to accept electrons (IE and EA). usually non-metals. Non-Metals transfer electrons share Metals electrons Bromine Iodine Valence electrons are the electrons in the outer Metals tend to loose (be oxidized) electrons while non- shell (highest value of n quantum number) of the metals tend to gain electrons (be reduced) to acquire atom that dictate chemistry & in chemical bonding. the noble gas configuration closest to them in the periodic table. Group e- configuration # of valence e- 1A ns1 1 2A ns2 2 Given by the Group 3A ns2np1 3 Number for Group A 4A ns2np2 4 5A ns2np3 5 6A ns2np4 6 7A ns2np5 7 • For B group elements, the valence electrons are in the highest value ns orbital and the (n-1)d orbitals. Lewis dot structures are used to depict atoms in Lewis structures are used to show how ionic bonds chemical bonding. Look at the Group number to get are conceptualized and modeled by chemists. the number of valence electrons. +1 +2 +3 -3 -2 -1 Formation of sodium chloride noble gas electronic •• configuration •Cl •• • •• - Mg• 2+ •• Formation of •• Mg 2 Cl•• •• •• magnesium Cl •• • •• chloride • A chemical symbol A single dot around • •• 2+ •• 2- represents the nucleus • Ba • • • •• + O Ba •• O Formation of - and all core e . the symbol represents •• •• barium oxide one valence e-. Chemists use different ways to conceptualize and Use partial orbital diagrams and Lewis symbols draw ionic bonding. Know them. to depict the formation of Na+ and O2- ions from the atoms, and determine the formula of the 1. Electron configurations compound. Li (1s22s1) + F (1s22s22p5) Li+ 1s2 + F- (1s22s22p6) Draw orbital diagrams for the atoms and then move electrons to make filled outer levels. It 2. Orbital diagrams can be seen that 2 sodiums are needed for Li+ each oxygen. Li 1s 2s 2p 1s 2s 2p - + F + F 1s 2s 2p 1s 2s 2p 3. Lewis electron-dot symbols .. + . Li + :: F: Li + :: F: Use partial orbital diagrams and Lewis symbols to Non-metal elements typically share electrons depict the formation of Na+ and O2- ions from the between atoms forming covalent bonded compounds atoms, and determine the formula of the compound. called molecules or molecular compounds. Example of Covalent Bonding SOLUTION: share electrons O2- forming chemical bond Na 2s 2p 3s 3p O 2s 2p 2 Na+ Na Na . 3s 3p : : + : : 2- H H H H + O. 2Na + O : . : Na . 2 H ATOMS 1 H2 MOLECULE • Covalent bonds are formed by the sharing at The formation of a covalent bond releases energy least one pair of electrons between non-metals. and reduces the potential energy (i.e minimizes Covalent compounds forms the largest class of energy). molecules on our planet. Atoms to Close Together ELECTRON Atoms Far Apart Attractive Forces CLOUD ELECTRON CLOUD • Every covalent bond has a Energy characteristic Released Repulsive Forces NUCLEUS bond length When Bond NUCLEUS that leads to Formed maximum H Atom H Atom stability. There is a balance of repulsive and attractive forces We use Lewis structures and the octet rule to show between nuclei and electrons such. covalent bonds and bonding between atoms. - F + F F F 8 e or an Nucleui electron density “octet” around 7e- 7e- 8e- 8e- each atom single covalent bond lone pairs F F lone pairs Lewis Dot Structure F F Lewis Skeletal Structure single covalent bond We use Lewis structures to predict molecular • What is the Lewis dot structure and dash geometries of molecules (organic). formula for water, H2O and for carbon dioxide? Lewis Structure is used to predict Molecule Geometry which can be related to how chemical bonds are formed through atomic orbitals or hybrids of them. • What is the Lewis dot structure and dash Bond order, which affects bond length in a formula for water, H2O and for carbon dioxide? compound, is the number of bonds between any two atoms. H H O H O H H O H • Bond Order is the number of bonds between two atoms. Lewis Dot Dash Formula – Single bond, order = 1 – Double bond, order = 2 H O H What is the Lewis dot structure and the dash formula for carbon •• •• – Triple bond, order = 3 O C O dioxide? •• •• • • • • • • •• • •• • • • O • C • O • • O C O Higher bond order means: • • • • • •• • •• • – Shorter bond lengths between bonded atoms • • • •• •• – Stronger bond energies (more energy needed to break) •• O C O •• O C O •• • •• •• •• – Bond Length = Distance between two bonded nuclei Double Bond-Octet Still Higher bond orders give shorter bond lengths and Bond length depends on the size of the bonded require more energy to break a bond. atoms. Internuclear distance Covalent Internuclear distance Covalent (bond length) radius (bond length) radius 72 pm 114 pm F2 72pm Br2 114pm Internuclear distance Covalent Internuclear distance Covalent (bond length) radius (bond length) radius 100 pm 133 pm Cl2 100pm I2 Bond Lengths 133pm Triple bond < Double Bond < Single Bond Comparing Bond Length and Bond Strength Comparing Bond Length and Bond Strength Using the periodic table, but not Tables 9.2 and Using the periodic table, but not Tables 9.2 and 9.3, 9.3, rank the bonds in each set in order of rank the bonds in each set in order of decreasing bond decreasing bond length and bond strength: length and bond strength: (a) S - F, S - Br, S - Cl (a) S - F, S - Br, S - Cl (b) C = O, C - O, C O (b) C = O, C - O, C O PLAN: (a) The bond order is one for all and sulfur is bonded to halogens; bond length should increase and bond strength should decrease with increasing atomic radius. (b) The same two atoms are bonded but the bond order changes; bond length decreases as bond order increases while bond strength increases as bond order increases. (a) Atomic size increases going down a group. (b) Using bond orders we get Bond length: S - Br > S - Cl > S - F Bond length: C - O > C = O > C O Bond strength: S - F > S - Cl > S - Br Bond strength: C O > C = O > C - O Electronegativity is an element’s inherent ability to Electronegativity is an inherent ability of an element draw electrons to itself when chemically bonded to to attract electron density---its’ “electron- another atom in a molecule (relative to Li). withdrawing power”. Highest Electronegativity electron poor electron rich region H F region F O !- N !+ Cl Br Electron affinity is the amount of energy absorbed (endothermic) when an electron is added to a gaseous atom to form an ion with a 1- charge. ! ! X (g) + e X (g) EA = - "H (kJ/mol)) Differences in electronegativity between bonding Unsymmetrical electron distribution gives polar atoms can blur the distinction between covalent, bonds and when considered with geometry may polar covalent and ionic bonding types. lead to polar molecule (dipole moment). The arrow is used to denote a polar covalent Symmetric charge distribution Polar Covalent no dipole moment Bonding bond. Arrow points negative Covalent Bonding Ionic Bonding Non-polar symmetrical molecules Un-symmetric charge distribution !+ !- has a dipole moment !+ !- Polar asymmetric molecule F2 HF LiF The delta’s are used to show a partial charge on atoms. How ionic or covalent a bond is between two atoms Electronegativity values can be used to judge the can be judged by looking at the the absolute value extent of ionic or covalent bond character in a of the difference in electronegativity. chemical bond. 3.0 χ = |ENX - ENM| absolute value of Mostly Ionic electronegativity difference between 2-bond atoms 2.0 χ = |ENX - ENM| Polar Covalent ! Mostly Covalent 0.0 Classify the following bonds as ionic, polar Write the Lewis dot and skeletals structure of nitrogen covalent, or covalent: The bond in CsCl; the bond trifluoride (NF3). in H2S; and the NN bond in H2NNH2. Cs – 0.7 Cl – 3.0 3.0 – 0.7 = 2.3 Ionic Write the Lewis dot and skeletal structures of the carbonate ion (CO 2-). H – 2.1 S – 2.5 2.5 – 2.1 = 0.4 Polar Covalent 3 N – 3.0 N – 3.0 3.0 – 3.0 = 0 Covalent Write the Lewis dot and skeletal structures structure of - the carbonate ion (BrO3 ). Write the Lewis dot and skeletal structures structure of the carbonate ion HCN? Write the Lewis structure of nitrogen trifluoride (NF3). Write the Lewis structure of nitrogen trifluoride (NF3). Step 1 – N is less electronegative than F --> N is central atom! Step 1 – N is less electronegative than F --> N is central atom! Step 2 – Count octet electrons = N = 4 - atoms X 8 e- = 32 electrons Step 2 - Count valence electrons = A; Nitrogen = 5, Fluorine = 3 X 7 = 21 N = 32 octet electrons A = 5 + 21 = 26 valence electrons Step 3 - Count valence electrons = A; Nitrogen = 5, Fluorine = 3 X 7 = 21 A = 5 + 21 = 26 valence electrons Step 3 - Write structure with N central and three bonds and rest non- Step 4 – Compute S = N -A = # bonding electrons = 32-26 = 6 bonding octet electrons around the central atom.
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