Unit IV. Covalent Bonding

Transcription

1 Unit IV. Covalent Bonding READING ASSIGNMENT 1: Read 16.1 pp Complete section review questions Lewis Theory of Covalent Bonding- The driving force of bond formation is the desire of each atom electrons in its valence shell. in a molecule to attain an octet of A. Covalent Bond- A chemical bond that is formed from a pair of shared electrons between two atoms 1. Caused by a small difference in electronegativities of the bonded atoms General Rule for covalent bonds: a. If electronegativities < 2.0 (1.7) b. If Allen Factor is < General Characteristics a. typically between 2 nonmetals b. occur from an overlapping of orbitals where shared electrons are attracted by both nuclei c. very strong bonds. TUTORIAL: Explanation of Covalent Bonding 3. Lewis Structures- Diagrams that show how the valence electrons are arranged among the atoms in the molecule showing how the atoms bond. a. The valence electrons about an atom in a molecule as either: 1. Shared electrons (bonded electrons) 2. Unshared electrons (lone pair electrons) b. Guidelines for writing Lewis Structures. 1.Draw a simple structural formula using single covalent bonds. The least electronegative atom serves as the central atom. ** Never use Hydrogen as the central atom** 2. Determine the total number of valence electrons in the molecule or ion. a. For molecules- sum of the valence electrons on each atom b. For cations- sum of the valence electrons on each atom the charge of the cation c. For anions- sum of the valence electrons on each atom + the charge of the anion 3.Deduct 2 valence electrons for each bond from 2., then distribute the remaining about each atom as lone pairs to fill the octet (8). If too few electrons exist, convert single bonds to multiple bonds. Most multiple bonds occur between Carbon(s), Oxygen(s), Nitrogen(s), Sulfur, & Phosphorus. TUTORIAL: Explanation of Lewis Structures 4. Formal Charge- The hypothetical charge assigned to each atom in a Lewis structure a. Rules for calculating Formal Charge 1. One half of the electrons in a bond are assigned to each atom. (1 each for single, 2 each for double, etc.) 2. Both electrons of an unshared pair are assigned to the their respective atom. b. Formal Charge = the number of valence electrons in the free atomic state minus the number of assigned electrons of the atom in a molecule. 1. the sum of the formal charges of a molecule should equal zero, while the sum of the formal charges of an ion should equal the charge of the ion. 2. formal charges are used to predict structural formulas for varying Lewis structures. Page 1

2 c. Rules for predicting Lewis Structures using Formal Charges 1. A Lewis structure in which all formal charges in a molecule are zero is preferable to one in which some formal charges are not zero. 2. If Lewis structures must have nonzero formal charges, the one with the lowest number of nonzero formal charges is preferable, and a Lewis structure with no more than one large formal charge (-2,+2,etc.) is preferable to one with several 3. Lewis structures should have adjacent formal charges of zero or have opposite sign. 4. When we must choose among several Lewis structures with similar distributions of formal charges, the structure with the negative formal charges on the more electronegative atoms is preferable. REVIEW: Comprehensive List of the Rules for Writing Lewis Structures ASSIGNMENT 1: Lewis Structure Worksheet. B. Molecular Formulas- Identifies the elements and bonds in a molecule 1.Writing & Naming molecular formulasa. Write the least electronegative atom first b. Use prefixes to designate the ratio of atoms in a molecule 1- mono 6- hexa 11-undeca 22-Docosa (etc.) 2- di 7- hepta 12-dodeca 30-Triacota 3- tri 8- octa 13-trideca (etc.) 31-Hentriacota 4- tetra 9- nona 20-eicosa 40-Tetracota (etc.) 5- penta 10- deca 21-heneicosa (etc.) 100- Hecta The italicized prefixes are rarely, if ever, used in non-organic covalent molecules. c. change second name ending to (-ide) PRACTICE: Naming compounds. Three practice pages; Chapter 3. a, b, & c. PRACTICE: Naming Compounds. Click on Names & Formulas, then click on Naming Compounds 2. Diatomic molecules- Those atoms that when in elemental state, exist as diatomic molecules. ex. (the "super seven"): H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2 REVIEW TUTORIAL: Quiz over Interpreting Chemical Formulas. ASSIGNMENT 2: Covalent Compound Nomenclature: Print off questions and write the answers for each. 3. Types of Covalent Bonds a. Single covalent- sharing of 1 pair of electrons between bonding nuclei b. Double covalent- sharing of 2 pairs of electrons between bonding nuclei c. Triple covalent- sharing of 3 pairs of electrons between bonding nuclei 4. Coordinate Covalent Bonds- A covalent bond where 1 atom provides both bonding electrons- Can be drawn as an arrow. 5. Resonance- A situation where two or more Lewis structures for a molecule can be written which only differ in the arrangement of electrons (shared and unshared) ASSIGNMENT 3: Resonance Worksheet: EXPLANATION: Bonding Analogies. An interesting explanation to covalent bonding principles. 6. Exceptions to the Octet Rule for Covalent Molecules a. Magnetic charges- due to unpaired electrons. Electron spin creates a magnetic field Page 2

3 1. Diamagnetic Compounds- compounds that have no unpaired electrons. These compounds are not affected by a magnetic field 2. Paramagnetic Compounds- compounds that have unpaired electrons therefore are affected by magnetic fields. Picture: Liquid oxygen exhibiting paramagnetic properties. b. Octet Expansion & Contraction 1. Elements in 3 rd or higher energy level can obtain more than 8 valence electrons in the valence shell of the central atom, due to d-orbital hybridization. 2. Boron and Aluminum can exhibit configurations with less than 8 valence electrons. 7. Correlating Oxidation Numbers in Ionic and Covalent Bonds a. The oxidation number of any atom in its elemental form is Zero b. The oxidation number of a monatomic ion is considered equal to the charge on the ion c. The oxidation number of Fluorine is always 1. d. Alkali Metals have an oxidation number of +1 e. Alkaline Earth Metals have an oxidation number of +2 f. Halogens have oxidation numbers of 1 when paired with less electronegative atoms g. Oxygen s oxidation number is usually 2 except: 1. When bonded with Fluorine, the oxidation number is positive. Ex. OF 2, oxygen is O In peroxides (O 2-2 ) the oxidation number is 1 3. In superoxides (O 2-1 ), the oxidation number is ½ Ex. KO 2. h. Hydrogen is assigned a 1 with less electronegative atoms and +1 with greater electronegative atoms i. The sum of the oxidation numbers of all the atoms in a compound is zero. The sum of the oxidation numbers in an ion is equal to the charge of the ion. ASSIGNMENT 4: Naming Compounds Worksheet READING ASSIGNMENT 2: Read p Answer Review Questions 13-19, 23, 24, & 26. Also, from the concept map on page 269, using two concepts, write one paragraph (5-7 complete sentences) explaining the relationship between the two. 8. Valence Shell Electron Pair Repulsion Theory- VSEPR -Used to predict geometric shape of a molecule due to the repulsion of bonded and lone pair electrons INTRODUCTION: TUTORIAL over VSEPR. a. Valence Bond Theory- Describes covalent bonds as the overlapping of orbitals which produces a mutual attraction on bonded electrons by adjacent nuclei 1. Sigma Bond (σ)- covalent bond where electron density is concentrated in the region along the inter-nuclear axis. Occurs between s/s orbitals, s/p orbitals, and directional p/p bonds 2. Pi Bond (π)- covalent bond resulting from the side-by-side bonding of p orbitals. The amount of overlap is less than a sigma bond; therefore the pi bond is weaker. 3. Multiple bondsa. single- sigma b. double- sigma & 1 pi c. triple- sigma & 2 pi bonds VSEPR Table and examples, Another view of molecular shapes (contains better pictures) b. Hybridization- Hybrid orbitals are created from the mixing of atomic orbitals that are covalently bonded Page 3

4 1. Facts about hybridization a. Hybrid orbitals are formed only in covalently bonded atoms b. Hybrid orbitals have different shapes & orientations than the atomic orbitals c. All orbitals in a set of hybrid orbitals are equivalent and form identical bonds d. The number of Hybrid orbitals is equal to the number of atomic valence orbitals that are used to form the hybrid orbitals e. The type of Hybrid orbitals on a bonded atom depends on the geometry of its unsharedelectron pairs and bonded atoms. f. Sigma bonds occur from the overlapping of hybridized orbitals and pi bonds occur from theoverlap of unhybridized orbitals 2. Types of hybridization a. sp 3-1 s and 3 p orbitals producing 4 hybrid orbitals -producing a tetrahedral arrangement b. sp 2-1 s and 2 p orbitals producing 3 trigonal planar hybridized orbitals and 1 unhybridized p c. sp- 1 s and 2 p orbitals producing 2 linear hybridized orbitals and 2 unhybridized orbitals d. others- for those of the octet exceptions, due to hybridization of n=3 (or higher) d-type orbitals 1.sp 3 d- (1 s, 3 p, & 1 d) producing trigonal bipyramidal shape 2.sp 3 d 2 - (1 s, 3 p, & 2 d) producing octahedral shape TUTORIAL: Hybrid Orbitals c. Molecular Structure- 3 dimensional arrangement of atoms 1. Bond distance (radii)- the distance between 2 bonded nuclei -measured in pm (10-12 m) or (10-10 m) 2. Bond angle- the angle produced between 2 bonded that share a common atom Using VSEPR for predicting shapes AX m (LP) n : A represents the central atom X peripheral (bonded) atoms. m is the number of peripherally bonded atoms LP Lone pairs about the central atom. n is the number of lone pairs -To predict the geometry you must 1 st draw proper Lewis structures PRACTICE: Review and Practice on VSEPR PRACTICE: VESPR Calculator. Click on Exercises on the menu 9. Polar Covalent Bonds- Covalent bonds that result in partially charged positive ends and partially charged negative ends due to differences in electronegativities. -Nonpolar Covalent Bond- No net partial charges due to similar electronegativities between bonded atoms a. Electronegativity - a measure of the attraction on an electron by an atom in a chemical bond electronegativity = e.n. 1 e.n. 2 gives a rough estimate of polarity of a bond Electronegativity Differences and Bond Types e.n Bond Type Nonpolar covalent Polar covalent a Strongly polar covalent > 2.0 a Ionic a -If the compound is made from a metal/nonmetal and e.n > 1.6 then it's ionic Additional Information on predicting bond polarity b. Dipolarity - Polarity in molecules (electronic lopsidedness) is dependent upon bonding and geometry Page 4

5 1. Dipole moment- (DeBye dipole moment µ D ) found by placing a molecule into an electric field. -A polar molecule will align itself to the polarity of the field - µ D = 0 for nonpolar molecules - µ D > 0 for polar molecules Using VESPR and Structural Formulas to predict Polarity Examples: Identifying Dipole moments Molecule Bond Polarity of molecule CO 2 Polar Nonpolar NH 3 Polar Polar H 2 O Polar Polar O 3 Nonpolar Polar 2. Bond dissociation energy- the energy required to break a single bond between two atoms. (typically calculated when molecule is in gaseous state). - Described as the enthalpy change of the endothermic reaction. Ex. XY (g) X (g) + Y (g) D X-Y = H (D X-Y is the bond energy) Kilojoule = 1000 joules, 1 cal = joules - The strength of a bond between two atoms increases as the number of electron pairs in the bond increases. 3. Comparison of Covalent vs. Ionic Compounds Covalent Ionic a. Are composed of discrete particles called a. Are composed of (+) and (-) ions stacked aroundeach molecules. The molecules are made up of atoms held together by covalent bonds (shared electrons) other. The ions are held together by ionicbonds (electrostatic attractions). b. May exist as solids, liquids, or gases b. Exist as brittle, crystalline solids at room temps. c. Electronegativity differences are small c. Electronegativity differences are high d. Composed of 2 non-metals d. Binary compounds are formed from the reaction of a metal and a non-metal. e. Contain directional bonds and have definite bond angles f. Usually have lower melting and boiling points Ternary and higher compounds are formed from polyatomic ions e.contain no directional bonds. Ionic compounds stacks of ions f. Exhibit high melting and boiling points ASSIGNMENT 5 : Identify Ionic vs. Covalent Compounds. Print off the questions and include the names for all of the compounds found in the answers. ASSIGNMENT 6: Ch. 16 Review Questions: p470; 29, 32, 35, 36, 40, 41, 42, 45, 47, 48, 50, 54, & 68 Review the Unit by taking one of several quizzes Page 5