Chapter 8 Chemical Bonding

Transcription

1 Chapter 8 Chemical Bonding Types of Bonds Ionic Bonding Covalent Bonding Shapes of Molecules 8-1 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Table 8.1 Two Carbon Compounds Compound Formula Calcium Carbonate CaCO 3 Carbon Dioxide CO 2 Physical State Molar Mass (g/mol) Density (g/ml) Melting Point ( C) Boiling Point ( C) Electrical Conductivity as a Liquid Dissolves in White solid (at high P) Decomposes High Acids Colorless gas (at 5.11 atm) Sublimes at 78.6 Very low H 2 O, CCl Table 8.2 General Properties of Ionic and Covalent Compounds Ionic Crystalline solids Hard, brittle solids Very high melting point Very high boiling point Good electrical conductor when molten or in solution Often soluble in water but not in carbon tetrachloride Covalent Gases, liquids, or solids Weak, brittle solids or soft and waxy solids Low melting point Low boiling point Poor conductor of electricity and heat Often soluble in carbon tetrachloride but not in water 8-3 1

2 Chemical Bonds Chemical bond A force that holds atoms together in a molecule or compound Two types of chemical bonds Ionic Bonds Covalent Bonds Figure Ionic Bond A bond created by electrostatic attraction between oppositely charged ions Occurs between a metal and a nonmetal Electrons transferred between the cation (positively charged ion) and the anion (negatively charged ion) Extremely strong bonds 8-5 Figure 8.2 or another molecule picture Covalent Bonds A bond created by the sharing of electrons between atoms Occurs between two nonmetals (resulting in a neutral overall charge) Electrons not transferred in this case Electrons shared in pairs typically Weaker bonds than ionic bonds 8-6 2

3 Practice Identifying Types of Bonding Identify the type of bonding in each of the following substances: 1. af 2. ClO 2 3. FeSO 4 4. SO 2 5. Ca(ClO 2 ) Practice Solutions Identifying Types of Bonding Identify the type of bonding in each of the following substances: 1. af Ionic bonding (metal + nonmetal) 2. ClO 2 Covalent bonding (2 nonmetals) 3. FeSO 4 Ionic bonding between the metal and nonmetal; Covalent bonding between the nonmetals in the polyatomic ion 4. SO 2 Covalent bonding 5. Ca(ClO 2 ) 2 - Ionic bonding between the metal and nonmetal; Covalent bonding between the nonmetals in the polyatomic ion 8-8 Polar vs. onpolar Two general types of covalent bonds: Polar covalent Unequal sharing (or a partial transfer) of electrons Occurs when different elements are covalently bonded to one another Why different elements?»because different elements have different electronegativities onpolar covalent Equal sharing (no transfer) of electrons Occurs only when all of the atoms in a molecule belong to the same element 8-9 3

4 Polar vs. onpolar (Figure 8.4) Bonds Ionic Covalent Polar onpolar Complete Transfer of Electrons Increasing electron transfer o Transfer of Electrons o Sharing of Electrons Increasing equality of sharing Equal Sharing of Electrons 8-10 Polar vs. onpolar Polar covalent bonds are: Typically shorter bonds Stronger bonds due to their increased ionic character onpolar covalent bonds are: Typically longer bonds Weaker bonds Polarity Occurs in polar covalent molecules Polarity is the degree of transfer of electrons in a covalently bonded molecule composed of different element s atoms Electronegativity Ability of an atom to attract bonding electrons Proposed by Linus Pauling in the early 1930 s A difference in electronegativity between the atoms in a covalent bond results in: A polar covalent bond Increased ionic character The greater the difference in electronegativity, the greater the ionic character and the more polar the bond that joins the atoms. Decreased bond length and increased bond strength o difference in electronegativity between atoms in a covalent bond results in a nonpolar covalent bond

5 Electronegativity Figure Trends in Electronegativity Figure Practice Polar Bonds Which of the following molecules have polar bonds? If a bond is polar, which atom has a partial negative charge? 1. SO PH 3 4. CCl 4 5. O

6 Practice Solutions Polar Bonds Which of the following molecules have polar bonds? If a bond is polar, which atom has partial negative charge? 1. SO 2 Polar covalent bonds O is more electronegative and has a partial negative charge 2. 2 onpolar covalent bonds 3. PH 3 Polar covalent bonds P is more electronegative and has a partial negative charge 4. CCl 4 Polar covalent bonds Cl is more electronegative and has a partial negative charge 5. O 3 onpolar covalent bonds 8-16 Ionic Bonding Formation of ions and ionic bonds relates to an element s electron configuration. Each element immediately following a noble gas is a metal. Metals lose electrons, forming a positive charge, to become cations. Each element immediately preceding a noble gas is a nonmetal. onmetals gain electrons, forming a negative charge, to become anions. Therefore, elements (main-group) either lose or gain electrons to become isoelectronic with a noble gas (i.e. have the same electron configuration) Ionic Bonding

7 Lewis Dot Symbols Lewis Dot symbol Electron dot symbol Dots placed around an element s symbol represent valence electrons Pair electrons as needed Octet rule Tendency of an atom to achieve an electron configuration having 8 valence electrons Same as the electron configuration of a noble gas The 8 electrons exist in 4 pairs Ions achieve 8 electrons by losing or gaining electrons 8-19 Practice Lewis Symbols for Ions Write the Lewis symbols for the beryllium and nitrogen ions. Then write a formula for the compound that would form between them, using their Lewis symbols Practice Solutions Lewis Symbols for Ions Write the Lewis symbols for the beryllium and nitrogen ions. Then write a formula for the compound that would form between them, using their Lewis symbols. Lewis Symbols for beryllium and nitrogen. Be Be Be Be 2 Be 2 Lewis Structures for beryllium and nitrogen ions Compound formula Be

8 Structures of Ionic Crystals Crystal lattice The pattern obtained when an ion, represented as a charged sphere, exerts a force equally in all directions. Thus, ions of equal and opposite charge surround it. Cations and anions must come into contact for a crystal lattice to form. Figure Structures of Ionic Crystals Ionic crystal Ions are arranged in a regular geometric pattern that maximizes the attractive forces and minimizes the repulsive forces. Hard and brittle Can shatter if struck forcefully The charges and sizes of ions largely determine the characteristic patterns of ionic crystals 8-23 Structures of Ionic Crystals

9 Octet Rule Octet rule Tendency of an atom to achieve an electron configuration having 8 valence electrons Same as the electron configuration of a noble gas Covalently bonded atoms achieve 8 valence electrons by sharing electrons The 8 electrons exist in 4 pairs H reacts to obtain a total of 2 electrons like He Covalent Bonding Single covalent bond A covalent bond that consists of a pair of electrons shared by two atoms Each atom contributes one electron to the bond The orbitals overlap to allow the electron pair to be located around both atoms Lewis formula The atoms are shown separately and the valence electrons are represented by dots Figure Covalent Bonding Multiple covalent bonds Covalent bonds that consist of more than one pair of electrons shared by two atoms Double bond Sharing of two pairs of electrons (4 electrons total) In Lewis Dot structures, a double bond is represented by 4 dots or 2 parallel lines. Triple bond Sharing of three pairs of electrons (6 electrons total) In Lewis Dot structures, a triple bond is represented by 6 dots or 3 parallel lines. Figure

10 Practice Lewis Formulas Determine the formula of a simple compound that follows the octet rule and is formed from nitrogen and fluorine atoms. Use electron dot structures to describe the bonding in this compound. Figure Practice Solutions Lewis Formulas Determine the formula of a simple compound that follows the octet rule and is formed from nitrogen and fluorine atoms. Use electron dot structures to describe the bonding in this compound. Lewis Symbols for nitrogen and fluorine. F F F F F F F F F F 8-29 Steps for Writing Lewis Dot Structures 1. Write an atomic skeleton. The arrangement of atoms is usually symmetrical. In a molecule of two different elements, the one with the greater number of atoms usually surrounds the one with the lesser number of atoms. The central atom, the one surrounded by the other atoms, tends to be the one that is less electronegative and is present in the least quantity. This atom usually forms the greater number of bonds and is found further toward the bottom left side of the periodic table. Hydrogen atoms are generally on the outside of the molecule. The chemical formula may give clues about the arrangement of atoms

11 Steps for Writing Lewis Dot Structures (Cont d) 2. Sum the valence electrons from each atom to get the total number of valence electrons. 3. Place two electrons, a single bond, between each pair of bonded atoms. 4. If you have not placed all the valence electrons in the formula, add any remaining electrons as unshared electron pairs, consistent with the octet rule. Add pairs of electrons first to complete the octet of atoms surrounding the central atom. Then add any remaining electrons in pairs to the central atom Steps for Writing Lewis Dot Structures (Cont d) 5. If necessary to satisfy the octet rule, shift unshared electrons from nonbonded position on atoms with completed octets to positions between atoms to make double or triple bonds Writing Lewis Dot Structures Write a Lewis formula for the formaldehyde, CH 2 O, molecule. CH 2 O Write a Lewis formula for cyanic acid. HC

12 Writing Lewis Dot Structures 1. Write an atomic skeleton for CH 2 O: O H C H 2. Sum the valence electrons from each atom to get the total number of valence electrons. Carbon is in Group IVA (14), so it has 4 valence electrons. Each hydrogen contributes 1 valence electron (H is in Group IA (1)). Oxygen contributes 6 valence electrons because it is in Group VIA (16). Total number of valence electrons = 4 + (1 x 2) + 6 = Writing Lewis Dot Structures 3. ext, bond the electrons around each atom in a single bond first, then use double bonds as necessary. O H C H H H C O H H C O 8-35 Writing Lewis Dot Structures 1. Write an atomic skeleton for HC: H C 2. Sum the valence electrons from each atom to get the total number of valence electrons. Carbon is in Group IVA (14), so it has 4 valence electrons. Hydrogen contributes 1 valence electron (H is in Group IA (1)). itrogen contributes 5 valence electrons because it is in Group VA (15). Total number of valence electrons = =

13 Writing Lewis Dot Structures 3. ext, bond the electrons around each atom in a single bond first, then use double and triple bonds as necessary. H C H C H C H C 8-37 Resonance Structures When there are several equally valid arrangements of bonding (i.e. Lewis Dot structures), then the concept of resonance helps explain why. Resonance The electron arrangement in molecules with several equally valid Lewis Dot structures is represented by them all, each showing a different arrangement of the true arrangement of electrons. Resonance hybrid Representation of the actual molecule A composite of the formulas drawn 8-38 Practice - Resonance Structures 1. Write an atomic skeleton for 2 O: O 2. Sum the valence electrons from each atom to get the total number of valence electrons. itrogen is in Group VA (15), so it has 5 valence electrons. Oxygen contributes 6 valence electrons because it is in Group VIA (16). Total number of valence electrons = (5 x 2) + 6 =

14 Practice - Resonance Structures 3. ext, bond the electrons around each atom in a single bond first, then use double bonds as necessary. O O O 8-40 Practice - Resonance Structures 4. To draw resonance structures, rearrange the electrons (and bonds) in the structures. O O 8-41 Exceptions to the Octet Rule Incomplete octets The central atom has less than eight electrons around it. Ex. BH 3 Expanded octets The central atom has greater than eight electrons around it. Ex. PH 5, SF 6 Odd-numbered Lewis Dot structures The total number of electrons is odd. The central atom has an odd number of electrons around it. Ex. O, O 2, ClO

15 Carbon Compounds Carbon has: Four valence electrons The ability to form four bonds The ability to catenate, i.e. bond to itself Very strong bonds when bonded to itself Carbon molecules are ubiquitous in nature Hydrocarbons Figure Hydrocarbons Compounds containing hydrogen and carbon Aliphatic hydrocarbons A class in which the bonds are all localized single, double, and triple bonds Alkanes Hydrocarbons which contain only carbon-carbon single bonds Alkenes Hydrocarbons which contain at least one carboncarbon double bond Alkynes Hydrocarbons which contain at least one carboncarbon triple bond

16 Hydrocarbons Aromatic hydrocarbons A class of hydrocarbons which has carbon atoms arranged in a six-atom ring with alternating single and double bonds Delocalized structures Figure Functional Groups Functional group A group that is introduced into or substituted in a hydrocarbon chain Gives the hydrocarbon its characteristic properties The group has a heteroatom, an atom other C and H Typically O, S, and Alcohol A hydroxyl group (-OH) replaces a hydrogen atom in the formula for a hydrocarbon Figure Table 8.4 Functional Groups in Hydrocarbons Class Alcohol Functional Group -OH Example Ethyl alcohol Formula C 2 H 5 -OH Ether -O- Diethyl ether H 5 C 2 -O-C 2 H 5 Aldehyde -C-H Acetaldehyde Ketone -C- Acetone Carboxylic Acid -C-OH Acetic acid Ester -C-O- Ethyl acetate Amine -- Methyl amine H 3 C-H

17 Odors and Carbon Compounds 8-49 Shapes of Molecules The relative locations of electron pairs around a central atom play a large role in determining a molecule s 3-D shape. egatively charged electrons repel one another, so electron pairs in different orbitals stay as far apart as possible Shapes of Molecules Valence shell electron pair repulsion (VSEPR) theory The tendency of electron pairs to adjust the orientation of their orbitals to maximize the distance between them The bonded atoms and unshared pairs are arranged around the central atom as far apart as possible Bond angle A shape is characterized by a bond angle between the central atom and the atoms bonded to it

18 VSEPR Parent Structures Table VSEPR Derivative Structures 8-53 Steps for VSEPR Structures 1. Draw a Lewis formula. 2. Count the number of atoms bonded to the central atom, and count unshared pairs on the central atom. 3. Add the number of atoms and the number of unshared electron pairs around the central atom. The total indicates the parent structure. 4. The molecular shape is derived from the parent shape by considering only the positions in the structure occupied by bonded atoms

19 Practice VSEPR Structures What is the shape of the nitrite ion (O 2-1 )? What is the O--O bond angle? 8-55 Practice Solutions VSEPR Structures 1. Draw the Lewis Dot Structure. O O O 2. Count the number of atoms bonded to the central atom and count unshared pairs on the central atom. O, which is the central atom in this case, has 2 atoms bonded to it, and 1 unshared pair on it Practice Solutions VSEPR Structures 3. If we add the number of atoms and unshared pairs around the central atom, we get the number 3. This indicates that the parent structure is trigonal planar. B B A 120 B

20 Practice Solutions VSEPR Structures 4. The derived structure, which only considers bonded atoms, is called bent (or angular) and has a bond angle equal to 118, since we take 2 off the parent structure s bond angle to obtain the derived structure s bond angle. O 118 O 8-58 atural Applications of VSEPR Theory Molecular shapes are important in living systems. Glycine molecules are typically found in proteins or gelatins. Figure atural Applications of VSEPR Theory Heme molecule Oxygen is carried throughout the body via red blood cells containing heme molecules. Histidine, an amino acid in the heme molecule, just fits into the space next to the oxygen molecule

21 Polarity of Molecules Diatomic molecules Polarity lies along the plane of the bond Polyatomic molecules A nonpolar molecule is one that has all nonpolar bonds or one that has polar bonds that cancel out Bonds that cancel out have equal polarities in opposite directions This happens when:» A central atom has no unshared electrons» The atoms around the central atom all have the same electronegativity A polar molecule is one that has polar bonds that DO OT cancel out 8-61 Polarity of Molecules Figure Practice Polarity of Molecules Predict whether C 2 H 6, O 2, CO 2, SO 2, and SO 3 are polar or nonpolar molecules

22 Practice Solutions Polarity of Molecules Predict whether C 2 H 6, O 2, CO 2, SO 2, and SO 3 are polar or nonpolar molecules. C 2 H 6 tetrahedral at each C nonpolar O 2 bent polar CO 2 linear nonpolar SO 2 bent polar SO 3 trigonal planar nonpolar 8-64 Like Dissolves Like Ionic salts and polar liquids dissolve better in polar liquids than in nonpolar liquids onpolar liquids dissolve better in other nonpolar liquids than in polar liquids Figure