AP CHEMISTRY CHAPTER 8 BONDING 8 1 Types

AP CHEMISTRY CHAPTER 8 BONDING

8. 1 Types of Chemical Bonds To study chemical bonds, we can look at physical or chemical properties like melting point, hardness, conductivity, solubility and bond energy- energy required to break a chemical bond we can measure bond energy to determine the strength of interaction between particles

Ionic compound- product when a metal reacts with a nonmetal

Bond Length Bonding occurs as atoms try to achieve their lowest possible energy state. Bond length is the distance where the energy is at a minimum. When two like atoms form a bond, there is a balance among proton-proton repulsion, electron-electron repulsion, and proton-electron attraction.

Bonding in Hydrogen Gas In H 2, the two e- will usually be found between the two H atoms because they are spontaneously attracted to both protons. Therefore, electrons are shared by both nuclei. This is called covalent bonding. + H F Polar covalent bonds occur when electrons are not shared equally. One end of the molecule may have a partial charge. This is called a dipole.

8. 2 Electronegativity- the ability of an atom in a molecule to attract shared electrons to itself Electronegativity generally increases across a period and decreases down a group. It ranges from 0. 79 for cesium to 4. 0 for fluorine.

Periodic Trends. Electronegativity generally increases across a period and decreases down a group. It ranges from 0. 79 for cesium to 4. 0 for fluorine.

Predict the polarity of the following. polar covalent due to a dipole moment Nonpolar covalent CH 4 H-F Polar covalent Nonpolar covalent H-H SO 3 NH 3 Nonpolar covalent polar with a dipole moment H 2 O

8. 3 Bond Polarity and Dipoles are affected by Moments As stated earlier, particles may have a positive area and a negative area due to unequal sharing of electrons. These particles are said to be dipolar or to have a dipole moment. This is represented by an arrow with a positive end a negative end. magnetic fields.

Dipole Moments Dipole moments may cancel themselves out, as in the case of CO 2 and CCl 4. Dipole moments can be shown by looking at differences in electronegativity of the bonding pairs, or by examining an electrostatic potential diagram.

8. 4 Ions: Electron Configurations and Sizes Stable compounds usually have atoms with noble gas electron configurations. Two nonmetals react to form a covalent bond by sharing electrons to obtain noble gas electron configurations.

When a nonmetal and a group A metal react to form a binary ionic compound, the ions form so that the valence electron configuration of the nonmetal is completed and the valence orbitals of the metal are emptied to give both noble gas configurations.

Ions form to get noble gas configurations. -exceptions in Group A metals: Sn 2+ & Sn 4+ Pb 2+ & Pb 4+ Bi 3+ & Bi 5+ Tl+ & Tl 3+ Metals with d electrons will lose their highest numerical energy level electrons before losing their inner d electrons. Example: Mn [Ar] 4 s 23 d 5 Mn 3+ [Ar] 3 d 4

Size of Ions Positive ions (cations) are smaller than their parent atoms since they are losing electrons. Negative ions (anions) are larger than their parent atoms since they are gaining electrons. Ion size increases going down a group.

Isoelectronic ions Ions that contain the same number of electrons. O 2 -, F-, Na+, Mg 2+, Al 3+ all have the electron configuration of neon. They are isoelectronic. For an isoelectronic series, size decreases as Z increases.

8. 5 Energy Effects in Binary Ionic Compounds Lattice energy The change in energy that takes place when separated gaseous ions are packed together to form an ionic solid. Na+(g) + Cl-(g) Na. Cl(s) If exothermic, the sign will be negative and the ionic solid will be the stable form. We can use a variety of steps to determine the heat of formation of an ionic solid from its elements. This is called the Born-Haber cycle.

The Born-Haber Process for Na. Cl Total LE = 107 + 502 + 121 + -355 + 411 = 786 KJ

The Structure of Lithium Fluoride

Lattice Energy Lattice energy can be calculated using the following: where k is a proportionality constant that depends on the structure of the solid and the electron configuration of the ions. Q 1 & Q 2 are the charges on the ions. r is the distance between the center of the cation and the anion.

Lattice Energy Since the ions will have opposite charges, lattice energy will be negative (exothermic). The attractive force between a pair of oppositely charged ions increases with increased charge on the ions or with decreased ionic sizes. There is far more energy released when Mg 2+ and O 2 - combine than when Na+ and F- combine.

Three Possible Types of Bonds Covalent Pol cov ar ale nt Io c i n

8. 6 Partial Ionic Character of Covalent Bonds There are probably no totally ionic bonds. Percent ionic character in binary compounds can be calculated. Percent ionic character increases with electronegativity difference. Compounds with more than 50% ionic character are considered to be ionic (electronegativity difference of about 1. 9).

Bonding Polyatomic ions are held together by covalent bonds. We call Na 2 SO 4 ionic even though it has 4 covalent bonds and 2 ionic bonds. Ionic compound- any solid that conducts an electrical current when melted or dissolved in water.

8. 7 The Covalent Chemical Bond: A Model A chemical bond is a model “invented” by scientists to explain stability of compounds. A bond really represents an amount of energy. The bonding model helps us understand describe molecular structure. It is supported by much research data. To fully understand a model, we must understand the data that prompted its description. However, models are not always correct and we can learn from their faults.

8. 8 Covalent Bond Energies and Chemical Reactions Single bond- one pair of shared electrons; long bond length Double bond- two pair of shared electrons; intermediate bond length Triple bond- three pair of shared electrons; short bond length

We can use bond energies to calculate heats of reaction. H = D(bonds broken)- D(bonds formed) 2 H 2 + O 2 2 H 2 O Ex. H = [2(432) + 495] –[4(467)] = -509 k. J 2 H-H O=O 4 H-O exothermic

Practice! Use the table of bond energies to find the ΔH of the following reaction. CH 4(g) + 2 Cl 2(g) + 2 F 2(g) CF 2 Cl 2(g) + 2 HF(g) + 2 HCl(g) Bond H (k. J/mol) C-H 413 C-Cl 339 Cl-Cl 239 H-F 565 F-F 154 H-Cl 427 C-F 485 H = D(bonds broken)- D(bonds formed) Broken = [4(413) + 2(239) + 2(154)] = 2438 k. J Formed = [2(485) + 2(339) + 2(565) + 2(427)] 2 C-F 2 C-Cl 2 H-F 2 H-Cl = 3632 H = 2438 k. J – 3632 k. J = -1194 k. J

8. 9 Bonding Models Molecular Orbital Model electrons occupy orbitals in a molecule in much the same way as they occupy orbitals in atoms electrons do not belong to any one atom very complex

Localized electron model molecules are composed of atoms that are bound together by sharing pairs of electrons using the atomic orbitals of the bound atoms traditional model lone pair- pair of electrons localized on an atom (nonbonding) shared pair or bonding pairelectrons found in the space between atoms

Lewis structure -shows how the valence electrons are arranged among the atoms in the molecule To draw a Lewis structure: 1. Count all valence electrons and add them together. 2. Draw a single bond between bonded atoms. 3. Fill in valence shells to fit duet or octet rules. 4. Place any extra electrons on the central atom.

Lewis Structures The most important requirement for the formation of a stable compound is that the atoms achieve noble gas configurations ionic-only valence electrons are included molecular H 2 O

duet rule -hydrogen forms stable molecules when it shares two electrons H: H -filled valence shell octet rule -most elements need 8 electrons to complete their valence shell Why does He not form bonds? Its valence orbitals are already filled.

Ex. H 2 S # of valence electrons: 1 + 6 = 8

Ex. CO 2 # of valence electrons: 4 + 6 = 16

Ex. HCN # of valence electrons: 1 + 4 +5 = 10

Ex. NH 3 # of valence electrons: 5 + 1 +1 = 8

Ex. NO+ # of valence electrons: 5 + 6 - 1 = 10

Ex. CO 32 - # of valence electrons: 4 + 18 + 2 = 24

8. 11 Exceptions to the Octet Rule Boron and beryllium tend to form compounds where the B or Be atom have fewer than 8 electrons around them. Ex. BF 3 Common AP equation: NH 3 + BF 3 H 3 NBF 3 C, N, O, F always obey the octet rule.

Elements that Exceed the Octet Rule Some elements in Period 3 and beyond exceed the octet rule. Ex. SF 6 S has 12 e- around it. d orbitals are used to accommodate the extra electrons. Elements in the 1 st or 2 nd period of the table can’t exceed the octet rule because there is no d sublevel. If the octet rule can be exceeded, the extra electrons are placed on the central atom. See examples of exceptions on pg 372. Ex. I 3 -, Cl. F 3, Rn. Cl 2

8. 12 Resonance occurs when more than one valid Lewis structure can be written for a particular molecule actual structure is an average of all resonance structures this concept is needed to bit the localized electron model (electrons are really delocalized) Ex. Benzene, C 6 H 6 All bond lengths and angles are the same. Ex. SO 3

Ex. Benzene, C 6 H 6 All bond lengths and angles are the same.

Ex. SO 3

Formal Charge Formal charge is the charge on an atom in a compound that is used to determine the most accurate Lewis structure. It is the difference between the # of valence electrons on the free atom and the # of valence electrons assigned to the atom in the molecule atoms try to achieve formal charges as close to zero as possible any negative formal charges are expected to reside on the most electronegative atoms Sum of the formal charges must equal the overall charge on the molecule (zero) or ion.

Valence circled

Ex. SO 4 Ex. 2 - NCS

8. 13 Molecular Structure: The VSEPR Model Stands for Valence Shell Electron Pair Repulsion allows us to use electron dot structures to determine molecular shapes the structure around a given atom is determined primarily by minimizing electron repulsions bonding and nonbonding pairs of electrons around an atom position themselves as far apart as possible Steps: Draw Lewis structure 2. Count effective electron pairs on central atom (double and triple bonds count as one) 3. Arrange the electron pairs as far apart as possible 1.

VSEPR Structures for Molecules and Ions Effective Bonding Nonbonding VSEPR Electron Formula Pairs Approx. Bond Angles Hybrid- 3 -D ization Structure Examples 2 3 3 4 4 4 5 2 3 2 4 3 2 5 0 0 1 2 0 AX 2 AX 3 AX 2 E AX 4 AX 3 E AX 2 E 2 AX 5 180 o 120 o <120 o 109. 5 o <109. 5 o 90 o, 120 o sp sp 2 sp 3 d linear trigonal planar bent tetrahedral trigonal pyramidal bent trigonal bipyramidal CO 2, Be. Cl 2 BF 3, CO 32 Sn. Cl 2, Pb. Br 2 CH 4, Sn. Cl 4 NH 3, PCl 3, H 3 O+ H 2 O, OF 2, SCl 2 PCl 5, Nb. Cl 5 5 4 1 AX 4 E <90 o, <120 o sp 3 d see-saw SF 4, Te. Cl 4 5 5 6 6 6 3 2 6 5 4 2 3 0 1 2 AX 3 E 2 AX 2 E 3 AX 6 AX 5 E AX 4 E 2 <90 o 180 o 90 o <90 o sp 3 d 2 sp 3 d 2 T-shaped linear octahedral square pyramidal square planar Cl. F 3 ICl 2 -, I 3 -, Xe. F 2 SF 6 Br. F 5 ICl 4 -, Xe. F 4

AX 2 Shapes (A represents central atom, X represents attached atom, E represents unshared electron pair) X – A – X linear 180 o bond angle

AX 3 Shape is trigonal planar 120 o bond angle Any resonance structure can be used to determine shape.

AX 2 E Shape is bent Bond angle is < 120 o Ex. Sn. Cl 2

AX 4 Shape is tetrahedral Bond angle is 109. 5 o Ex. CH 4

Figure 8. 14 The Molecular Structure of Methane

AX 3 E Shape is trigonal pyramidal Bond angle is < 109. 5 o Ex. NH 3

AX 2 E 2 Shape is bent Bond angle is < 109. 5 o Unshared electron pairs repel more than shared pair. Lone pairs require more space than share pairs. Ex. H 2 O

Figure 8. 17 The Bond Angles in the CH 4, NH 3, and H 2 O Molecules

AX 5 Shape is trigonal bipyramidal Bond angles are 120 o(equatorial) and 90 o(axial) Ex. PCl 5

Happy Mole Day!!

AX 4 E Shape is see-saw Bond angles are <90 o and <120 o Ex. SF 4 34 electrons

AX 3 E 2 Shape is T-shaped Bond angle is <90 o Ex. Cl. F 3

AX 2 E 3 shape is linear bond angle is 180 o Ex. Xe. F 2

Three Possible Arrangements of the Electron Pairs in the I 3 - Ion

AX 6 Shape is octahedral Bond angle is 90 o Ex. SF 6

AX 5 E Shape is square pyramidal Bond angle is <90 o Ex. Br. F 5

AX 4 E 2 Shape is square planar. Bond angle is 90 o. animated vsepr table

Possible Electron Pair Arrangements for Xe. F 4

AP CHEMISTRY CHAPTER 9 BONDING

9. 1 Hybridization and the Localized Electron Model When drawing Lewis structures to explain bonding, we have been using the Localized Electron Model of bonding. This assumes that the electrons stay with (or close to) the atoms from which they originated and that bonds are formed by the overlap of atomic orbitals. This model needs to be developed a little further to explain experimental data.

In methane, CH 4, carbon has one 2 s and three 2 p orbitals available for bonding. Hydrogen has one 1 s orbital available. We could imagine that one of the bonds in methane would be formed from the overlap of a hydrogen 1 s orbital and a carbon 2 s orbital and the other three bonds would be formed from the overlap of a hydrogen 1 s orbital and a carbon 2 p orbital. This would make 2 different kinds of bonds. Bonding in Methane

Experimental data shows that all four bonds are identical, so how do we resolve this? If the bonds instead were an overlap of the hydrogen’s 1 s and a new kind of orbital from the carbon, then the bonds would all be equal in length. The “new” orbital from the carbon is created by utilizing carbon’s 2 s and all three of its 2 p orbitals. These orbitals are blended (hybridized) into a mixture that produces four orbitals that look a lot like p orbitals, with a hint of s.

These are called sp 3 orbitals, to indicate what they are made from. The hybrid sp 3 orbitals each overlap with the 1 s orbitals from each hydrogen atom and make four identical bonds having 109. 5 o bond angles. These overlapping single bonds are called sigma bonds (σ). Atoms with four effective electron pairs have sp 3 hybridization, even if they have unshared pairs as in H 2 O and NH 3. The electron pairs have a tetrahedral arrangement.

Atoms with four effective electron pairs have sp 3 hybridization, even if they have unshared pairs as in H 2 O and NH 3. The electron pairs have a tetrahedral arrangement.

2 sp Hybridization We can look at the hybridization in the bonding of ethene (C 2 H 4). These bonds are going to happen when a 1 s orbital from hydrogen overlaps with an orbital from carbon.

2 sp Hybridization This time, each carbon has to form three bonds, one with the other carbon, and two with hydrogen. We will discuss the double bond in a moment. This time, each carbon will use one s and two p orbitals to make these three bonds. This forms an sp 2 hybrid orbital. This type of hybridization results in trigonal planar electron pair arrangements and 120 o bond angles.

The overlap of the 1 s orbitals from each hydrogen with the sp 2 hybrid orbitals of carbon results in sigma bonds. In a sigma bond, the orbitals overlap head-on and the electrons are in the region between the two nuclei. All single bonds are sigma bonds. Each double bond and each triple bond contains one sigma bond.

The Double Bond Notice that when this occurs, one p orbital from each carbon is not used to form the hybrid orbitals. These p orbitals actually have one promoted electron in each that form areas of stability above and below the molecule. This is what makes up the double bond. A double bond is called a pi bond (π). Because the electrons in a pi bond are not directly between the nuclei, a pi bond is weaker than a sigma bond. A double bond contains one sigma plus one pi bond and is thus stronger and shorter than a single bond.

What happened to the p orbital that wasn’t hybridized? The remaining p orbitals from each carbon atom overlap above and below the plane of the nuclei resulting in a sideways overlap that is called a pi bond.

A pi bond can form only if there is also a sigma bond between the same two atoms. Because the electrons in a pi bond are not directly between the nuclei, a pi bond is weaker than a sigma bond. A double bond contains one sigma plus one pi bond and is thus stronger and shorter than a single bond.

Bonding in Ethyne Let’s look at the hybridization and bonding in ethyne (acetylene), C 2 H 2. In ethyne, each carbon is forming two bonds, one with hydrogen, one with the other carbon. We will discuss the triple bond in a moment. H-C C-H

sp Hybridization Carbon will utilize one s and one p to form these bonds, so the hybrid orbitals are called sp. Linear molecules and 180 o bond angles result from sp hybridization.

sp Hybridization The two p orbitals and their electrons interact around the C-C sigma bond above and below the plane of the molecule to form two pi bonds. This completes the triple bond. A triple bond is one sigma and two pi bonds and is thus stronger and shorter than either a single or a double bond.

Exceeding the Octet We learned that certain elements can exceed the octet rule by utilizing unfilled d orbitals. These d orbitals are also involved in hybridization. For example, PF 5 has five effective electron pairs. These five pairs require five equal bonds. We must throw one s, three p and 1 d orbital into the blender to get five dsp 3 (or sp 3 d) orbitals. This form of hybridization results in trigonal bipyramidal shapes and 90 o and 120 o bond angles.

With SF 6 we have six effective electron pairs. We must hybridize one s, three p, and two d orbitals to get six d 2 sp 3 or sp 3 d 2 orbitals. This results in the octahedral shape with 90 o bond angles.

Single bonds= 1 sigma bond Double bonds = 1 sigma bond + 1 pi bond Triple bonds = 1 sigma bond + 2 pi bonds

Practice: For each of the following, determine: a)hybridization of each atom b)bond angles c)molecular shape d)# of sigma bonds and # of pi bonds

Sb. Cl 5 a)hybridization of each atom- Sb-sp 3 d Cl-sp 3 b)bond angles- 90° and 120° c)molecular shape- trigonal bipyramidal d)# of sigma bonds and # of pi bonds- 5σ 0π Sb

NO 3 - N-sp 2 O-sp 3 or sp 2 a)hybridization of each atomb)bond angles-120° c)molecular shape- trigonal planar d)# of sigma bonds and # of pi bonds-3σ 1π

CHCl 3 a)hybridization of each atom- C-sp 3 Cl-sp 3 b)bond angles- 109. 5° c)molecular shape- tetrahedral d)# of sigma bonds and # of pi bonds-4σ 0π

a)hybridization of each atom-C sp or sp 2 N sp b)bond angles- 120 or 180 c)molecular shape- trigonal pyramidal or linear d)# of sigma bonds and # of pi bonds-9σ 9π : N C C=C C N:

9. 2 The Molecular Orbital Model (MO) The molecule is assumed to be a new entity consisting of positively charged nuclei and electrons. Molecular orbitals (MOs) are constructed from the valence orbitals of the atoms. (Some texts use core electrons also) Like atomic orbitals, MOs can hold two electrons

MOs fall into two classes: bonding & antibonding A bonding MO is lower in energy than the atomic orbitals of which it is composed. An antibonding MO is higher in energy than the atomic orbitals of which it is composed. In a bonding orbital, electrons have the greatest probability of being between the nuclei. In an antibonding orbital, electrons are mainly outside the space between the nuclei. We can have and π MOs. Orbitals are conserved. The number of MOs will be the same as the number of AOs used to construct them.

When H 2 is formed from two H atoms, its 1 s orbitals combine to form molecular orbitals as follows: H 2 MO configuration for H 2 = ( 1 s 2) Since both electrons are in the bonding MO, H 2 is very stable.

The nd 2 Energy Level

For diatomic molecules with valence electrons in the first energy level, the molecular orbitals are: * 1 s___ 1 s ____ For diatomic molecules with valence electrons in the second energy level, the molecular orbitals are: * 2 p ____ ____ * 2 s ____ There are 2 pi orbitals, so 2 p and * 2 p can each hold 4 electrons.

Bond order- an indication of bond strength. Bond order = # bonding electrons- # antibonding electrons 2 For C 2 = (6 -2)/2 = 2 A bond order equal to zero means that the molecule has equal numbers of electrons in bonding MOs and in antibonding MOs. Such a molecule would be no more stable than separate atoms, so it would not exist. The greater the bond order, the more stable we predict the molecule or ion to be. As bond order increases, the bond energy increases and the bond length decreases.

Molecular Orbital Summary of Second Row Diatomics

PARAMAGNETISM AND DIAMAGNETISM Substances that contain unpaired electrons are weakly attracted into magnetic fields and are said to be paramagnetic. Substances in which all of the electrons are paired are very weakly repelled by a magnetic field and are said to be diamagnetic. Paramagnetism is much weaker than ferromagnetism (Fe, Co, Ni)

C 2 Carbon has 4 valence electrons, so C 2 has 8. MO configuration = ( 2 s 2)( * 2 s 2)( 2 p 4) All electrons are paired so C 2 is diamagnetic. O 2 Oxygen has 6 valence electrons. O 2 would have 12. MO configuration= ( 2 s 2)( * 2 s 2)( 2 p 4)( * 2 p 2) The last two electrons are unpaired, this explains the experimental results that show that oxygen is paramagnetic. Bond order = (8 -4)/2 = 2

Oxygen The Lewis structure of a molecule does not always correctly predict paramagnetism. The oxygen molecule is an example of this. Experimental data shows that oxygen is paramagnetic (has unpaired electrons). The more complex molecular orbital model will correctly predict oxygen’s paramagnetism.