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Electron Configurations of Ions of the Representative Elements
Sample Exercise 8.2
Predict the ion generally formed by (a) Sr (b) S (c) Al
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Transition Metal Ions
Practice Exercise 8.2 Page 307
Predict the charges on the ions formed when magnesium reacts with nitrogen
• Configuration for iron (II) vs iron (III)
• Which element forms a 1+ ion that has the electron configuration [Kr]
4d8 ?
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Covalent Bonding
Polyatomic Ions
• Simplest example is H 2 molecule
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Lewis Structures
• Hydrogen molecule
• Chlorine molecule
Sample Exercise 8.3
Given the Lewis symbols for nitrogen and fluorine shown in Table 8.1, predict the formula of the stable binary compound (a compound composed of two elements) formed when nitrogen reacts with fluorine, and draw its Lewis structure.
EXAMPLES:
• Hydrogen fluoride, Water, ammonia, methane
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Practice Exercise 8.3 (page 311)
Compare the Lewis symbol for neon with the Lewis structure for methane (CH 4). In what important way are the electron arrangements about the neon and the carbon alike? In what important way are they different?
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Multiple Bonds:
EXAMPLES:
carbon dioxide
nitrogen molecule
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Bond Polarity and Electronegativity:
Bond Length of Covalent Compounds:
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Polarity of Bonds:
Electronegativity
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Electronegativity Scale:
Electronegativity and Bond Polarity
Electronegativity Difference Type of Bond
< 0.5 nonpolar
0.5 ­ 2.0 polar
> 2.0 ionic
• Consider: F 2 vs. HF vs. LiF
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Sample Exercise 8.4
In each case, which bond is more polar: (a) B­Cl or C­Cl (b) P­F or P­
Cl? Indicate in each chase which atom has the partial negative charge.
Practice Exercise 8.4 (page 314)
Which of the following bonds is most polar: S­Cl, S­Br, Se­Cl, Se­Br?
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Dipole Moments:
• EXAMPLE: HF (polarity is indicated in two ways)
• μ = Qr
Q is the magnitude of the charge and r is the separation of the
charges
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Bond Types and Nomenclature
A Little More on Nomenclature:
• Example TiO2
• Reason?
• Question: The compounds MnO 3 and OsO 4 are more properly named molybdenum (VI) oxide and osmium tetroxide, respectively. Which of these compounds do you think has the higher melting point?
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Steps for Drawing Lewis Structures:
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Practice Exercise 8.6 (page 319)
a) How many valence electrons should appear in the Lewis structure for CH 2Cl2?
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Sample Exercise 8.6
Draw the Lewis structure for phosphorus trichloride.
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Sample Exercise 8.7
Draw the Lewis structure for HCN.
b) Draw the Lewis structure
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Practice Exercise 8.7 (page 319)
Draw the Lewis structure for (a) NO + ion and (b) C 2H4
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Sample Exercise 8.8
Draw the Lewis structure for the BrO 3­ ion.
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Formal Charge
Practice Exercise 8.8 (page 320)
Draw the Lewis structure for (a) ClO 2­ ion and (b) PO 43­ ion.
• To calculate formal charge:
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Calculate the formal charge on the atoms in the CN ­ ion:
To choose the most correct structure: 1) Choose the structure in which the atoms bear formal charges closet to zero
Sulfate ion:
2)Choose the structure in which any negative charges reside on the more electronegative atoms
3) Choose the structure that obeys the octet rule if possible
Note that the sum of the formal charges equals the overall charge of the ion.
IMPORTANT REMINDER:
Formal charges do not represent real charges on atoms
Calculate the formal charge of the CO 2 structures that obey the octet rule (1 with 2 double bonds and 1 with 1 triple bond)
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Sample Exercise 8.9
The following are three possible structures for the thiocyanate ion (SCN­)
Practice Exercise 8.9 (page 321)
The cyanate ion (CNO ­), like the thiocyanate ion, has three possible Lewis structures. (a) Draw these three Lewis structures, and assign formal charges to the atoms in each structure. (b) Which Lewis structure is the preferred one?
(a) Determine the formal charges of the atoms in each structure (b) Which Lewis structure is the preferred one?
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Green Paint Analogy
Resonance Structures
• Consider the structure of the ozone molecule
• Resonance structures are Oct 23­7:58 PM
Resonance structures of the nitrate ion:
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Sample Exercise 8.10
Which is predicted to have shorter sulfur­oxygen bonds, SO 3 or SO32­ ?
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Practice Exercise 8.10 (page 324)
Draw two equivalent resonance structures for the formate ion, HCO
2
­
.
Resonance in Benzene
Benzene is an aromatic organic compound with the formula C 6H6. The six C atoms are bonded in a hexagonal ring and a H is bonded to each C.
Two resonance structures for benzene that satisfy the octet rule:
Shorthand version:
The bonding in benzene gives it a special stability. This results in millions of organic compounds having the characteristic 6­C ring structure. These compounds are important in pharmaceuticals, biochemistry, and production of modern materials.
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Odd Number of Electrons Occurs in:
Exceptions to the Octet Rule:
The octet rule fails in many situations involving covalent bonds:
ClO2
NO
NO2
O2­
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Less than 8 Valence Electrons:
• Most often found in compounds of Boron and Beryllium
• Consider boron trifluoride (B has only 6 electrons)
More than 8 Valence Electrons:
• PCl 5 requires 10 electrons around the central phosphorus atom
• This bonds easily with molecules having an unshared pair of electrons (like ammonia)
• Will form NH 3BF3
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Sample Exercise 8.11
Draw the Lewis structure for ICl 4­.
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Strength of Covalent Bonds:
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Practice Exercise 8.11 (page 328)
(a) Which of the following atoms is never found with more than an octet of electrons around it: S, C, P, Br? (b) Draw the Lewis structure for XeF2.
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Bond Enthalpy and Enthalpy of Reaction:
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Determining Enthalpies of Reactions:
Consider two steps:
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Consider the reaction:
H­CH3(g) + Cl­Cl(g) ­­> Cl­CH 3(g) + H­Cl(g) ΔHrxn =?
Sample Exercise 8.12
Using Table 8.3, estimate ΔH for the following reaction (where we
explicitly show the bonds involved in the reactants and products):
Important factors to remember: Bond enthalpies are given for gaseous molecules and are averaged values
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Practice Exercise 8.4 (page 332)
Using Table 8.4, estimate ΔH for the reaction:
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Bond Enthalpy and Bond Length
In general, as the number of bonds between two atoms increases, the bond grows shorter and stronger.
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Sample Integrative Exercise Page 334
Phosgene, a substance used in poisonous gas warfare in World War I, is so named because it was first prepared by the action of sunlight on a mixture of carbon monoxide and chlorine gases. Its name comes from the Greek word phos (light) and genes (born of). Phosgene has the following elemental composition: 12.14%C, 16.17%O, and 71.69% Cl by mass. Its molar mass is 98.9 g/mol. (a) Determine the molecular formula of this compound. (b) Draw three Lewis structures for the molecule that satisfy the octet rule for each atom..the Cl and O atoms bond to the C. (c) Using formal charges determine which Lewis structure is the most important one. (d) Using average bond enthalpies, estimate ΔH for the formation of gaseous phosphene from CO(g) and Cl2(g).
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