11/14/2014. Chemical Bonding. Richard Philips Feynman, Nobel Laureate in Physics ( )

Transcription

1 Chemical Bonding Lewis Theory Valence Bond VSEPR Molecular Orbital Theory 1 "...he [his father] knew the difference between knowing the name of something and knowing something" Richard Philips Feynman, Nobel Laureate in Physics ( ) 2 BONDING MODELS 1

2 4 Formation of dihydrogen, H 2 2

3 nonpolar covalent bond polar covalent bond 8 9 3

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5 The element with the larger electronegativity will carry the partial negative charge 13 The degree of polarity, or ionic character, varies continuously with the electronegativity difference 14 Figure 9.19 Percent ionic character of electronegativity difference ( EN). 15 5

6 BEGIN 11/18 16 Bond Type and Electronegativity ΔEN = nonpolar covalent ΔEN = polar covalent ΔEN = ionic 17 H F EN 2.1 EN 4.0 HF H F 18 6

7 Bond Polarity EN Cl = = 0 Pure Covalent EN Cl = 3.0 EN H = = 0.9 Polar Covalent EN Cl = 3.0 EN Na = = 2.1 Ionic Bond Dipole Moments the dipole moment is a quantitative way of describing the polarity of a bond a dipole is a material with positively and negatively charged ends measured dipole moment,, is a measure of bond polarity it is directly proportional to the size of the partial charges and directly proportional to the distance between them = (q)(r) measured in Debyes, D the percent ionic character is the percentage of a bond s measured dipole moment to what it would be if full ions 20 Dipole Moments 7

8 Polarity of Molecules in order for a molecule to be polar it must 1) have polar bonds electronegativity difference theory bond dipole moments measured 2) have an unsymmetrical shape vector addition polarity affects the intermolecular forces of attraction therefore boiling points and solubilities like dissolves like nonbonding pairs affect molecular polarity, strong pull in its direction 22 Molecule Polarity The H Cl bond is polar. The bonding electrons are pulled toward the Cl end of the molecule. The net result is a polar molecule. 23 Vector Addition 24 8

9 25 Molecule Polarity The O C bond is polar. The bonding electrons are pulled equally toward both O ends of the molecule. The net result is a nonpolar molecule. 26 Molecule Polarity The H O bond is polar. The both sets of bonding electrons are pulled toward the O end of the molecule. The net result is a polar molecule. 27 9

10 Water a VERY Polar Molecule stream of water attracted to a charged glass rod stream of hexane not attracted to a charged glass rod Molecule Polarity The H N bond is polar. All the sets of bonding electrons are pulled toward the N end of the molecule. The net result is a polar molecule. 29 Molecular Polarity Affects Solubility in Water polar molecules are attracted to other polar molecules since water is a polar molecule, other polar molecules dissolve well in water and ionic compounds as well some molecules have both polar and nonpolar parts 30 10

11 A Soap Molecule Sodium Stearate 31 Practice Decide Whether the Following Are Polar O O N Cl O S O EN O = 3.5 N = 3.0 Cl = 3.0 S = Practice Decide Whether the Following Are Polar O O N Cl Trigonal O S O Bent N 3.5 O Trigonal Planar Cl O S ) polar bonds, N O O O 2) asymmetrical shape 1) polar bonds, all S O polar 2) symmetrical shape nonpolar 33 11

12 VIDEO 34 Lewis Structures of Molecules shows pattern of valence electron distribution in the molecule useful for understanding the bonding in many compounds allows us to predict shapes of molecules allows us to predict properties of molecules and how they will interact together 35 Lewis Structures use common bonding patterns C = 4 bonds & 0 lone pairs, N = 3 bonds & 1 lone pair, O= 2 bonds & 2 lone pairs, H and halogen = 1 bond, Be = 2 bonds & 0 lone pairs, B = 3 bonds & 0 lone pairs often Lewis structures with line bonds have the lone pairs left off their presence is assumed from common bonding patterns structures which result in bonding patterns different from common have formal charges B C N O F 36 12

13 Practice Lewis Structures O CO 2 16 e H 3 PO 4 H O P O H :O::C::O: 32 e O H SeOF 2 SO 2 3 O O 26 e 26 e F Se F O S O NO 1 2 P 2 H 4 H H 18 e O N O 14 e H P P H 37 Formal Charge during bonding, atoms may wind up with more or less electrons in order to fulfill octets this results in atoms having a formal charge FC = valence e nonbonding e ½ bonding e left OFC = 6 4 ½ (4) = 0 O S FC = 6 2 ½ (6) = +1 S O right O FC = 6 6 ½ (2) = 1 sum of all the formal charges in a molecule = 0 in an ion, total equals the charge 38 Common Bonding Patterns B C N O C + N + O + F F + B - C - N - O - F 13

14 Resonance when there is more than one Lewis structure for a molecule that differ only in the position of the electrons, they are called resonance structures the actual molecule is a combination of the resonance forms a resonance hybrid it does not resonate between the two forms, though we often draw it that way look for multiple bonds or lone pairs O S O O S O Dinitrogen monoxide 42 14

15 43 H O : O : S O H : O : Structure I H O : O : S O H : O : Structure II Structure I obeys the octet rule, but is not consistent with experiment Structure II violates the octet rule, but is consistent with experiment