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1、Chemical Bonding,Chemical bond:attractive force holding two or more atoms together.Covalent bond results from sharing electrons between the atoms.Usually found between nonmetals.Ionic bond results from the transfer of electrons from a metal to a nonmetal.Metallic bond:attractive force holding pure m
2、etals together.,Chemical Bonds,Lewis Symbols,and the Octet Rule,Figure 8.3:Ionic Bonding,Figure 8.5:Covalent Bonding,Chemical Bonds,Strengths of Covalent Bonds,Lewis Symbols,Chemical Bonds,Lewis Symbols,and the Octet Rule,The Octet RuleAll noble gases except He has an s2p6 configuration.Octet rule:a
3、toms tend to gain,lose,or share electrons until they are surrounded by 8 valence electrons(4 electron pairs).Caution:there are many exceptions to the octet rule.,Chemical Bonds,Lewis Symbols,and the Octet Rule,Bond Polarity and Electronegativity,ElectronegativityElectronegativity:The ability of one
4、atoms in a molecule to attract electrons to itself.Pauling set electronegativities on a scale from 0.7(Cs)to 4.0(F).Electronegativity increases across a period and down a group.,Figure 8.6:Electronegativities of Elements,Electronegativity,Bond Polarity and Electronegativity,Figure 8.7:Electronegativ
5、ity and Bond PolarityThere is no sharp distinction between bonding types.The positive end(or pole)in a polar bond is represented+and the negative pole-.,HyperChem,Drawing Lewis Structures,Follow Step by Step Method(See Ng Web-site)Total all valence electrons.Consider ChargeWrite symbols for the atom
6、s and guess skeleton structure define a central atom.Place a pair of electrons in each bond.Complete octets of surrounding atoms.H=2 only Place leftover electrons in pairs on the central atom.If there are not enough electrons to give the central atom an octet,look for multiple bonds by transferring
7、electrons until each atom has eight electrons around it.,HyperChem,CyberChem(Lewis)video,Lewis Structures Examples-I,Lewis Structures Examples-II,Lewis Structures Examples-III,Lewis Structures Examples-IV,Exceptions to the Octet Rule,Central Atoms Having Less than an OctetRelatively rare.Molecules w
8、ith less than an octet are typical for compounds of Groups 1A,2A,and 3A.Most typical example is BF3.Formal charges indicate that the Lewis structure with an incomplete octet is more important than the ones with double bonds.,Exceptions Central Atoms-Less than an Octet,Exceptions to the Octet Rule,Ce
9、ntral Atoms Having More than an OctetThis is the largest class of exceptions.Atoms from the 3rd period onwards can accommodate more than an octet.Beyond the third period,the d-orbitals are low enough in energy to participate in bonding and accept the extra electron density.,HyperChem,Exceptions Cent
10、ral Atoms-Greater than an Octet,There are five fundamental geometries for molecular shape:,Molecular Shapes:VSEPR,Molecular Shapes 3D Notations,VSEPR(Ballons)-Movie Clip,Figure 9.3,HyperChem,Summary of VSEPR Molecular Shapes,See Ng Web-site,HyperChem,CyberChm Gems,Examples:VSEPR Molecular Shapes-I,E
11、xamples:VSEPR Molecular Shapes I F08,Examples:VSEPR Molecular Shapes-II,Examples:VSEPR Molecular Shapes II F08,Examples:VSEPR Molecular Shapes-III,Examples:VSEPR Molecular Shapes III F08,HyperChem,Examples:VSEPR Molecular Shapes-IV,Examples:VSEPR Molecular Shapes-IV,The Effect of Nonbonding Electron
12、sBy experiment,the H-X-H bond angle decreases on moving from C to N to O:Since electrons in a bond are attracted by two nuclei,they do not repel as much as lone pairs.Therefore,the bond angle decreases as the number of lone pairs increases,VSEPR Model,HyperChem,Figure 9.10:Shapes of Larger Molecules
13、In acetic acid,CH3COOH,there are three central atoms.,VSEPR Model,HyperChem,Shapes of Larger MoleculesIn glycine(simplest amino acid),NH2CH2CO2H,there are four possible central atoms.Draw the Lewis Structure and the 3D VSEPR Molecular Geometry for glycine.Indicate the name of the shape for all possi
14、ble central atoms,including estimation of bond angles.Hint 1:Designate the 2nd carbon in the formula as the central atom in skeleton structure.Hint 2:The acid portion of glycine is the same as that of acetic acid.,Lewis-VSEPR HW for 10/30/09.Due 11/2/09.,HyperChem,Figure 8.10:Drawing Lewis Structure
15、s,Resonance Structures,Figure 9.12,HyperChem,Figure 9.11:Molecular Shape and Molecular Polarity,HyperChem,Figure 9.13:Molecular Shape and Molecular Polarity,HyperChem,Lewis structures and VSEPR do not explain why a bond forms.How do we account for shape in terms of quantum mechanics?What are the orb
16、itals that are involved in bonding?We use Valence Bond Theory:Bonds form when orbitals on atoms overlap.There are two electrons of opposite spin in the orbital overlap.,Covalent Bonding and Orbital Overlap,Gems-Movie Clip,Figure 9.14:Covalent Bonding and Orbital Overlap,To determine the electron pai
17、r geometry:draw the Lewis structure,count the total number of electron pairs around the central atom,arrange the electron pairs in one of the above geometries to minimize e-e-repulsion,and count multiple bonds as one bonding pair.,VSEPR Model(Figure 9.6),VSEPR Model,Drawing Lewis Structures,Formal C
18、hargeConsider:For C:There are 4 valence electrons(from periodic table).In the Lewis structure there are 2 nonbonding electrons and 3 from the triple bond.There are 5 electrons from the Lewis structure.Formal charge:4-5=-1.,Drawing Lewis Structures,Formal ChargeConsider:For N:There are 5 valence electrons.In the Lewis structure there are 2 nonbonding electrons and 3 from the triple bond.There are 5 electrons from the Lewis structure.Formal charge=5-5=0.We write:,CyberChm Gems,Chemical Bonding,Lewis,AXE notation,VSEPR shapes,Polarity,
