Chemical Bonding Types of Bonds Ionic Bonding Lewis Structures Covalent Bonding Resonance Structures Octet Rule Polar Molecules Molecular Geometries VSEPR Basic Shapes 3-D Notation Hybridization (Lab)
Chemical Bonds, Lewis Symbols, and the Octet Rule 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. Ionic compounds that form electrolytes conduct electricity. Metallic bond: attractive force holding pure metals together. Sea of electrons.
Figure 8.3: Ionic Bonding
Figure 8.5: Covalent Bonding
Chemical Bonds Bond Type Single Double Triple # of e s 2 4 6 Notation = Bond order 1 2 3 Bond strength Bond length Increases from Single to Triple Decreases from Single to Triple
Strengths of Covalent Bonds
Chemical Bonds, Lewis Symbols, and the Octet Rule Lewis Symbols
Chemical Bonds, Lewis Symbols, and the Octet Rule The Octet Rule All noble gases except He has an s 2 p 6 configuration. Octet rule: atoms 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.
Bond Polarity and Electronegativity Electronegativity Electronegativity: The ability of one 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: Electronegativity and Bond Polarity There 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) 1. Total all valence electrons. [Consider Charge] 2. Write symbols for the atoms and guess skeleton structure [ define a central atom ]. 3. Place a pair of electrons in each bond. 4. Complete octets of surrounding atoms. [ H = 2 only ] 5. Place leftover electrons in pairs on the central atom. 6. If there are not enough electrons to give the central atom an octet, look for multiple bonds by transferring electrons until each atom has eight electrons around it. CyberChem (Lewis) video HyperChem
Lewis Structures Examples - I
Lewis Structures Examples - II
Exceptions to the Octet Rule Relatively rare. Central Atoms Having Less than an Octet Molecules with less than an octet are typical for compounds of Groups 1A, 2A, and 3A. Most typical example is BF 3. Formal charges indicate that the Lewis structure with an incomplete octet is more important than the ones with double bonds.
Exceptions to the Octet Rule Central Atoms Having More than an Octet This is the largest class of exceptions. Atoms from the 3 rd 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
Molecular Shapes: VSEPR There are five fundamental geometries for molecular shape:
Molecular Shapes 3D Notations VSEPR (Ballons)-Movie Clip
Figure 9.3 HyperChem
Summary of VSEPR Molecular Shapes e-pairs Notation Name of VSEPR shape Examples 2 AX 2 Linear HgCl 2, ZnI 2, CS 2, CO 2 3 AX 3 Trigonal planar BF 3, GaI 3 AX 2 E Non-linear (Bent) SO 2, SnCl 2 4 AX 4 Tetrahedral CCl 4, CH 4, BF - 4 AX 3 E (Trigonal) Pyramidal NH 3, OH - 3 AX 2 E 2 Non-Linear (Bent) H 2 O, SeCl 2 5 AX 5 Trigonal bipyramidal PCl 5, PF 5 AX 4 E Distorted tetrahedral (see-sawed) TeCl 4, SF 4 AX 3 E 2 T-Shaped ClF 3, BrF 3 AX 2 E 3 Linear I 3-, ICl - 2 6 AX 6 Octahedral SF 6, PF - 6 AX 5 E Square Pyramidal IF 5, BrF 5 AX 4 E 2 Square Planar ICl 4-, BrF - 4 HyperChem CyberChm Gems See Ng Web-site
Examples: VSEPR Molecular Shapes - I # electron pairs on Central Atom A Notation Example Lewis VSEPR & Name of Shape 2 AX 2 2 bp on A AX 3 3 bp on A 3 AX 2 E 2 bp and 1 lp on A
Examples: VSEPR Molecular Shapes I F08
Examples: VSEPR Molecular Shapes - II # electron pairs on Central Atom A Notation Example Lewis VSEPR & Name of Shape AX 4 4 bp on A 4 AX 3 E 3 bp and 1 lp on A AX 2 E 2 2 bp and 2 lp on A
Examples: VSEPR Molecular Shapes II F08
Examples: VSEPR Molecular Shapes - III # electron pairs on Central Atom A Notation Example Lewis VSEPR & Name of Shape AX 5 5 bp on A 5 AX 4 E 4 bp and 1 lp on A AX 3 E 2 3 bp and 2 lp on A AX 2 E 3 2 bp and 3 lp on A
Examples: VSEPR Molecular Shapes III F08 HyperChem
Examples: VSEPR Molecular Shapes - IV # electron pairs on Central Atom A Notation Example Lewis VSEPR & Name of Shape AX 6 6 bp on A 6 AX 5 E 5 bp and 1 lp on A AX 4 E 2 4 bp and 2 lp on A
VSEPR Model The Effect of Nonbonding Electrons By experiment, the H-X-H bond angle decreases on moving from C to N to O: H H C H109.5 H O H H N H 107 O O H H 104.5 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 HyperChem
VSEPR Model HyperChem Figure 9.10: Shapes of Larger Molecules In acetic acid, CH 3 COOH, there are three central atoms.
Lewis-VSEPR HW assigned 10/29/10. Due 11/1/10. HyperChem Shapes of Larger Molecules In glycine (simplest amino acid), NH 2 CH 2 CO 2 H, 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 possible central atoms, including estimation of bond angles. Hint 1: Designate the 2 nd 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. Solution Key
Figure 8.10: Drawing Lewis Structures Resonance Structures
HyperChem Figure 9.12
Figure 9.11: Molecular Shape and Molecular Polarity HyperChem
Figure 9.13: Molecular Shape and Molecular Polarity HyperChem
Covalent Bonding and Orbital Overlap Gems - Movie Clip 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 orbitals 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.
Figure 9.14: Covalent Bonding and Orbital Overlap
VSEPR Model (Figure 9.6) To determine the electron pair 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
Drawing Lewis Structures Consider: For C: C N Formal Charge 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 Consider: C For N: There are 5 valence electrons. Formal Charge 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: C N N CyberChm Gems
Easy ways to remember shapes Steric number- the number of atoms bonded to the central atom of a molecule plus the number of lone pairs on the central atom. It is often used in VSEPR theory (valence shell electron-pair repulsion theory) in order to determine the particular shape, or molecular geometry, that will be formed.
Molecular Geometry Steps 1. Memorize the structures in groups according to steric number/ Bonding number. A. Steric number 2 = 1 structure B. Steric number 3 = 2 structures C. Steric number 4 = 3 structures D. Steric number 5 = 4 structures E. Steric number 6 = 3 structures.
Molecular Geometry Steps 2. Now you have to memorize the structures in each steric number group. A. Steric number 2 = Linear (180 degrees) B. Steric number 3 = Bent and Trigonal Planar (Both 120 degrees) C. Steric number 4 = Tetrahedral, Trigonal Bipyramidal and Bent again. (109.5, 107.3 and 104.5 degrees) D. Steric number 5 = Trigonal Bipyramidal, See-Saw, T- Shaped and Linear (Mixture of 90, 120 and 180 degrees) E. Steric number 6 = Square Planar, Square Pyramid and Octahedral (All 90 degrees) (Last one kind of breaks the rule but I still find this helps. )
Molecular Geometry Steps 3. Try to find a relationship between all the groups. A. For groups 2 and 3, shapes pretty straightforward. B. Group 4 are all tetrahedral shaped molecules (Think group "4" / "Tetrahedral") with slight variations on the bond angles based on the amount of lone pairs. Tetrahedral is the most uniform shape and it has a bond angle closest to 120, with a slight repulsion from the uppermost atom making a bond angle of 109.5. Trigonal Bipyramidal has a lone pair of electrons which pushes the "side atoms" a little further away from 120 forming an angle of 107.3. Lastly, this variation of bent has two lone pairs of electrons pushing the "side atoms" even further away making an angle of 104.5.
Molecular Geometry Steps cont. C. Group 5, are the "weird shapes". This includes the trigonal bipyramidal, see-saw, T-shape and linear. T shape is 90 degrees because in the letter T, only 90 degree angles are present. Linear is 180 degrees. See-saw has 3 angles, 1 angle along the top of the seat (180), 1 between the seats and the legs (90), and one between the two legs (120). and trigonal Bipyramidal has 2 angles, 90 and 120 degrees. D. Group 6 is the "square group" including: Square planar, square pyramid and octahedral. (I know the last one isn t a square but, close enough). All squares consist of only 90 degree angles so these all only have 90 degree angles. Using All of this info you can intuitively put together the notation for each structure.
Chemical Bonding Types of Bonds Ionic Bonding Lewis Structures Covalent Bonding Resonance Structures Octet Rule Polar Molecules Lewis AXE notation Molecular Geometries VSEPR Basic Shapes 3-D Notation Hybridization (Lab) VSEPR shapes Polarity