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. • Metallic bond: attractive force holding pure metals together.
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 s2p6 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 atomsin 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 UP 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) • Total all valence electrons. [Consider Charge] • Write symbols for the atoms 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 electrons until each atom has eight electrons around it. HyperChem CyberChem (Lewis) video
Exceptions to the Octet Rule Central Atoms Having Less than an Octet • Relatively rare. • Molecules with 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 to the Octet Rule Central Atoms Having More than an Octet • This 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
Molecular Shapes: VSEPR • There are five fundamental geometries for molecular shape: VSEPR (Ballons)-Movie Clip
Molecular Shapes – 3D Notations VSEPR (Ballons)-Movie Clip
Figure 9.3 Vote HyperChem
Summary of VSEPR Molecular Shapes HyperChem CyberChm Gems See Ng Web-site
VSEPR Model The Effect of Nonbonding Electrons • By 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 HyperChem
VSEPR Model HyperChem Figure 9.10: Shapes of Larger Molecules • In acetic acid, CH3COOH, there are three central atoms.
Lewis-VSEPR HW assigned 11/?/1? . Due 11/?/1?. HyperChem Shapes of Larger Molecules • In 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 possible 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. Solution Key
Figure 8.10: Drawing Lewis Structures Resonance Structures
HyperChem Figure 9.12
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.