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Valence Bond Theory Lecture 18 - Valence Bond Theory

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Valence Bond Theory Lecture 18 - Valence Bond Theory Chem 103, Section F0F Lecture 18 - Covalent Bonding Unit VI - Compounds Part II: Covalent Compounds Reading in Silberberg Lecture 18 • Chapter 11, Section 1 - VAlence Bond (VB) Theory and Orbital Hybridization • Chapter 11, Section 2 Valence Bond Theory and Orbital Hybridization • - The Mode of Orbital Overlap and the Types of Covalent Bonds • The Mode of Orbital Overlap and the Types of Covalent Bonds 2 Lecture 18 - Introduction Lecture 18 - Valence Bond Theory In this lecture we will look into theorys of covent bonding. Valence bond theory proposes that covalent bonds form when the atomic orbitals of two atoms overlap. • We will look at the Valence Bond (VB) Theory, which • The combined, overlapping orbitals, contain a total of two resolves the inconsistentcy of what we know about the electrons. shapes and orientations of the atomic orbitals (s, p, and d) - Which, according Pauli’s exclusion principle, have opposing spins with the molecular shapes predicted by the VSEPR Theory. • We discuss how covalent bonds form when atomic orbitals from different atoms overlap. 3 4 Lecture 18 - Valence Bond Theory Lecture 18 - Valence Bond Theory Valence bond theory proposes that covalent bonds form Valence bond theory proposes that covalent bonds form when the atomic orbitals of two atoms overlap. when the atomic orbitals of two atoms overlap. • Every attempt is made to maximize the orbital overlap • Hybridization of atomic orbitals provides for molecular shapes that cannot be accommodated by using s, p, and d orbitals. - For example, we have seen that the VSEPR theory predicts a linear molecular geometry for BeCl2. Cl Be Cl - It is hard to see how this can be accommodated using s, and p orbitals. 5 6 Lecture 18 - Valence Bond Theory Lecture 18 - Valence Bond Theory 2 Be: [He]2s Hybridization of the 2s with one of the empty 2p orbitals in Be provides the solution to this problem. The 2s orbital of Be already has two electrons in it. Cl: [Ne]3s23p5 7 8 Lecture 18 - Valence Bond Theory Lecture 18 - Valence Bond Theory Hybridization of the 2s with one of the empty 2p orbitals in Be Hybridization of the 2s with one of the empty 2p orbitals in Be provides the solution to this problem. provides the solution to this problem. • The half-filled 3p orbitals from each of the Cl atoms can now each overlap with one of Be’s half-filled 2sp orbitals: 9 10 Lecture 18 - Valence Bond Theory Lecture 18 - Valence Bond Theory Hybridization: sp2 Hybridization: • The number of hybrid orbitals obtained equals the number • Example BF3 of atomic orbitals mixed. - VSEPR Theory predicts a trigonal planar molecular shape. • The type of hybrid orbitals obtained varies with the types of atomic orbitals mixed. F B F F We have just seen an example of sp hybridization with BeCl2, we will now go on to look at some other examples. 11 12 Lecture 18 - Valence Bond Theory Lecture 18 - Valence Bond Theory sp2 Hybridization: sp3 Hybridization: • Example BF3 • Example CH4 - VSEPR Theory predicts a trigonal planar molecular shape - VSEPR Theory predicts a tetrahedral molecular shape. - B: [He]2s22p1 - F: [He]2s22p5 H H H C H C H H H H 13 14 Lecture 18 - Valence Bond Theory Lecture 18 - Valence Bond Theory sp3 Hybridization: sp3 Hybridization: • Example CH4 • Example NH3 - VSEPR Theory predicts a tetrahedral molecular shape - VSEPR Theory predicts a tetrahedral molecular shape. - C: [He]2s22p2 - H: 1s1 H N H N H H H H 15 16 Lecture 18 - Valence Bond Theory Lecture 18 - Valence Bond Theory sp3 Hybridization: sp3 Hybridization: • Example CH4 • Example H2O - VSEPR Theory predicts a tetrahedral molecular shape - VSEPR Theory predicts a tetrahedral molecular shape. - N: [He]2s22p3 - H: 1s1 O H O H H H • Hybridized orbitals can also be used to hold non-bonding (lone pair) electrons. 17 18 Lecture 18 - Valence Bond Theory Lecture 18 - Valence Bond Theory sp3 Hybridization: sp3d Hybridization: • Example CH4 • To go beyond 4 hybridized orbitals the empty d orbtals can - VSEPR Theory predicts a tetrahedral molecular shape also be included 2 3 - N: [He]2s 2p Example PCl4 1 • - H: 1s - VSEPR Theory predicts a trigonal bipyramidal molecular shape. Cl Cl Cl P Cl Cl • Hybridized orbitals can also be used to hold non-bonding (lone pair) electrons. 19 20 Lecture 18 - Valence Bond Theory Lecture 18 - Valence Bond Theory sp3d Hybridization: sp3d2 Hybridization: • Example PCl5 • Example SF6 - VSEPR Theory predicts a trigonal bipyramidal molecular shape - VSEPR Theory predicts an octahedral molecular shape. - P: [Ne] 3s23p3 - Cl: [Ne]3s23p5 F F F S F F F 21 22 Lecture 18 - Valence Bond Theory Lecture 18 - Valence Bond Theory sp3d2 Hybridization: Hybridization and Molecular Shapes • Example SF6 - VSEPR Theory predicts an octahedral molecular shape - S: [Ne] 3s23p4 - F: [He]2s22p5 23 24 Lecture 18 - Valence Bond Theory Lecture 18 - Valence Bond Theory Hybridization and Molecular Shapes Hybridization and Molecular Shapes 25 26 Lecture 18 - Valence Bond Theory Lecture 18 - Types of Covalent Bonds Sometimes hybridization does not apply !-Bonds: x • Example H2S • Arise from end-to-end overlap of s, p, and hybridized sp or - VSEPR Theory predicts a tetrahedral molecular shape, like H2O. sp3dy orbitals - However, the bond angle is closer to that predicted for overlap of the • All single bonds are !-bonds. hydrogen 1s orbtals with non-hybridized 3p orbitals from the sulfur. S H 92° H 27 28 Lecture 18 - Types of Covalent Bonds Lecture 18 - Types of Covalent Bonds "-Bonds "-Bonds • Multiple bonds involve "-bonds in addition to one !-bond • Mutiple bonds involve "-bonds in addition to one !-bond • "-Bonds form from side-to-side overlap of p or d orbitals • "-Bonds form from side-to-side overlap of p or d orbitals • For example: • For example: - Double-bonds comprise 1 !-bond and 1 "-bond - Triple-bonds comprise 1 !-bond and 2 "-bonds 29 30 Lecture 18 - Types of Covalent Bonds Lecture 18 - Types of Covalent Bonds "-Bonds "-Bonds restrict rotation about double and triple bonds. • Have two lobes; the two lobes together make a single "- • This can lead to molecules that are configurational isomer. bond. - The isomers are distinguished as cis-trans isomers H H C C Cl Cl cis 1,2-dichloroethene H Cl C C Cl H trans 1,2-dichloroethene 31 32 Lecture 18 - Question 1 Lecture 18 - Question 2 What is the orbital hybridization of t a central atom that has What is the hybridization of the chlorine atom in each of the one lone pair and bonds to following polyatomic ions: - A) two other atoms? A) ClO2 - B) three other atoms? B) ClO3 - C) four other atoms? C) ClO4 D) five other atoms? 33 34 Lecture 18 - Question 3 Lecture 18 - Question 4 Is this statements true or false: Is this statements true or false: Two !-bonds comprise a double bond. A triple bond consists of one "-bond and two !-bonds. A) True A) True B) False B) False 35 36 Lecture 18 - Question 4 Unit VII - Up Next Is this statements true or false: Lecture 19 - Physical States and Intermolecular Forces A "bond consists of two pairs of electrons. • Overview of physical states and changes A) True • Description of phase changes B) False • Types of intermolecular forces 37 38 The End.
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