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General Chemistry II 102 Chem.

General Chemistry II 102 Chem. Prof. Hassan A. Mohammed. Chapter 1: Covalent Bonding. 1.1 Hybridization and the Localized Electron Model 1.2 The Molecular Orbital Model 1.3 Bonding in Homonuclear Diatomic Molecules 1.4 Bonding in Heteronuclear Diatomic Molecules

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General Chemistry II 102 Chem.

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  1. General Chemistry II102 Chem. Prof. Hassan A. Mohammed Prof.Haasan A.Mohammed

  2. Chapter 1: Covalent Bonding • 1.1 Hybridization and the Localized Electron Model • 1.2 The Molecular Orbital Model • 1.3 Bonding in Homonuclear Diatomic Molecules • 1.4 Bonding in Heteronuclear Diatomic Molecules • 1.5 Combining the Localized Electrons • and Molecular Orbital Models Prof.Haasan A.Mohammed

  3. 1.1Hybridization and the Localized Electron Model • 1.1.1 Introduction: • Lewis Dot Structures: Gilbert N. Lewis (1916) • Valence shell: • The outermost occupied electron shell of an atom. • Valence electrons: • Electrons in the valence shell of an atom; these electrons are used to form chemical bonds and in chemical reactions. • Lewis dot structure: • The symbol of an element represents the nucleus and all inner shell electrons. • Dots represent electrons in the valence shell of the atom. • Lewis Dot Structures: Prof.Haasan A.Mohammed

  4. Lewis Dot Diagrams of Selected Elements Prof.Haasan A.Mohammed

  5. 1.1.2 Lewis Model of Bonding: • To write a Lewis structure • Determine the number of valence electrons. • Determine the arrangement of atoms. • Connect the atoms by single bonds. • Arrange the remaining electrons so that each atom • has a complete valence shell. • Show a bonding pair of electrons as a single line. • Show a nonbonding pair of electrons (a lone pair) • as a pair of dots. • In a single bond atoms share one pair of electrons, in a double bond they share two pairs of electrons and in a triple bond they share three pairs of electrons. Prof.Haasan A.Mohammed

  6. In neutral molecules • hydrogen has one bond. • carbon has 4 bonds and no lone pairs. • nitrogen has 3 bonds and 1 lone pair. • oxygen has 2 bonds and 2 lone pairs. • halogens have 1 bond and 3 lone pairs. Prof.Haasan A.Mohammed

  7. 1.1.3The Valence Bond Theory(VSEPR) model: According to this theory a covalent bond is formed between the two atoms by the overlap of half filled valence atomic orbitals of each atom containing one unpaired electron. Valence bond theory considers that the overlappingatomic orbitals of the participating atoms form a chemical bond. Because of the overlapping, it is most probable that electrons should be in the bond region. Valence bond theory views bonds as weakly coupled orbitals (small overlap). Valence bond theory is typically easier to employ in ground state molecules. Prof.Haasan A.Mohammed

  8. The two types of overlapping orbitals are sigma and pi. Sigma bonds occur when the orbitals of two shared electrons overlap head-to-head. Pi bonds occur when two orbitals overlap when they are parallel. Prof.Haasan A.Mohammed

  9. In a polyatomic molecule, where there are two or more bonds between the central atom and the surrounding atoms, the repulsion between electrons in different bonding pairs causes them to remain as far apart as possible. This approach to the study of molecular geometry is called the valence-shell electron-pair repulsion (VSEPR) model. Two general rules govern the use of the VSEPR model: As far as electron-pair repulsion is concerned, double bonds and triple bonds can be treated like single bonds. If a molecule has two or more resonance structures, we can apply the VSEPR model to any one of them. Prof.Haasan A.Mohammed

  10. Based on the twin concepts that • atoms are surrounded by regions of electron density. • regions of electron density repel each other Prof.Haasan A.Mohammed

  11. 1.1.4 Hybridization of Atomic Orbital'sThe localized Electron Model Hybridization: The mixing of atomic orbitals to form special orbitals for bonding. The atoms are responding as needed to give the minimum energy for the molecule. A sigma () bondcenters along the internuclear axis. A pi () bondoccupies the space above and below the internuclear axis. Prof.Haasan A.Mohammed

  12. When one s orbital and one p orbital are hybridized, a set of two sp orbitals oriented at 180 degrees results. Prof.Haasan A.Mohammed

  13. Procedure for Hybridizing Atomic Orbitals: To assign a suitable state of hybridization to the central atom in a molecule, we must have some idea about the geometry of the molecule. The steps are as follows: 1. Draw the Lewis structure of the molecule. 2. Predict the overall arrangement of the electron pairs (both bonding pairs and lone pairs) using the VSEPR model (see Table 10.1). 3. Deduce the hybridization of the central atom by matching the arrangement of the electron pairs with those of the hybrid orbitals Prof.Haasan A.Mohammed

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  17. The localized Electron Model We will discuss three examples of VB treatment of bonding in polyatomic molecules sp3 Hybridization Consider the CH4 molecule. Focusing only on the valence electrons, we can represent the orbital diagram of C as: To account for the four C-H bonds in methane, we can try to promote (that is, energetically excite) an electron from the 2s orbital to the 2p orbital: Prof.Haasan A.Mohammed

  18. VB theory uses hypothetical hybrid orbitals, which are atomic orbitals obtained when two or more nonequivalent orbitals of the same atom combine in preparation for covalent bond formation. We can generate four equivalent hybrid orbitals for carbon by mixing the 2s orbital and the three 2p orbitals: Because the new orbitals are formed from one s and three p orbitals, they are called sp3 hybrid orbitals. These four hybrid orbitals are directed toward the four corners of a regular tetrahedron. Prof.Haasan A.Mohammed

  19. Formation of four bonds between the carbon sp3 hybrid orbitals and the hydrogen 1s orbitals in CH4. The smaller lobes are not shown. Another example of sp3 hybridization is ammonia (NH3): so that the orbital diagram for the sp3 hybridized N(The ground-state electron configuration of N is 1s22s22p3) atom is Prof.Haasan A.Mohammed

  20. Sp2 hybridization : Next we will look at the BF3 (boron trifluoride) molecule, known to have a planar geometry. Considering only the valence electrons, the orbital diagram of B(1s2,2s2,2p1) is First, we promote a 2s electron to an empty 2p orbital. Mixing the 2s orbital with the two 2p orbitals generates three sp2 hybrid orbitals: Prof.Haasan A.Mohammed

  21. Mixing the 2s orbital with the two 2p orbitals generates three sp2 hybrid orbitals: These three sp2 orbitals lie in the same plane, and the angle between any two of them is 120‘ . Each of the BF bonds is formed by the overlap of a boron sp2hybrid orbital and a fluorine 2p orbital . This result conforms to experimental findings and also to VSEPR predictions. Prof.Haasan A.Mohammed

  22. The orbital arrangement for an sp2 hybridized oxygen atom. (a) The orbitals used to form the bonds in carbon dioxide. Note that the carbon-oxygen double bonds each consist of one s bond and one p bond. (b) The Lewis structure for carbon dioxide. Prof.Haasan A.Mohammed

  23. Sp Hybridization  The beryllium chloride (BeC12) molecule is predicted to be linear by VSEPR. The orbital diagram for the valence electrons in Be is First, we promote a 2s electron to a 2p orbttal, resulting in Thus, the 2s and 2p orbitals must be mixed, or hybridized, to form two equivalent sp hybrid orbitals: Prof.Haasan A.Mohammed

  24. When one s orbital and one p orbital are hybridized, a set of two sp orbitals oriented at 180 degrees results. Prof.Haasan A.Mohammed

  25. (a) An sp hybridized nitrogen atom.(b) The s bond in the N2 molecule. (c) The two p bonds in N2 are formed when electron pairs are shared between two sets of parallel p orbitals. (d) The total bonding picture for N2. Prof.Haasan A.Mohammed

  26. The orbitals of an sp hybridized carbon atom. Prof.Haasan A.Mohammed

  27. Hybridization of s, p, and d Orbitals: (Sp3 d2 ) hybridization Consider the SF6 molecule as an example: The ground-state electron configuration of S is [Ne]3s2 3p4 . Focusing only on the valence electron, w e have the orbital diagram Because the 3d level is quite close in energy to the 3s and 3p levels, we can promote 3s and 3p electrons to two of the 3d orbitals: Mixing the 3s, three 3p, and two 3d orbitals generates six sp3d2 hybrid orbitals Prof.Haasan A.Mohammed

  28. An octahedral set of d2sp3 orbitals on a sulfur atom. The small lobe of each hybrid orbital has been omitted for clarity. Prof.Haasan A.Mohammed

  29. (Sp3 d ) hybridization: we shall consider PBr5 Prof.Haasan A.Mohammed

  30. Prof.Haasan A.Mohammed

  31. Figure 9.21: A set of dsp3 hybrid orbitals on a phosphorus atom. Note that the set of five dsp3 orbitals has a trigonalbipyramidal arrangement. (Each dsp3 orbital also has a small lobe that is not shown in this diagram.) Prof.Haasan A.Mohammed

  32. (a) The structure of the PCI5 molecule. (b) The orbitals used to form the bonds in PCl5. The phosphorus uses a set of five dsp3 orbitals to share electron pairs with sp3 orbitals on the five chlorine atoms. The other sp3 orbitals on each chlorine atom hold lone pairs. Prof.Haasan A.Mohammed

  33. Hybridization in Molecules Containing Double and Triple Bonds (a) Hybridization in Molecules Containing Double Bonds Consider the ethylene molecule, C2H2which contains a carbon-carbon double bond and has planar geometry. Each carbon atom is sp2-hybridized. We assume that only the 2px and 2py orbital combine with the 2s orbital, and that the 2pz orbital remains unchanged. Each carbon atom uses the three sp2- hybrid orbitals to form two bonds with the two hydrogen 1s orbitals and one bond with the sp2 hybrid orbital of the adjacent C atom. In addition, the two unhybridized 2pz orbitals of the C atoms form another bond by overlapping sideways(π-bonds) Prof.Haasan A.Mohammed

  34. The hybridization of the s, px, and py atomic orbitals results in the formation of three sp2 orbitals centered in the xy plane. The large lobes of the orbitals lie in the plane at angles of 120 degrees and point toward the corners of a triangle. Prof.Haasan A.Mohammed

  35. When an s and two p orbitals are mixed to form a set of three sp2 orbitals, one p orbital remains unchanged and is perpendicular to the plane of the hybrid orbitals. Note that in this figure and those that follow, the orbitals are drawn with narrowed lobes to show their orientations more clearly. Prof.Haasan A.Mohammed

  36. (a) The orbitals used to form the bonds in ethylene. (b) The Lewis structure for ethylene. Prof.Haasan A.Mohammed

  37. (a) Hybridization in Molecules Containing TribleBonds The acetylene molecule (C2H2) contains a carbon-carbon triple bond. Because the molecule is linear. Each C atom is sp-hybridized by mixing the 2s with the 2px orbital . The two sp hybrid orbitals of each C atom form one sigma bond with a hydrogen 1s orbital and another sigma bond with the other C atom. In addition, two pi bonds are formed by the sideways overlap of the unhybridized 2py and 2pz orbitals. Thus, the C C bond is made up of one sigma bond and two pi ( π)bonds. Prof.Haasan A.Mohammed

  38. Prof.Haasan A.Mohammed

  39. 1.2 The Molecular Orbital Model: • Bonding and Antibonding Molecular Orbitals • According to MO theory, the overlap of the 1s orbitals of two hydrogen atoms leads to the formation of two molecular orbitals: one bonding molecular orbital and one antibonding molecular orbital. A bonding molecular orbital has lower energy and greater stability than the atomic orbitals from which it was formed. Prof.Haasan A.Mohammed

  40. An antibonding molecular orbital has higher energy and lower stability than the atomic orbitals from which it was formed. As the names "bonding" and "antibonding” suggest, placing electrons in a bonding molecular orbital yields a stable covalent bond, whereas placing electrons in an antibonding molecular orbital results in an unstable bond. Prof.Haasan A.Mohammed

  41. In the bonding molecular orbital, the electron density is greatest between the nuclei of the bonding atoms. In the antibonding molecular orbital, on the other hand, the electron density decreases to zero between the nuclei. We can understand this distinction if we recall that electrons in orbitals have wave characteristics. Prof.Haasan A.Mohammed

  42. The formation of bonding molecular orbitals corresponds to constructive interference (the increase in amplitude is analogous to the buildup of electron density between the two nuclei). The formation of antibonding molecular orbitals corresponds to destructive interference (the decrease in amplitude is analogous to the decrease in electron density between the two nuclei). The constructive and destructive interactions between the two 1s orbitals in the H2 molecule, then, lead to the formation of a sigma bonding molecular orbital б1s and a sigma antibonding molecular orbital б*1s Prof.Haasan A.Mohammed

  43. A property unique to waves enables waves of the same type to interact in such a way that the resultant wave has either enhanced amplitude or diminished amplitude. In the former case, we call the interaction constructive(a) interference; in the latter case, it is destructive interference(b) Prof.Haasan A.Mohammed

  44. In a pi(π) molecular orbital (bonding or antibonding), the electron density is concentrated above and below a line joining the two nuclei of the bonding atoms. Two electrons in a pi molecular orbital form a pi bond (see Section 10.5). A double bond is almost always composed of a sigma bond and a pi bond; a triple bond is always a sigma bond plus two pi(π) bonds. where the star denotes an antibonding molecular orbital. ln a sigma molecular orbital (bonding or antibonding) the electron density is concentrated symmetrically around a line between the two nuclei of the bonding atoms. Two electrons in a sigma molecular orbital form a sigma bond Prof.Haasan A.Mohammed

  45. (a) Energy levels of bonding and antibonding molecular orbitals in the H2 molecule. Note that the two electrons in the б1s , orbital must have opposite spins in accord with the Pauli exclusion principle. Keep in mind that the higher the energy of the molecular orbital, the less stable the electrons in that molecular orbital. • (b) Constructive and destructive interferences between the two hydrogen 1s orbitals lead to the formation of a bonding and an antibonding molecular orbital. ln the bonding molecular orbital, there is a buildup between the nuclei of electron density, which acts as a negatively charged "glue" to hold the positively charged nuclei together. ln the antibonding molecular orbital, there is a nodal plane between the nuclei where the electron density is zero. Prof.Haasan A.Mohammed

  46. Prof.Haasan A.Mohammed

  47. Two 2p orbital can approach each other end-to-end to produce a sigma bonding and a sigma antibonding molecular orbital. Alternatively, the two p orbitals can overlap sideways to generate a bonding and an antibonding pi molecular orbital In a pi molecular orbital (bonding or antibonding), the electron density is concentrated above and below a line joining the two nuclei of the bonding atoms. Prof.Haasan A.Mohammed

  48. 1.3 Bonding in Homonuclear Diatomic Molecules Hydrogen and Helium Molecules: Later in this section we will study molecules formed by atoms of the second-period elements. Before we do, it will be instructive to predict the relative stabilities of the simple species H+2, H2, He+2, and He2, using the energy-level diagrams shown Prof.Haasan A.Mohammed

  49. The orbitals can accommodate a maximum of four electrons. The total number of electrons increases from one for H+2 to four for He2. To evaluate the stabilities of these species we determine their bond order, defined as: The difference between the number of bonding electrons and the number of antibonding electrons divided by 2 The bond order indicates the approximate strength of a bond. For example, if there are two electrons in the bonding molecular orbital and none in the antibonding molecular orbital, the bond order is one, which means that there is one covalent bond and that the molecule is stable. Note that the bond order can be a fraction, but a bond order of zero (or a negative value) means the bond has no stability and the molecule cannot exist. Prof.Haasan A.Mohammed

  50. To summarize, we can rearrange our examples in order of decreasing stability: Our simple molecular orbital method predicts that H+2 and He+2 also possess some stability, because both have bond orders of 1/2. Indeed, their existence has been confirmed by experiment. It turns out that H+2 is somewhat more stable than He+2, because there is only one electron in the hydrogen molecular ion and therefore it has no electron-electron repulsion. Furthermore, H+2 also has less nuclear repulsion than He+2 Our prediction about He2 is that it would have no stability, but in 1993 He2 gas was found to exist. The "molecule" is extremely unstable and has only a transient existence under specially created conditions. Prof.Haasan A.Mohammed

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