CHEMISTRY - ZUMDAHL 2E CH.4 - MOLECULAR STRUCTURE AND ORBITALS.

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2 CONCEPT: ELECTRONIC GEOMETRY When drawing a compound you have to take into account two different systems of geometrical shape. The simpler system known as electronic geometry or shape treats lone pairs (nonbonding electrons) and surrounding elements as the same. Key: A = Central Element X = Lone Pairs and Surrounding Elements X A X X! AX 2 = Linear (2 Groups) AX 3 = Trigonal Planar (3 Groups) O C O C N B Sn AX 4 = Tetrahedral (4 Groups) AX 5 = Trigonal Bipyramidal (5 Groups) N C Xe P AX 6 = Octahedral (6 Groups) Xe S Page 2

3 PRACTICE: ELECTRONIC GEOMETRY EXAMPLE: Draw each of the following compounds and determine their electronic geometries. P 3 Be 2 PRACTICE 1: Draw the following compound and determine its electronic geometry. SBr4 PRACTICE 2: Draw the following compound and determine its electronic geometry. I3 PRACTICE 3: Draw the following compound and determine its electronic geometry. 2 S PRACTICE 4: Draw the following compound and determine its electronic geometry. PO 4 3- Page 3

4 CONCEPT: MOLECULAR GEOMETRY When drawing a compound you have to take into account two different systems of geometrical shape. With the molecular geometry you treat lone pairs (nonbonding electrons) and surrounding elements as different. X A X X Key: A = Central Element X = Surrounding Element E = Lone Pair 2 Groups 3 Groups O C O C N AX 2 - Linear B AX 3 - Trigonal Planar Sn AX 2 E 1 - Bent, Angular or V-Shaped 4 Groups C AX 4 - Tetrahedral N AX 3 E 1 - Trigonal Pyramidal O AX 2 E 2 - Bent, Angular or V-Shaped 5 Groups P AX 5 - Trigonal Bipyramidal S AX 4 E 1 - Seesaw AX 3 E 2 - T-Shaped Xe AX 2 E 3 - Linear 6 Groups S AX 6 - Octahedral S AX 5 E 1 - Square Pyramidal Xe AX 4 E 2 - Square Planar Page 4

5 PRACTICE: MOLECULAR GEOMETRY EXAMPLE: Draw each of the following compounds and determine their molecular geometries. P 2 Xe 2 PRACTICE 1: Draw the following compound and determine its molecular geometry. OBr2 PRACTICE 2: Draw the following compound and determine its molecular geometry. SO 4 2- Page 5

6 CONCEPT: IDEALIZED BOND ANGLES According to the (VSEPR) model bond and lone electron pairs will position themselves around the central element so that they are as far apart as possible. Page 6

7 PRACTICE: IDEALIZED BOND ANGLES EXAMPLE: Determine the bond angles of each of the following compounds. CO 2 Br 4 + PRACTICE 1: Determine the bond angle of the following compound. As5 PRACTICE 2: Determine the bond angle of the following compound. I3 Page 7

8 CONCEPT: YBRIDIZATION Covalent bonds are formed when atomic orbitals on different atoms overlap and electrons are shared. But what happens when we need to form covalent bonds with different atomic oribitals, for example Be2? Be 2 [e]2s 2 [Ne]3s 2 3p 5 2s 2p 2s 2p Promotion 2s & ybridization 2p sp 2p Be Page 8

9 PRACTICE: YBRIDIZATION EXAMPLE: or each of the given covalent compounds draw out the Lewis Structure and answer the questions C22 ybridization: Xe5 + ybridization: Unhybridized Orbitals: Bonding orbitals (C ): Unhybridized Orbitals: Bonding orbitals (Xe ): PRACTICE 1: or the given covalent compound draw out the Lewis Structure and answer the questions. I2 ybridization: Unhybridized Orbitals: Bonding orbitals (I ): PRACTICE 2: or the given covalent compound draw out the Lewis Structure and answer the questions. C3 + ybridization: Unhybridized Orbitals: Bonding orbitals (C ): Page 9

10 CONCEPT: MO TEORY In the molecular orbital theory electrons are seen as being, or spread out over a molecule instead of concentrated in a covalent bond. A(n) orbital is the region of high electron density between nuclei where a bond forms. A(n) orbital is the region that has zero electron density (a node) between the nuclei where a bond can t form. EXAMPLE: Use a MO diagram to write the electron configuration of each of the following: a. C2 2- b. 2 + Page 10

11 PRACTICE: MO TEORY The MO diagram can be connected to the MO bond order: Bond Order = 1 2 (# of e in bonding MO # of e in anti-bonding MO) A bond order zero means that the compound is stable and exists. A bond order zero means the compound is unstable and does not exist. In general, the the bond order, the the bond. PRACTICE: Use a MO diagram to determine if the following compound exists or not. a. O2 2- b. B2 - Page 11

12 CONCEPT: ETERONUCLEAR DIATOMIC MOLECULES Molecular Orbital Theory can also be applied to heteronuclear diatomic molecules, which are composed of two different elements covalently bonded together. The key differences it has from a homonuclear diatomic molecule include: The electronegative element determines the particular MO diagram used. The more electronegative element will possess atomic orbitals that are in energy. The energy orbital contributes more to the bonding molecular orbital, while the energy orbital contributes more to the antibonding molecular orbital. Atomic Orbitals Molecular Orbitals Atomic Orbitals Atomic Orbitals Molecular Orbitals Atomic Orbitals s* 2p s* 2p p* 2p p* 2p 2p 2p 2p 2p s 2p p 2p p 2p s 2p s* 2s s* 2s 2s 2s 2s 2s ydrogen s 2s Nitrogen Oxygen s 2s Neon EXAMPLE: Using your knowledge of molecular orbital diagrams, determine the bond order of the NO ion. Page 12