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Covalent Bonding Sec. 8.4: Molecular shape

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Objectives n Define hybridization n Discuss the VSEPR bonding theory n Predict the shape of and the bond angles in a molecule

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Hybridization n When 2 of the same type of object combine, a hybrid results that has characteristics of both objects. n During bonding, orbitals undergo hybridization. –Hybridization is a process in which atomic orbitals are mixed to form new, identical hybrid orbitals.

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Hybridization n For example, think about the carbon atom. Its 4 valence electrons are located in the s and p orbitals. n In reality, the s and p orbitals hybridize. n All 4 valence electrons, instead of being located in separate s and p orbitals, are found in 4 orbitals that are identical in shape to each other. These are called hybrid orbitals. hybrid orbitals. hybrid orbitals.

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Molecular Shape n The number and size of the hybrid orbitals (which overlap with those of another atom to form a bond) determines the location of atoms relative to each other in a compound. n The location of the atoms in space, called molecular geometry or molecular shape, determines many physical and chemical properties of compounds.

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VSEPR Model n Many theories have been developed to explain and predict the shape of molecules. n alence Shell lectron air epulsion model uses Lewis structures to determine molecular geometry (shape). n The Valence Shell Electron Pair Repulsion model uses Lewis structures to determine molecular geometry (shape).

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VSEPR Model n Molecular shape is determined by the overlap of hybrid orbitals that are sharing electrons. n Atoms/orbitals assume a shape that minimizes the repulsion of shared and unshared pairs of electrons around the central atom.

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VSEPR Model n Orbitals containing shared pairs (covalent bonds) repel one another. n Orbitals with lone pairs also repel one another n In addition, lone pairs repel shared pairs and push shared pairs closer to each other (because of the relatively large orbitals of the lone pairs) n Consider an analogy...

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VSEPR Model n Repulsions of electron pairs for each other results in atoms being at fixed angles to each other. –The angle formed by 2 terminal atoms and the central atom is a bond angle. n Table 6 (pg. 263) summarizes molecular shapes and angles predicted by the VSEPR theory.

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Molecular shape: Linear The central atom has no lone pairs present. The central atom has 2 bonding pairs Maximum separation is attained at a bond angle of 180 0. Ex. BeCl 2

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Molecular shape: Linear Double and triple bonds have a sigma and 2 pi bonds. Only the sigma bonds occupy hybrid orbitals. Thus, a linear configuration is seen between atoms with double & triple bonds.

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Trigonal Planar n There are no lone pairs present on central atom. n There are 3 bonding pairs present. –Maximum separation is attained at bond angles of 120 0 Ex. AlCl 3

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Tetrahedral n No lone pairs are present on the central atom. n 4 bonding pairs are present. –Maximum separation is attained at bond angles of 109.5 0 Ex. CH 4

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Trigonal Pyramidal n 1 lone pair is present on the central atom. n 3 bonding pairs are present –A lone pair takes up more space than a bonding pair. –The bonding pairs are pushed closer together as a result. –Bond angles are 107.3 0

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Bent n 2 lone pairs are present on the central atom. n 2 bonding pairs are also present. –Lone pairs take up more space than bonding pairs. –Bond angles are 104.5 0

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Trigonal Bipyramidal n No lone pairs are present n 5 bonding pairs are present –Bond angles are 90 0 vertical to horizontal –Bond angles are 120 0 horizontal to horizontal

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Octahedral n No lone pairs are present n 6 bonding pairs are present –Bond angles are 90 0

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More on hybridization... n Recall that carbon has 4 valence electrons: [He] 2s 2 2p 2 n The electrons in the s orbital unpair: [He] 2s 1 2p 3 n Therefore, the hybrid orbitals that are formed in bonding come from the 1 “s” and 3 “p” orbitals. n The 4 identical hybrid orbitals formed are called sp 3. CH 4 (Methane)

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Hybridization n Notice that the number of atomic orbitals that hybridize equals the total number of bonding pairs of electrons –CH 4 has 4 hybrid orbitals for the 4 bonding pairs, formed from a mix of 1 “s” and 3 “p” orbitals

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Hybridization n Lone pairs also occupy hybrid orbitals. n H 2 O has 4 electron pairs available for bonding - 2 lone pairs and 2 shared pairs n 4 hybrid orbitals are formed - all sp 3 Review hybridizations given on pg. 263

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Practice Problems n Determine the shape, bond angles, and hybridization of –PH 3 –BF 3 –NH 4 + –OCl 2 –KrF 2

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