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General, Organic, and Biological Chemistry Fourth Edition Karen Timberlake 5.8 Shapes and Polarity of Molecules Chapter 5 Compounds and Their Bonds © 2013 Pearson Education, Inc. Lectures

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 2 Valence-Shell Electron-Pair Repulsion Theory (VSEPR) In the valence-shell electron-pair repulsion (VSEPR) theory, the electron groups around a central atom are arranged as far apart from each other as possible. have the least amount of electron-electron repulsion. are used to predict the molecular shape.

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 3 Shapes of Molecules The three-dimensional shape of a molecule can be predicted by the number of bonding groups and lone-pair electrons around the central atom VSEPR theory (valence-shell-electron-pair repulsion).

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 4 Two Electron Groups In a molecule of BeCl 2, there are two electron groups bonded to the central atom, Be (Be is an exception to the octet rule). to minimize repulsion, the arrangement of two electron groups is 180 , or opposite each other. the shape of the molecule is linear.

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 5 Three Electron Groups In a molecule of BF 3, three electron groups are bonded to the central atom B (B is an exception to the octet rule). repulsion is minimized with 3 electron groups at angles of 120 . the shape is trigonal planar.

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 6 Three Electron Groups with a Lone Pair In a molecule of SO 2, S has 3 electron groups; 2 electron groups bonded to O atoms and one lone pair. repulsion is minimized with the electron groups at angles of 120 , a trigonal planar arrangement. the shape is determined by the two O atoms bonded to S, giving SO 2 a bent (~120 ) shape.

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 7 Four Electron Groups In a molecule of CH 4, there are four electron groups around C. repulsion is minimized by placing four electron groups at angles of 109 , which is a tetrahedral arrangement. the four bonded atoms form a tetrahedral shape.

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 8 Four Electron Groups with a Lone Pair In a molecule of NH 3, three electron groups bond to H atoms, and the fourth one is a lone (nonbonding) pair. repulsion is minimized with 4 electron groups in a tetrahedral arrangement. the three bonded atoms form a pyramidal (~109 ) shape.

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 9 Four Electron Groups with Two Lone Pairs In a molecule of H 2 O, two electron groups are bonded to H atoms and two are lone pairs (4 electron groups). four electron groups minimize repulsion in a tetrahedral arrangement. the shape with two bonded atoms is bent (~109 ).

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 10 Molecular Shapes for 2 and 3 Bonded Atoms

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 11 Molecular Shapes for 4 Bonded Atoms

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 12 Guide to Predicting Molecular Shape (VSEPR Theory)

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 13 Predicting the Molecular Shape of H 2 Se Step 1 Draw the electron-dot formula. In the electron-dot formula for H 2 Se, there are four electron groups, including two lone pairs of electrons around Se.

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 14 Predicting the Molecular Shape of H 2 Se Step 2 Arrange the electron groups around the central atom to minimize repulsion. The four electron groups around Se would have a tetrahedral arrangement. Step 3 Use the atoms bonded to the central atom to determine the molecular shape. Two bonded atoms give H 2 Se a bent shape with a bond angle of ~109 .

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 15 Learning Check What is the shape of a molecule of NF 3 ? A. linear B. trigonal pyramidal C. bent (120 )

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 16 Solution What is the shape of a molecule of NF 3 ? B. trigonal pyramidal Step 1 Draw the electron-dot formula. In the electron-dot structure for NF 3, there are three electron groups and a lone pair.

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 17 Solution What is the shape of a molecule of NF 3 ? B. trigonal pyramidal Step 2 Arrange the electron groups around the central atom to minimize repulsion. The four electron groups around N would have a tetrahedral arrangement. Step 3 Use the atoms bonded to the central atom to determine the molecular shape. Three bonding groups and a lone pair give NF 3 a trigonal pyramidal shape with a bond angle of 109 .

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 18 Polar Molecules A polar molecule contains polar bonds. has a separation of positive and negative charge. called a dipole, indicated with + and –. has dipoles that do not cancel. + – H–Cl NH 3 dipole Dipoles do not cancel.

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 19 Nonpolar Molecules A nonpolar molecule contains nonpolar bonds Cl–Cl H–H or has a symmetrical arrangement of polar bonds.

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 20 Guide to Determination of Polarity

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 21 Molecular Polarity, H 2 O Determine the polarity of the H 2 O molecule. Step 1 Determine if the bonds are polar covalent or nonpolar covalent. From the electronegativity table, O 3.5 and H 2.1 gives a difference of 1.4, which makes the O — H bonds, polar covalent.

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 22 Molecular Polarity, H 2 O Determine the polarity of the H 2 O molecule. Step 2 If the bonds are polar covalent, draw the electron-dot formula and determine if the dipoles cancel or not. The four electron groups of oxygen are bonded to two H atoms. Thus, the H 2 O molecule has a net dipole, which makes it a polar molecule.

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 23 Learning Check Determine the shape of each of the following molecules and whether they are polar or nonpolar. Explain. 1. PBr 3 2. HBr 3. Br 2 4. SiBr 4

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© 2013 Pearson Education, Inc. Chapter 5, Section 8 24 Solution Determine the shape of each of the following molecules and whether they are polar or nonpolar. Explain. 1. PBr 3 pyramidal; polar; dipoles don’t cancel 2. HBr linear; polar; one polar bond (dipole) 3. Br 2 linear; nonpolar; nonpolar bond 4. SiBr 4 tetrahedral; nonpolar; dipoles cancel

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