Chapter 4 Compounds and Their Bonds 4.7 Shapes and Polarity of Molecules 1 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh.

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Presentation transcript:

Chapter 4 Compounds and Their Bonds 4.7 Shapes and Polarity of Molecules 1 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

VSEPR Theory Valence Shell Electron-Pair Repulsion Theory (VSEPR) describes the orientation of electron groups around the central atom states that electron groups are arranged as far apart as possible around the central atom states that specific shape of a molecule is determined by the number of atoms attached to the central atom 2 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

Central Atom with Two Electron Groups In a molecule of BeCl 2 there are only two electron groups around the central atom, Be, which is an exception to octet rule repulsion is minimized by placing the two groups on opposite sides of the Be atom, giving this a linear arrangement with bond angles of 180 ° the shape with two electron groups around the central atom is linear 3 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

Central Atom with Three Electron Groups In a molecule of BF 3 there are only three electron groups around the central atom, B, which is an exception to octet rule repulsion is minimized by placing the three groups as far apart as possible at bond angles of 120 ° the shape with three electron groups around the central atom is trigonal planar 4 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

Central Atom with Three Electron Groups In a molecule of SO 2 there are three electron groups around the central atom S: a single-bonded O atom, a double-bonded O atom, and a lone pair of electrons repulsion is minimized by placing the three groups as far apart as possible since one group is a lone pair, the shape is determined by the two O atoms bonded to S, giving the molecule its bent shape 5 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

Central Atom with Four Electron Groups In a molecule of CH 4 there are four electron groups attached to H atoms around the central atom, C repulsion is minimized by placing the four groups at corners of a tetrahedron with bond angles of 109 ° the shape with four bonds on the central atom is called tetrahedral 6 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

Central Atom with Four Electron Groups In a molecule of NH 3 there are three electron groups attached to H atoms and a lone pair around the central atom, N repulsion is minimized by placing the four groups at corners of a tetrahedron with bond angles of 109 ° the shape with three bonds on the central atom is called trigonal pyrimidal 7 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

Central Atom with Four Electron Groups In a molecule of H 2 O there are two electron groups attached to H atoms and two lone pairs around the central atom, O repulsion is minimized by placing the four groups at corners of a tetrahedron with bond angles of 109 ° the shape with two bonds on the central atom is called bent 8 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

Molecular Shapes for Molecules 9 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

Guide to Predicting Molecular Shape 10 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

Predict Molecular Shape of H 2 S Predict the shape of H 2 S. Step 1Draw the electron-dot formula. Step 2Arrange electron groups around the central atom to minimize repulsion. To minimize repulsion, electron groups have a tetrahedral arrangement. Step 3Use the atoms bonded to the central atom determine the molecular shape. With two bonds and two lone pairs, the shape is bent. 11 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

Learning Check State the number of electron groups and lone pairs and use VSEPR theory to determine the shape of the following molecules or ions as tetrahedral, trigonal pyramidal, or bent. A. PF 3 B. H 2 S C. CCl 4 12 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

Solution State the number of electron groups and lone pairs and use VSEPR theory to determine the shape of the following molecules or ions as tetrahedral, trigonal pyramidal, or bent. Step 1 Draw the electron-dot formula. A. PF 3 B. H 2 SC. CCl 4 13 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

Solution State the number of electron groups, and lone pairs and use VSEPR theory to determine the shape of the following molecules or ions as tetrahedral, trigonal pyramidal, or bent. Step 2Arrange electron groups around the central atom to minimize repulsion. A.PF 3 To minimize repulsion electron groups have a tetrahedral arrangement. B.H 2 STo minimize repulsion electron groups have a tetrahedral arrangement. C.CCl 4 To minimize repulsion electron groups have a tetrahedral arrangement. 14 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

Solution State the number of electron groups and lone pairs and use VSEPR theory to determine the shape of the following molecules or ions as tetrahedral, trigonal pyramidal, or bent. Step 3Use the atoms bonded to the central atom to determine the molecular shape. A.PF 3 With three bonds and one lone pair, the shape is trigonal planar. B.H 2 SWith two bonds and two lone pairs, the shape is bent. C.CCl 4 With four bonds the shape is tetrahedral. 15 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

Nonpolar Molecules Nonpolar molecules such as H 2, Cl 2, and O 2 are nonpolar because they contain nonpolar bonds with polar bonds such as CO 2 can be nonpolar if the polar bonds cancel each other in a symmetrical arrangement dipoles cancel out, which makes the molecule nonpolar 16 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

Polar Molecules Polar molecules such as HCl are polar because one end of the molecule is more negatively charged than the other when polar bonds in the molecule do not cancel each other because the electrons are shared unequally in the polar covalent bond 17 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

Polar Molecules In polar molecules with three or more atoms, the shape of the molecule determines whether the dipoles cancel or not there are often lone pairs around the central atom such as H 2 O, the dipoles do not cancel, making the molecule positive at one end and negative at the other end such as H 2 O, there is a dipole on the central atom 18 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

Learning Check Identify each of the following molecules as polar or nonpolar. Explain. A. PBr 3 B. HBr C. CF 4 19 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.

Solution Identify each of the following molecules as polar or nonpolar. Explain. Number BondsLone PairsGeometry A.PBr 3 31trigonal pyrimidal The dipoles do not cancel; the molecule is polar. B.HBr 10linear The bond has a dipole; the molecule is polar. C.CF 4 40tetrahedral The bonds have dipoles; the dipoles cancel and the molecule is nonpolar. 20 Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc.