Chapter 6 – Covalent Compounds Section 3 – Molecular Shapes

Determining Molecular Shapes Lewis structures are two-dimensional and do not show the three dimensional shape of a molecule. However, the three-dimensional shape of a molecule is important in determining the molecule’s physical and chemical properties.

Chapter 6 – Covalent Compounds Section 3 – Molecular Shapes A Lewis Structure Can Help Predict Molecular Shape There is a model based on the valence shell electron pair repulsion (VSEPR) theory Using this model, you can predict the shape of a molecule by examining the Lewis structure of the molecule. Unshared electrons are called lone pair electrons and influence the shape of a molecule. Lone pair electrons will push bonded pair electrons away from themselves.

Chapter 6 – Covalent Compounds Section 3 – Molecular Shapes Shared Pairs Lone PairShape of Molecule FormulaExample LinearAB 2,A 2,ABCO 2 F 2 HBr BentAB 2 H2OH2O Triginal Planer AB 3 BCl 3 Triginal Pyramidal AB 3 NH 3 Tetrahedral AB Only two atoms bonded Or double bonds to side atoms as in CO2 CCl 4

Chapter 6 – Covalent Compounds Section 3 – Molecular Shapes NameShape of MoleculeNameShape of Molecule HBrNH 3 CF 4 BI 3 SiO 2 CCl 4 BF 3 H2OH2O CO 2 N2N2 H2SH2SPCl 3 SCl 2 AsI 3 BBr 3 H 2 Te Linear Tetrahedral LinearTetrahedral Bent Triginal Planer Bent Triginal Planer Linear TriginalPyramidal

Chapter 6 – Covalent Compounds Section 3 – Molecular Shapes Molecular Shape Affects a Substance’s Properties The polarity of a molecule that has more than two atoms depends on the polarity of each bond and the way the bonds are arranged in space. If the bonds create a molecule that is symmetrical the partial polar charges (  ) will cancel each other out. This will create a molecule that has polar covalent bonds but the molecule overall will be nonpolar.

Chapter 6 – Covalent Compounds Section 3 – Molecular Shapes O=C=O Oxygen has a higher electronegativity than carbon and therefore should have a negative  and carbon a positive . -- -- ++ However because CO 2 is symmetrical the  will cancel each other out and make the molecule nonpolar. (without poles)

Chapter 6 – Covalent Compounds Section 3 – Molecular Shapes O H Place the partial polar charges on H 2 O. Look up electronegativity. ++ -- ++ Look to see if H 2 O symmetrical. H 2 O is not symmetrical, therefore has polar bonds and the molecule remains polar.

Chapter 6 – Covalent Compounds Section 3 – Molecular Shapes The stronger the poles of a molecule the higher the melting and boiling point of the molecule. To determine the strength of the pole find the electronegativity difference. Which of the following solids has the highest melting point? 1. H 2 O( s ) 2. Na 2 O( s ) 3. SO 2 ( s ) 4. CO 2 ( s )

Chapter 6 – Covalent Compounds Section 3 – Molecular Shapes The electrons in a bond between two iodine atoms (I 2 ) are shared 1. equally, and the resulting bond is polar 2. equally, and the resulting bond is nonpolar 3. unequally, and the resulting bond is polar 4. unequally, and the resulting bond is nonpolar Which electron-dot formula represents a polar molecule?

Chapter 6 – Covalent Compounds Section 3 – Molecular Shapes Which structural formula represents a nonpolar molecule? Which structural formula represents a dipole?

Chapter 6 – Covalent Compounds Section 3 – Molecular Shapes Which type of molecule is CF 4 ? 1. polar, with a symmetrical distribution of charge 2. polar, with an asymmetrical distribution of charge 3. nonpolar, with a symmetrical distribution of charge 4. nonpolar, with an asymmetrical distribution of charge Which formula represents a nonpolar molecule? 1. CH 4 2. HCl 3. H 2 O 4. NH 3