Shape of Molecules and their Interactions Chapter 3
Lewis Structures • Predicts what molecules look like in three dimension. • The 3-Dimensional structure determines the physical properties such as B.P., M.P., miscibility, vapor pressure, heat of evaporation, etc. • Determined by the number of atoms and lone pairs connected to the central atom but not the number of bonds. • Atoms want to be as far away from each other as possible to minimize repulsion. (VSEPR)
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Electronic Geometries Number of Atoms Connected
Shape
Geometry
2 AX2
Linear
3 AX3
Trigonal Planar
4 AX4
Tetrahedral
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Electronic Geometry • SO2
• SO3 -2
Shapes of Molecules - Based on what the bonds look like. Cover the lone pairs and look at the bonds. 4
Electronic Geometry • SO2
• SO3 -2
Shapes of Molecules - Based on what the bonds look like. Cover the lone pairs and look at the bonds. 5
Molecular Geometries (Shape) Number of Atoms and Lone Pairs Connected
Shape
Geometry
3 AX3
Trigonal Planar
3 AX2E
Bent
Molecular Geometries (Shape) Number of Atoms and Lone Pairs 4 AX4 4 AX3E
Shape of Larger Molecules • For the molecule below predict the electronic geometry and the molecular geometry (Shape) for each central atom
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Predicting Type of Covalent Bond When the sharing of electrons is equal the bond is considered non polar covalent. • Unequal sharing of electrons generates a polar covalent bond. (Dipole) • Polar covalent bonds might result in polar molecules.
Electronegativity • Tendency of some atoms to pull electrons toward themselves. • Right side of the periodic table will have the largest electronegativity. • Top and right side contains the most selfish electron huggers. • F, O, and N are the three most electronegative elements.
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Electronegative
Polar Covalent Bond • Atoms with different electronegativities result in a polar covalent bond. • C-H is considered a non polar bond • In a polar covalent bond, the unequal sharing of electrons gives the bond a partially positive and partially negative end (dipole).
Think of a magnet
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Polar Molecules • Polarity in a molecule depends on its molecular geometry. • Perfect geometries are non polar. • Presence of one lone pair around the central atom makes the molecule polar. • CO2 vs. SO2
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Let’s Try It! • Are the molecules polar or not? • A) CH2Cl2
a) A only
b) B only
c) Both
d) neither
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Molecule Polarity H2O
The H-O bond is polar. The both sets of bonding electrons are pulled toward the O end of the molecule. The net result is a polar molecule.
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Noncovalent Interactions
Intermolecular Forces
Intermolecular Forces • Weaker forces than covalent bonds that are responsible for: • Physical state of substance • Boiling and melting points • Viscosity, solubility, density • 3-D structure and function of proteins and DNA
Types of Intermolecular Forces • Ion-Ion Interaction – ▫ Present in all ionic compounds. ▫ Strongest of all types of intermolecular forces.
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Types of Intermolecular Forces • Dipole-Dipole ▫ Present in polar molecules ▫ A polar molecule could be represented as a small dipole ( tiny magnet)
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Types of Intermolecular Forces • Ion-Dipole Interaction ▫ Present when ionic compounds are in solution.
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Types of Intermolecular Forces • Hydrogen Bond ▫ Present in F, O, and N when they have a hydrogen directly attached to them due to the very polar bond. ▫ The hydrogen bond is formed between a H attached to an F, O, or N and the F, O, or N of a neighboring molecule. ▫ Stronger than dipole-dipole interaction.
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The Hydrogen Bond
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Hydrogen Bonding • The “stickiness" of water molecules, resulting in beads of
water is due to hydrogen bonds.
• They also cause surface tension in water. (insects walking
on water)
• Their order in solid water (ice) results in a lower density, so
ice floats on water; other solid states generally sink in their liquid states.
• Water being a liquid and boil at 100 C, while similarly sized
methane is a gas.
• Water controls global temperatures. • Read Hydrogen bond in DNA
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London Dispersive Forces aka Induced Dipole-Induced Dipole Forces • Important for nonpolar molecules. • Compare the physical state of the halogens: ▫ F2
Cl2
Br2
I2
• Something has to be present that makes the molecules feel some form of intermolecular attraction
London Dispersion Forces • Attraction is due to instantaneous, temporary dipoles formed due to electron motions.
dispersion forces increase when the number of electrons in a molecule increases, and the surface area of a molecule increases Larger the molecule, more dominant London dispersive forces become (stronger). 25
Intermolecular Forces • Is it ionic? • yes-IONIC ; in solution? Yes (ion-dipole) ; no (ion-ion • No-COVALENT; polar? Yes (dipole-dipole); • No; London dispersion • H-bonding?
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Molecular Polarity Affects Solubility in Water • polar molecules are attracted to other polar molecules • since water is a polar molecule, other polar molecules dissolve well in water ▫ and ionic compounds as well • some molecules have both polar and nonpolar parts
