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The VSEPR model can be extended to molecules by treating atoms in succession as central atoms.

Shapes of Larger Molecules. The VSEPR model can be extended to molecules by treating atoms in succession as central atoms. trigonal planar geometry, bond angle is >120° (due to double bond). tetrahedral geometry, bond angle is 109.5°. bent geometry, bond angle is <109.5°.

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The VSEPR model can be extended to molecules by treating atoms in succession as central atoms.

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  1. Shapes of Larger Molecules The VSEPR model can be extended to molecules by treating atoms in succession as central atoms. trigonal planar geometry, bond angle is >120° (due to double bond) tetrahedral geometry, bond angle is 109.5° bent geometry, bond angle is <109.5°

  2. Shapes of Larger Molecules

  3. Shapes of Larger Molecules The VSEPR model can be extended to molecules by treating atoms in succession as central molecules. bent geometry, bond angle is <109.5° tetrahedral geometry, bond angle is 109.5°

  4. Shapes of Larger Molecules The VSEPR model can be extended to molecules by treating atoms in succession as central molecules. linear geometry, 180° H | H — C — C ≡ C — H propyne | H tetrahedral geometry, 109.5°

  5. Determining the Polarity of a Molecule • The polarity of a molecule dictates many important physical properties and also much of the chemical behavior. • Determining the polarity of a molecule • no polar bonds: molecule is nonpolar • 1 polar bond: molecule is polar • 2 or more polar bonds: polarity is a function of the geometry of the polar bonds in the molecule. • First we will see how to determine if a bond is polar.

  6. Bond Polarity and Dipole Moments Combining the shape of the molecule with the polarities of the bonds lets us determine whether the molecule is polar or nonpolar. Bond Polarity – A measure of how equally the electrons in a bond are shared. Equal sharingnonpolar bond Unequal sharingpolar bond Dipole moment (μ) – A quantitative measure of the amount of charge separation in a bond or molecule. μ = δ d where δ is the amount of partial charge and d is the bond length.

  7. Dipole Moments Dipole moment – A quantitative measure of the amount of charge separation in a bond or molecule. Electronegativities can be used to determine the direction of the dipole moment in a bond. The arrow points to the more electronegative element. C 2.5 N 3.0 O 3.5 F 4.0 H 2.1 O—H C≡N C—O C=O N—H C—H H—H H—F The C-H bond is considered to be nonpolar.

  8. Determining the Polarity of a Molecule no polar bonds: molecule is nonpolar 1 polar bond: molecule is polar 2 or more polar bonds: polarity is a function of the geometry of the polar bonds in the molecule.

  9. Case 1: No polar bonds Molecules with only C and H atoms are nonpolar. H | H — C — C ≡ C — H propyne is nonpolar | H

  10. Case 2: One polar bond Almost any heteronuclear diatomic molecule will be polar.

  11. Case 3a: Two or more polar bonds symmetrically arranged • In molecules with three or more atoms, the dipole moment of the molecule depends on the • polarities of the individual bonds, and • geometry of the molecule. A molecule can have polar bonds but no overall dipole moment if the molecular symmetry causes the dipoles to cancel each other.

  12. Case 3a: Two or more polar bonds symmetrically arranged A molecule can have polar bonds but no overall dipole moment if the molecular symmetry causes the dipoles to cancel each other.

  13. Case 3b: Two or more polar bonds that are not symmetrically arranged Polar

  14. Case 3b: Two or more polar bonds that are not symmetrically arranged δ- δ+ δ+ acetic acid is polar δ- The dipole moment will point toward the O atoms, so the O atoms will each have a partial negative charge.

  15. Molecular Properties that Depend on Polarity Nonpolar compounds Polar compounds • Intermolecular attractions significant • dipole-dipole attractions • hydrogen bonds • Higher melting points • Higher boiling points • Will dissolve other polar compounds • Can dissolve ionic compounds • Will not dissolve in nonpolar compounds • Much smaller intermolecular attractions • instantaneous dipoles • (London forces) • Lower melting points • Lower boiling points • Will dissolve other nonpolar compounds • Will not dissolve in polar compounds

  16. Detergents – A Bridge Between Polar and Nonpolar Compounds The rule for solutions is “like dissolves like.” Soaps and detergents allow water to dissolve oils and greases by having a polar end (also called hydrophilic or water-loving) and a nonpolar end (also called hydrophobic or water-fearing). The polar end dissolves in water and the nonpolar end dissolves the oil or grease. sodium stearate CH3(CH2)16COONa O ║ CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CO- Na+

  17. Detergents – A Bridge Between Polar and Nonpolar Compounds

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