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Polarity in Covalent Bonds

Learn about polar and nonpolar covalent bonds, bond polarity, and how to predict the polarity of a bond. Understand the difference between symmetric and asymmetric molecules and their impact on molecular polarity.

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Polarity in Covalent Bonds

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  1. Bonding Movie Polarity in Covalent Bonds

  2. Polar Bond • Polar – has poles • North pole / South pole = magnetic poles • The ends are different • In bonds: the difference is in how the electrons are distributed • Polar bond: one end has more electrons than the other end

  3. Nonpolar Bond • Nonpolar = No poles • The electron cloud on one end of the bond is the same as the other end

  4. H2 is symmetric: Both ends are the same; electron cloud is football-shaped HCl is asymmetric: electron cloud is lopsided; Cl has more electrons than H

  5. Which bond(s) are polar? Which are nonpolar? Polar = LiH and HF Nonpolar = H2 Red = electron rich Blue = electron poor

  6. How can you predict if a bond is polar or nonpolar? Compare the electronegativities of the two atoms in the bond

  7. Found inTable S! Electronegativity • Ability of an atom to attract electrons in a bond

  8. Electronegativities of the Elements

  9. Bond Polarity • The more electronegative atom pulls the bonding electrons more strongly and gets more than its fair share of the electron cloud • Leads to separation of charge • electron-rich side is partially negative (-) • electron-poor side is partially positive (+)

  10. Delta notation

  11. Bond Polarity • Depends on the difference in electronegativity between two atoms in the bond A B EA - EB  • - care only aboutthe size of thedifference, not the • sign • - bigger the difference, the more polar the bond

  12. 2.1 – 1.0 1.1 2.1 – 2.1 0 2.1 – 4 1.9 LiH : H2 : HF: Calculate the electronegativity difference for each of these bonds

  13. Bond Polarity • What can you say about the polarity of any bond where atom A = atom B? If both atoms in the bond are the same element: the electronegativity difference = 0

  14. ElectronegativityDifference Type of Bond 0.0 to 0.4 Nonpolar Covalent 0.5 to 1.7 Polar Covalent > 1.7 Ionic Electronegativity Difference

  15. Electronegativity & Bond Type

  16. 2.1 – 3.2 = -1.1 = 1.1 Review • Which of the following bonds is the most polar? • A) O2 • B) HCl • C) NH in NH3 • D) HBr 3.4 – 3.4 = 0 3.0 – 2.1 = 0.9 3.0 – 2.1 = 0.9 Answer = (B) HCl

  17. 0.8 – 3.2 = -2.4 = 2.4 2.1 – 3.2 = -1.1 = 1.1 3.2 – 3.2 = 0 2.6 – 3.2 = -0.6 = 0.6 Answer = (A) KCl Review • Which substance contains a bond with the greatest ionic character? • A) KCl • B) HCl • C) Cl2 • D) CCl4

  18. Summary • Nonpolar covalent bonds form: • between atoms having equal or closeelectronegativity values: 0.0 - 0.5 • Polar covalent bonds form: • between atoms with an electronegativity difference between 0.5 and 1.7

  19. Polar Polar Nonpolar Polar Polar Predict the Polarity • N2 • HF • HCl • O2 • Cl2 • HI • HBr Nonpolar Nonpolar

  20. Molecular Polarity • Diatomic molecules: • molecular polarity is same as bond polarity • Molecules with 3 or more atoms: • polarity depends on: • types of bonds and • shape of the molecule

  21. Molecular Polarity • Depends on 2 factors 1.Type of bonds in the molecule 2. Arrangement of bonds or shape of molecule Think SYMMETRY!!

  22. For larger molecules: • look at the kind & arrangement of bonds to determine overall polarity of molecule

  23. Polarity of Molecules • A molecule may contain polar bonds, but not be polar! • Depends on the geometry of the molecule • If molecule is symmetric: • the “pull” of one polar bond is offset by the “pull” of another polar bond • It’s a tug-of-war that no one can win!

  24. Symmetric Molecules • Contain at least one mirror plane

  25. Symmetric vs. Asymmetric CO2 is nonpolar - electron cloud is symmetric H2O is polar - electron cloud is lop-sided (asymmetrical)

  26. Polarity of CO2? Draw an arrow along each bond pointing to the more electronegative atom. If the arrows cancel out, the molecule is NONPOLAR

  27. Polarity of H2O? The green arrows do not cancel out - water is polar!

  28. Use the structural formula to predict Molecular Polarity! H H  C  H H  

  29. Symmetry of Larger Molecules CF4 is fairly symmetric so it is nonpolar

  30. Ethane = C2H6 Ethene = C2H4 Ethyne = C2H2 These molecules are symmetric and the electron cloud is the same on both ends: they are nonpolar

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