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AP Chemistry Unit 4 – Molecular Structure

AP Chemistry Unit 4 – Molecular Structure. Lesson 3 – Lewis Structures for Molecular Compounds Book Section: 8.3-8.5, 8.7. Covalent Bonding. In covalent bonds atoms share electrons. There are several electrostatic interactions in these bonds: Attractions between electrons and nuclei

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AP Chemistry Unit 4 – Molecular Structure

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  1. AP ChemistryUnit 4 – Molecular Structure Lesson 3 – Lewis Structures for Molecular Compounds Book Section: 8.3-8.5, 8.7

  2. Covalent Bonding • In covalent bonds atoms share electrons. • There are several electrostatic interactions in these bonds: • Attractions between electrons and nuclei • Repulsions between electrons • Repulsions between nuclei

  3. Polar Covalent Bonds • Though atoms often form compounds by sharing electrons, the electrons are not always shared equally. • Fluorine pulls harder on the electrons it shares with hydrogen than hydrogen does. • Therefore, the fluorine end of the molecule has more electron density than the hydrogen end.

  4. Polar Covalent Bonds • When two atoms share electrons unequally, a bond dipole results. • The dipole moment, μ, produced by two equal but opposite charges separated by a distance r, is calculated: μ = Qr • It is measured in debyes (D).

  5. Polar Covalent Bonds • The greater the difference in electronegativity, the more polar is the bond.

  6. 1999 MC #40 • Of the following molecules, which has the largest dipole moment? • CO • CO2 • O2 • HF • F2

  7. 1999 MC #40 • Of the following molecules, which has the largest dipole moment? • CO • CO2 • O2 • HF – 54% correct, medium • F2

  8. Lewis Structures • Lewis structures are representations of molecules showing all electrons, bonding and nonbonding.

  9. Writing Lewis Structures • 1) Find the sum of valence electrons of all atoms in the polyatomic ion or molecule. • If it is an anion, add one electron for each negative charge. • If it is a cation, subtract one electron for each positive charge. PCl3 5 + 3(7) = 26

  10. Writing Lewis Structures • 2) The central atom is the least electronegative element that isn’t hydrogen. Connect the outer atoms to it by single bonds. • Keep track of the electrons (1 bond = 2 electrons) • 26 – 6 = 20 e-

  11. Writing Lewis Structures • 3) Fill the octets of the outer atoms. • Keep track of the electrons: • 26 – 6 = 20 e- • 20 – 18 = 2 e-

  12. Writing Lewis Structures • 4) Fill the octet of the central atom. • Keep track of the electrons: • 26 – 6 = 20 e- • 20 – 18 = 2 e- • 2 – 2 = 0 e-

  13. Writing Lewis Structures • 4) Fill the octet of the central atom. • Keep track of the electrons: • 26 – 6 = 20 e- • 20 – 18 = 2 e- • 2 – 2 = 0 e-

  14. Writing Lewis Structures • 5) If you run out of electrons before the central atom has an octet… • …form multiple bonds until it does. • Double and triple bonds are shorter than single bonds.

  15. Writing Lewis Structures • Then assign formal charges. • For each atom, count the electrons in lone pairs and half the electrons it shares with other atoms. • Subtract that from the number of valence electrons for that atom: the difference is the formal charge.

  16. Writing Lewis Structures • The best Lewis structure… • …is the one with the fewest charges. • …puts a negative charge on the most electronegative atom. • Ions get put in brackets with the charge in the upper-right.

  17. 1994 MC #62 • The electron-dot structures (Lewis structure) for which of the following molecules would have two unshared pairs of electrons around the central atom? A) H2S B) NH3 C) CH4 D) HCN E) CO2

  18. 1994 MC #62 • The electron-dot structures (Lewis structure) for which of the following molecules would have two unshared pairs of electrons around the central atom? A) H2S – 64% correct, easy B) NH3 C) CH4 D) HCN E) CO2

  19. 1989 MC #17 • The Lewis dot structure of which of the following molecules shows only one unshared pair of valence electrons? • Cl2 • N2 • NH3 • CCl4 • H2O2

  20. 1989 MC #17 • The Lewis dot structure of which of the following molecules shows only one unshared pair of valence electrons? • Cl2 • N2 • NH3 - 76% correct, easy • CCl4 • H2O2

  21. 1984 MC #41 • Which of the following molecules has the shortest bond length? • N2 • O2 • Cl2 • Br2 • I2

  22. 1984 MC #41 • Which of the following molecules has the shortest bond length? • N2 – 49% correct, medium • O2 • Cl2 • Br2 • I2

  23. Homework: 8.30, 8.32, 8.34, 8.40, 8.42, 8.44, 8.46, 8.60, 8.62 • Next week: • Monday: Resonance Structures (8.7) • Tuesday: VSEPR (9.1-9.2), Stoichiometry Lab Due • Wednesday: Polarity (9.3) • Thursday: Hybridization (9.4-9.5) • Friday: No Class (Early Dismissal)

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