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Lecture 9

Lecture 9. Lewis Structures and Covalent Bonds 2.5-2.8 10 -September Assigned HW 2.33, 2.34, 2.35, 2.36, 2.38, 2.43, 2.48, 2.50, 2.52 Due : Monday 13-Sept. Review 2.1-2.4. Ions interact in specific ways that form crystal lattices

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Lecture 9

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  1. Lecture 9 Lewis Structures and Covalent Bonds 2.5-2.8 10-September Assigned HW 2.33, 2.34, 2.35, 2.36, 2.38, 2.43, 2.48, 2.50, 2.52 Due: Monday 13-Sept

  2. Review 2.1-2.4 • Ions interact in specific ways that form crystal lattices • The energy that stabilizes these lattices, called lattice energy is dependent on • The charges of the ions • The ionic radius • Madelung constant (A)  dependent on the orientation of the ions • Including an additional term allows us to accurately predict the lattice energy • Lewis Representation of atoms and Ions • Valence electrons are represented by dots • Single dot = unpaired electron • Pair of dots = paired electrons within an atomic orbital

  3. Compounds Compound Bond Type Inorganic Compound Organic Compound Covalent Composed of Charged components Ionic Bonds Contains Carbon Cations Anions

  4. Lewis Representation – Ionic Bond • American chemist G.N. Lewis had a profound impact on our understanding of chemical bonding. One of his many contributions was a simple way to represent chemical bonds. We call these Lewis Symbols. • Valence Electrons are represented by dots. • A single dot represents an electron • A pair of dots represents two paired electrons sharing an orbital H He N O Cl K Mg I 1s1 1s2 2s22p3 2s22p4 2s22p5 3s1 5s25p5 3s2

  5. Ionic Bonds – CaCl2 Ion Ca2+Cl- Electron Configuration [Ar] [Ne]2s22p6 Lewis Symbol Ionic bonds do not share electrons – bond due to coulombic attraction

  6. Lewis Representation – Covalent Bonds Covalent bonds  electrons shared between two atoms. This involves an overlap of atomic orbitals Let’s consider H2 - two hydrogen atoms share electrons. H• H H + H• Commonly represented as: H H Solid line represents covalent bond

  7. Lewis Representation – Covalent Bonds F2 Only the unpaired electron is involved in bonding. F• F F + F• F F How would this look if we were making Cl2?

  8. Lewis Representation – Covalent Bonds HF Start Finish The Octet Rule – non-metals share electrons until each has a complete octet (or duplet for H and He).

  9. Lewis Representation – Covalent Bonds CH4 Start Finish C H H H H Have we satisfied the octet rule?

  10. Common Representations of Compounds

  11. Common Representations of Compounds Ethanol Chemical Formula: C2H6O Tells us what is there Molecular Formula: CH3CH2OH Gives us information about the arrangement of the atoms OH Stick Ball and stick Spacefill

  12. Lewis Structures

  13. Lewis Structures Ethanol CH3CH2OH C  4 x 2 = 8 H  1 x 6 = 6 O  6 x 1 = 6 20

  14. Lewis Structures Ethanol CH3CH2OH C  4 x 2 = 8 H  1 x 6 = 6 O  6 x 1 = 6 20 20 – 16 = 4 left

  15. Lewis Structures – common groups

  16. Lewis Structures Formaldehyde CH2O C  4 x 1 = 4 H  1 x 2 = 2 O  6 x 1 = 6 12 Make skeleton Fill boxes with electron pair Fill terminal atoms octet Make multiple bonds 12 – 12 = 0 left Are all octets full? Are all octets full?

  17. Lewis Structures Ethyl Aldehyde CH3CHO 1 C  4 x 2 = 8 H  1 x 4 = 4 O  6 x 1 = 6 18 2 4 3 18 – 18 = 0 left Are all octets full?

  18. Bond Order Bond Order  the number of bonds that link a specific pair of atoms. BO = ½ x (# of shared electrons) 1  single bond 2  double bond 3  triple bond 1 1 1 1 1 2 3 1

  19. Lewis Structures Carbonic Acid H2CO3 C 4 x 1 = 4 H  1 x 2 = 2 O  6 x 3 = 18 24 When you see hydrogen first in molecular formula, think acid. H is attached to an Oxygen Make skeleton Fill boxes with electron pair Fill terminal atoms octet Fill central atoms octets 24 – 16 = 8 left 24 – 24 = 0 left Are all octets full? Are all octets full?

  20. Lewis Structures Carbonic Acid H2CO3 24 – 24 = 0 left Are all octets full? Make a double bond

  21. Formal Charge We’ll determine which structure is the most stable by inspecting the charge on each atom. Formal Charge = V – (LP + B) # valence electrons in free atom Total Bond Order # electrons in lone pair

  22. Formal Charge We’ll determine which structure is the most stable by inspecting the charge on each atom. Formal Charge = V – (LP + B) # valence electrons in free atom Total Bond Order # electrons in lone pair +1 0 0 0 0 0 -1 -1 0 0 0 0 0 0 0 +1 0 0

  23. Formal Charge We’ll determine which structure is the most stable by inspecting the charge on each atom. Formal Charge = V – (LP + BO) # valence electrons in free atom Total Bond Order # electrons in lone pair Carbon and Hydrogen same 3 types of Oxygens in these structures +1 0 0 0 -1 -1 0 0 0 0 +1 0

  24. Formal Charge Carbonic Acid H2CO3 Most stable structure has minimized charge Net charge on the molecule is the sum of all formal charges. +1 0 0 0 0 0 -1 -1 0 0 0 0 0 0 0 +1 0 0

  25. Formal Charge We’ll determine which structure is the most stable by inspecting the charge on each atom. Most stable structure has minimized charge 0 0 0 0 0 0

  26. Formal Charge and Ions What happens when we remove 2 H+? Carbonate CO32- 2H+

  27. Formal Charge and Ions What happens when we remove 2 H+? Carbonate CO32- 2H+ So the 2- charge on carbonate is localized to two oxygens?

  28. Formal Charge and Ions Carbonate CO32- We can draw this structure three equivalent ways! RESONANCE STRUCTURES We can conclude that 2- charge on carbonate is SHARED amongst the oxygens

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