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Lewis Dot Formulas (LDFs)

Lewis Dot Formulas (LDFs). Chapter 7 Take Home Test # 4 due next Tuesday 10/24/06. HW Due 10/17/06. Read Chapter 7 P 300, # 2, 4, 6 Also, give me LDFs for all elements in groups 1, 2 and 13 -18. If you use more than one color, Extra credit!.

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Lewis Dot Formulas (LDFs)

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  1. Lewis Dot Formulas (LDFs) Chapter 7 Take Home Test # 4 due next Tuesday 10/24/06

  2. HW Due 10/17/06 • Read Chapter 7 • P 300, # 2, 4, 6 • Also, give me LDFs for all elements in groups 1, 2 and 13 -18. If you use more than one color, Extra credit!

  3. All atoms have electrons organized in energy levels • 7 possible energy levels • Named by numbers: 1 through 7 • Electrons at EL 1 are closest to nucleus, at EL 7 they are farthest away • Each energy level has 1 or more sublevels • Named by letters: s, p, d, f • Each holds different # electrons: • s (2), p (6), d (10), f (14)

  4. Valence electrons • The electrons in the energy level farthest from the nucleus (s and p sublevels) • These are the electrons that participate in chemical reactions (they are shared, gained or lost) • We will only look at elements from groups 1, 2, 13 – 18

  5. Lewis Dot Formulas of atoms • Show one element’s valence electrons • First 4 are drawn alone in a side of an imaginary square around element’s symbol: • Second 4 are filled in as pairs of the first 4 electrons:

  6. Lewis Dot Formulasof compounds • Show all valence electrons in a compound • Covalent • All elements want to be like noble gases, who have full octet (8) of valence electrons • When elements get into compounds, it is to share/lose or gain electrons to feel like noble gases. • But! Why will they never be exactly like nobles? • Use this information to draw LDD’s for compounds

  7. LDFs of Covalent Compounds When there are just two atoms: • Count total # of available valence electrons from all elements = A • Count total # of valence electrons needed from all elements to achieve stable octet = N • S = # electrons shared  S = N – A

  8. LDFs of Covalent Compounds When there are just two atoms: • Place S electrons as dashes • Each dash = a shared pair of 2 electrons • Place any leftover electrons as unshared pairs (as dots) so that each element (that can) has full octet • Examples: CO, Br2, Cl2

  9. HW For Monday • P 301: 14, 34, 36, 40, 42, 52

  10. LDFs of Covalent Compounds Arranging atoms when more than 2 atoms: • Center atom usually least electronegative • Electronegativity = how much an element wants electrons. Ranges from 0.8 – 4. • Page 250 = table of electronegativity values • Oxygen usually not bonded to itself • Exceptions: O2, O3, H2O2 • In ternary acids, hydrogen usually does not bond to central atom (bonds to oxygen) • Exceptions: phosphoric and phosphorous acid

  11. LDFs of Covalent Compounds When more than 2 atoms: • Calculate S by subtracting Ntotal – Atotal • Place S electrons as dashes • Each dash = a shared pair of 2 electrons • Place any leftover electrons as unshared pairs (as dots) so that each element (that can) has full octet • Total # of electrons (dashes plus dots) should = A

  12. Hydrogen! • Only needs 2 electrons to fill its valence energy level. (1s)

  13. LDFs of Covalent Compounds When more than 2 atoms: • Examples: • NH3 – C3H8 • CO2 – H2SO4 • CH4 - C2H4 • CS2 • CHCl3 • CH2O

  14. HW For Tuesday • P 301 # 14, 34 and 36 d, 40 and 52

  15. Exceptions • See section in your book for section on exceptions to octet rule.

  16. Lewis Dot Diagramsof compounds • Ionic: • Between a metal and a non-metal • Put brackets and write charge around each ionic species and do each separately • Give/take away electrons from each species according to their charges • Don’t show electrons of positive cation • Examples: • NaCl – SrO • LiCl – MgCl2

  17. Lewis dot formulas of polyatomic ions • Put brackets & write charge on outside right hand corner • If negative charge, add value to A • If positive charge, subtract value from A • Examples • CO32-

  18. Resonance • When there is more than one possible Lewis dot formula, molecule or ion will be an “average” of all possible formulas. • Resonance = lowers energy = more stable compound Examples: • NO3- – CO32- • SO2 • O3

  19. Checking your Lewis Dot Formula Formal Charge! FC = # valence – [(# bonds) + (# unshared)] electrons electrons • Σ of FC in a covalent molecule = zero • Σ of FC in an ion = charge of the ion • Negative FC on more electronegative elements • FC represented by + or -

  20. Radicals • Compounds or elements where A (total valence electrons) is an odd # • Energetically unstable! • Radical elements  reactive with other elements to form compounds where A is an even # • Groups 1, 13, 15, 17 • Radical compounds: odd electron will be unshared & unpaired on the radical element (and ΣFC ≠ 0) • NO & NO2

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