Lewis Dot Formulas (LDFs)

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Lewis Dot Formulas (LDFs)

• Chapter 7
• Take Home Test 4 due next Tuesday
• 10/24/06

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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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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)

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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

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Lewis Dot Formulas of atoms

• Show one elements valence electrons
• First 4 are drawn alone in a side of an imaginary
  square around elements symbol
• Second 4 are filled in as pairs of the first 4
  electrons

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Lewis Dot Formulas of 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 LDDs for compounds

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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

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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

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HW For Monday

• P 301 14, 34, 36, 40, 42, 52

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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

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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

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Hydrogen!

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

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LDFs of Covalent Compounds

• When more than 2 atoms
• Examples
• NH3 C3H8
• CO2 H2SO4
• CH4 - C2H4
• CS2
• CHCl3
• CH2O

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HW For Tuesday

• P 301 14, 34 and 36 d, 40 and 52

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Exceptions

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

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Lewis Dot Diagrams of 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
• Dont show electrons of positive cation
• Examples
• NaCl SrO
• LiCl MgCl2

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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-

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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

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Checking your Lewis Dot Formula

• Formal Charge!
• FC valence ( bonds) ( unshared)
• electrons electrons
• S of FC in a covalent molecule zero
• S of FC in an ion charge of the ion
• Negative FC on more electronegative elements
• FC represented by or -

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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 S FC ? 0)
• NO NO2