# Lewis Dot Diagrams _LDDs_ by dffhrtcv3

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```									Lewis Dot Formulas (LDFs)

Chapter 7

Take Home Test # 4 due next Tuesday
10/24/06
HW Due 10/17/06

• 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!
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)
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
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:
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 LDD’s for
compounds
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
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
HW For Monday
• P 301: 14, 34, 36, 40, 42, 52
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
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
Hydrogen!
• Only needs 2 electrons to fill its valence
energy level. (1s)
LDFs of Covalent Compounds
When more than 2 atoms:
• Examples:
– NH3         – C3H8
– CO2         – H2SO4
– CH4         - C2H4
– CS2
– CHCl3
– CH2O
HW For Tuesday
• P 301 # 14, 34 and 36 d, 40 and 52
Exceptions
• See section in your book for section on
exceptions to octet rule.
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
– Don’t show electrons of positive cation
• Examples:
– NaCl       – SrO
– LiCl       – MgCl2
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-
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
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 -
• 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 Σ FC ≠ 0)
– NO & NO2

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