Drawing Lewis Structures of Molecules

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Drawing Lewis Structures of Molecules

• Chapter 4 Section 4

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Step 1

• Use chemical symbols for the various elements to
  draw the skeleton structure of the compound.
• Place least electronegative atom in the center.
• Hydrogen, fluorine, and other halogens occupy end
  positions.
• Carbon forms chains.

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Step 2

• Determine the number of valence electrons
  associated with each atom and combine them to
  determine the total number of valence electrons.
• Example 1 F2 2 F atoms x 7 electrons 14
• Example 2 CH3Cl 1 C atom x 4 e- 4 e-
• 3 H atoms x 1 e- 3 e-
• 1 Cl atom x 7 e- 7 e-
• 14 e-

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Step 2 (cont.)

• If the molecule has a charge, add or subtract
  electrons accordingly.
• (add electrons for a - charge, subtract them for
  a charge)
• SO4-2 (sulfate) NH41 (ammonium)
• 1 S 6 e- 1 N 5 e
• 4 O 24 e- 4 H 4 e
• charge 2 e- charge -1 e
• 32 e- 8 e

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Step 3

• Connect the central atom to each of the
  surrounding atoms using electron pairs.
• All atoms want 8 electrons except H which wants
  2.
• Add electron pairs around the end atoms first.
  If any are left over, then put them around the
  central atom.
• Check to make sure that the number of electrons
  drawn matches the number of total valence
  electrons calculated in step 2.

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Step 4

• Check the OCTET RULE !
• If the octet rule is not satisfied for any atom,
  move one or more electron pairs from the
  surrounding atoms to create double or triple
  bonds.
• Halogens NEVER multiple bond.

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Step 5

• After you are satisfied with the Lewis structure
  you have constructed, perform a final electron
  count verifying the total number of electrons AND
  that they all have 8 (or 2 if its hydrogen).

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Example CH4

• Step 1 Draw a skeleton structure.
• H is never the central atom.
• H
• H C H
• H

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Example CH4

• Step 2 determine the number of TOTAL valence
  electrons
• H 1 C 4 v.e.s
• H C H 4 H 4 v.e.s
• H 8 total v.e.s

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Example CH4

• Step 3 Connect central atom with surrounding
  atoms by electron pairs

Make sure the number of electrons drawn equals
the total number of electrons calculated in
step 2
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Example CH4

• Step 4 Check the OCTET RULE

Do all Hydrogens have 2 electrons and all other
atoms have 8?
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Example CH4

• Step 5 Verify that the total number of
  electrons is correct

8 total electrons
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Now, with a partner, try these!

• NH3
• CF4
• CH3Cl
• C3H8

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HOMEWORKDraw Lewis Structures for the following
molecules

• Water H2O
• Ethane C2H4
• Nitrogen N2
• Carbon monoxide CO