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Guidelines: Drawing Lewis Structures

Guidelines: Drawing Lewis Structures . Count total valence electrons in the molecule. Arrange atoms into a "skeleton" molecule . The less electronegative element is usually the central atom. The element written first in the formula is usually the central atom.

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Guidelines: Drawing Lewis Structures

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  1. Guidelines: Drawing Lewis Structures • Count total valence electrons in the molecule. • Arrange atoms into a "skeleton" molecule. • The less electronegative element is usually the central atom. • The element written first in the formula is usually the central atom. • C bonds to other C atoms readily and forms 4 bonds. • H forms 1 bond only • Connect all atoms with single bonds (dashes). • Distribute e-s to fill octets for all atoms (except H). • recount your e-s • If total e-s used (bonding and nonbonding) = valence e-s counted in step #1 you're done • If you used more e-s to fill octets than were counted in step #1 create double or triple bond • Make sure to check your structure!!! • Total e-s used in the Lewis Struct. should = total valence e-s counted in step #1

  2. N 1 5e- 5e- = x F 7e- 3 = 21e- x Examples: Drawing Lewis Structures Molecular formula NF3 Sum of valence e- Atom placement Total 26e- : : Remaining valence e- : F : : F : : N Lewis structure Total e-s needed = 26 : F : :

  3. PROBLEM: Write a Lewis structure for CCl2F2, one of the compounds responsible for the depletion of stratospheric ozone. Cl Cl C F : F : : Cl : : : : Cl C F : : : : F : Examples: Drawing Lewis Structures SOLUTION: Step 1:C has 4 valence e-, Cl and F each have 7. The sum is 4 + 4(7) = 32 valence e- Steps 2-4:Carbon has the lowest EN and is the central atom. The other atoms are placed around it. Make bonds and fill in remaining valence electrons placing 8 e- around each atom.

  4. H : H C O H : H Examples: Drawing Lewis Structures PROBLEM: Write the Lewis structure for methanol (molecular formula CH4O), an important industrial alcohol that is being used as a gasoline alternative in car engines. SOLUTION: Hydrogen can have only one bond so C and O must be next to each other with H filling in the bonds. There are 4(1) + 4 + 6 = 14 valence e- C has 4 bonds and O has 2. O has 2 pair of nonbonding e- 14 e-s total used to fill all atom's octets

  5. H H H H : C C C C H H H H Examples: Drawing Lewis Structures (cont) PROBLEM: Write Lewis structures for ethylene ( C2H4 ), the most important reactant in the manufacture of polymers PLAN: C atoms easily bond to each other and H gets 1 bond only so the C atoms are central and H are terminal If you do not have enough e-s to fill all atoms octets (exc H), create multiple bonds. Form a double bond if short 2 e-s, and 2 double or a triple bond if short 4 e-s. Dbl bonds between C, N, S, O and triple bonds between C, N, O SOLUTION: There are 2(4) + 4(1) = 12 valence e- You need 14 e- but only have 12. You’re short 2 e-s and have to create a double bond :

  6. Examples: Drawing Lewis Structures PROBLEM: Write Lewis structures for the following: Nitrogen ( N2), the most abundant atmospheric gas SOLUTION: N2 has 2(5) = 10 valence e- 14 e- are needed to fill octets 10 e- – 14 e- = 4 e-s short to fill octets A triple bond is required to make the octet around each N : : N N : : N N : : : :

  7. Examples: Drawing Lewis Structures Examples: SO2 CCl4 NH3 SO3 H2O CO2 PBr3 C2H5OH H2CO3 H2CO BF3 BeF2

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