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

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

Examples: Drawing Lewis Structures PROBLEM: Write a Lewis structure for CCl2F2, one of the compounds responsible for the depletion of stratospheric ozone. SOLUTION: Step 1: C has 4 valence e-, Cl and F each have 7. The sum is 4 + 4(7) = 32 valence e- C Cl F 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. C Cl F :

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- C O H : 14 e-s total used to fill all atom's octets

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 C H : C H :

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 : : :

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