LEWIS STRUCTURES
***showing how covalent compounds share electrons between atoms***
Molecular compound: compound composed of only nonmetals
Covalent bond: a bond between two nonmetals in which electrons are shared by atoms
To draw a Lewis dot structure:
1. Add up total valence electrons (add more for anions, subtract some for cations).
2. Determine a central atom. If carbon is present, it will be central. If not, the least
electronegative element is the central atom, but never hydrogen.
3. Connect the outer atoms with lines to represent a bonded electron pair (1 line = 2 e-s).
Distribute remaining electrons around non-central atoms so that they have a full octet, except
for hydrogen whose first energy level is maxed out with a single bond. Place any remaining
electrons on the central atom.
4. Check for the octet rule. Create double and triple bonds if necessary so that atoms (other than
hydrogen) have 8 electrons surrounding them.
5. Calculate the formal charge on each atom:
FC = valence - # of bonds – lone electrons
Rearrange the structure so that all formal charges are as close to zero as possible. This may
involve using lone pairs of electrons to create double and triple bonds.
General rule of thumb: If you are dealing with an anion, the negative FC should be on the
most electronegative element. If you are dealing with a cation, the positive FC should be on
the least electronegative element.
CAUTION: Some elements can have an expanded octet (they’re okay with more than 8
electrons) while some elements like B and Be are okay with less than an octet (B: 6 electrons,
Be: 4 electrons). These elements will always be your central atoms. Allow them to be strange.
The rest of the atoms will want the octet.
1. Write lewis structures that obey the octet rule for each of the following:
a. HCN
d. ammonium
g. CO2
b. PH3
e. H2CO
h. O2
c. CHCl3
f. SeF2
i. HBr
2. Write lewis structures for each of the following:
a. POCl3, XeO4, phosphate, perchlorate
b. NF3, phosphite, chlorate
c. Chlorite, SCl2, PCl23. Write lewis structures for the following molecules or ions, which have central atoms that do
not obey the octet rule:
a. PF5
d. Br3g. ClF5
b. BeH2
e. SF4
h. SF6
c. BH3
f. XeF4
4. ClF3 and BrF3 are both used to fluorinate uranium to produce UF6 in the processing of
nuclear fuel. Write lewis structures for ClF3 and BrF3.
5. Write lewis structures for the following:
a. BF2Br
d. TeH2
g. N2O
b. SiO2
e. AlH3
h. BCl3
c. CF4
f. CH3Cl
i. SCN6. Write lewis structures for sulfate, carbonate, and sulfite.
1