Honors Chemistry
Name: __________________________________________ Date: _____________ Mods: _________
Ch. 8: Rules for Drawing Covalently Bonded Lewis Structures
1) Sum the valence electrons from all atoms in the molecule or ion

For an ANION  add one electron to the total for each negative charge

For a CATION  subtract one electron from the total for each positive charge
2) Determine the central atom in the molecule and attach all other atoms to it

The central atom is the least electronegative element in the compound excluding hydrogen –
hydrogen can NEVER be central! (Note: typically, the central atom is the one written first in
the molecular formula)

Write the symbols for all the other atoms around the central atom. Connect the atoms to the
central with a single bond (a dash). Keep track of the electrons being used. Each single bond
made uses 2 electrons.

Chemical formulas are often written in the order in which the atoms are connected in the
molecule (ex: HCN  carbon is the central atom)
3) Complete the octets around all the outer atoms bonded to the central using nonbonding
electrons ( : )

Note: Hydrogen atoms may only ever have 2 electrons associated with them at any given time

Keep track of the electrons being used to complete the octets
4) Place any leftover electrons on the central atom, even if doing so results in more than an
octet of electrons around the atom.

Note: Expanded octets occur when more than 8 electrons surround an atom. This exception
to the octet rule is allowed for any atom in the 3rd row of the periodic table and after!

Keep track of the electrons – make sure the total number of valance electrons available were
used in the Lewis structure
5) If there are not enough electrons to give the central atom an octet, multiple bonds are
needed.
Single Bond (
):
Double Bond (
Triple Bond (
):
):

Remove one of the nonbonding pairs of electrons on one of the outer atoms and draw a
double bond (2nd dash) connecting the outer atom to the central. If need be, a triple bond (3rd
dash) may be formed by removing another nonbonding pair from the same outer atom.

Keep track of the electrons – make sure the total number of valance electrons available were
used in the Lewis structure
Drawing Covalent Lewis Structures – Examples
Lewis Dot Structures:
1) Methane: CH4
4) Sulfite ion: SO32-
2) Hydrochloric Acid: HCl
5) Hydrocyanic Acid: HCN
3) Ammonia: NH3
6) Methanal (aka: formaldehyde): H2CO
Resonance Structures:
Multiple Lewis structures are used to describe molecules which have double or triples bonds that
can be moved to different sides of the central atom and still stay bonded to an identical outer atom.
7) Nitrite ion: NO2 –
Exceptions to the Octet Rule:
Less than an Octet  central
atoms which have FEWER than
8 electrons (seen with B only)
8) BF3
Expanded Octet  central atoms which have MORE
than 8 electrons (this is allowed for central atoms
found in the 3rd period and after)
9) SBr6
10) XeI4
Drawing Lewis Diagrams - Practice
# of valence e-‘s:________
1. SF4
3. PO43-
5. N2
2. CH2F2
# of valence e-‘s:________
# of valence e-‘s:________
7. CO32-
# of valence e-‘s:________
4. CBr4 # of valence e-‘s:________
6. CCl2O
# of valence e-‘s:________
# of valence e-‘s:________
8. ClO21–
10. H2S
12. NH4+
# of valence e-‘s:________
# of valence e-‘s:________
# of valence e-‘s:________
11. C2H4 # of valence e-‘s:________
(attach carbons together – make symmetrical)
# of valence e-‘s:________
14. NO31-
9. OF2
13. BCl3
# of valence e-‘s:________
# of valence e-‘s:________
15. IBr5
# of valence e-‘s:________
17. C2H2 # of valence e-‘s:________
(attach carbons together – make symmetrical)
19. PCl3 # of valence e-‘s:________
21. PF5
# of valence e-‘s:________
16. SO42-
# of valence e-‘s:________
18. CO # of valence e-‘s:________
20. SiO2
# of valence e-‘s:________
22. PH3 # of valence e-‘s:________
23. XeF4
# of valence e-‘s:________
25. CN- # of valence e-‘s:________
27. C2Br2 # of valence e-‘s:________
(attach carbons together – make symmetrical)
29. SeF4
# of valence e-‘s:________
24. SeH2
# of valence e-‘s:________
26. IF3 # of valence e-‘s:________
28. ClO1– # of valence e-‘s:________
30. AsF3 # of valence e-‘s:________