Steps to a correct Lewis dot structure
1. Know the molecular formula.
Ammonia = NH3
Nitrate = NO3
-
2. Determine the total number of valence electrons.
5 + 3(1) = 8
5 + 3(6) + 1 = 24
3. Proper atom placement
a. The atom with the highest bonding potential is placed in the middle
b. What if there are two atoms with the same bonding potential? The least electronegative
element is placed in the middle.
H
O
N
N
H
O
H
O
4. Attach outer atoms to center atom with single bond.
H
O
N
N
H
O
H
O
5. Place remaining electrons on the outer atoms, any additional electrons are placed on the
center atom.
a. No more that 8 electrons total on any outer atom.
b. No more than 2 electrons on hydrogen.
O
H
N
N
H
O
H
O
6. Satisfy the octet rule for Carbon, Nitrogen, Oxygen, and Fluorine.
-
Already done
O
-
-
O
O
O
N
N
N
O
O
O
O
O
7. Optimize formal charges.
Already done
H
N
0
H
Already done
0
O
0
0
H
0
-1
O
N
-1
-
-1
-
O
+1
-1
O
-1 N
O
-
O
+1
0
O
0
O
N
+1
-1
O
8. Final structural representation.
-
H
H
O
N
N
H
O
O
Valence Electrons: (1) Electrons in the outermost n level and any electrons in any unfilled d or f
orbitals, (2) Electrons involved in chemical bonding
Bonding Potential: The number of unpaired electrons in the atomic Lewis dot structure.
Octet Rule: A proper Lewis dot structure will have eight electrons around Carbon, Nitrogen,
Oxygen, and Fluorine.
Formal Charge: The charge an atom would have if the electrons within a bond were shared
evenly.
Formal Charge = Number of valence electrons - [(2)Number of Lone Pairs + Number of Bonded Pairs]
Criteria for selecting the best Lewis dot structure based Formal Charge
1. Smaller (or fewer) formal charges are preferred.
2. Like charges on adjacent atoms leads to an unstable structure
3. The more negative formal charge should fall on the most electronegative element.
Resonance Structure: One of multiple Lewis dot structures used to describe a molecule that
cannot be adequately described using a single structure.
Resonance Hybrid: The weighted average of all resonance structures.
Bond Order: The number of chemical bonds between bonded atoms. In the case of a
resonance hybrid, the following formula can be used to determine the bond order if more than one
of the same element is connected to a center atom.
Bond Order =
number of bonded pairs around center atom
number of atoms bonded to center atom