Editing Lewis structure (section)

== Construction and electron counting ==
{{Main article|Electron counting}}
[[File:Ammonia Lewis structure.svg|class=skin-invert-image|thumb|Comparison between electron dot diagrams and Lewis structure]]
For a neutral molecule, the total number of electrons represented in a Lewis structure is equal to the sum of the numbers of [[valence electron]]s on each individual atom, not the maximum possible. Non-valence electrons are not represented in Lewis structures as they do not bond.

Once the total number of valence electrons has been determined, they are placed into the structure according to these steps:
# Initially, one line (representing a single bond) is drawn between each pair of connected atoms.
# Each bond consists of a pair of electrons, so if ''t'' is the total number of electrons to be placed and ''n'' is the number of single bonds just drawn, ''t''&minus;2''n'' electrons remain to be placed. These are temporarily drawn as dots, one per electron, to a maximum of eight per atom (two in the case of hydrogen), minus two for each bond.
# Electrons are distributed first to the outer atoms and then to the others, until there are no more to be placed.
# Finally, each atom (other than hydrogen) that is surrounded by fewer than eight electrons (counting each bond as two) is processed as follows: For every two electrons needed, two dots are deleted from a neighboring atom and an additional line is drawn between the two atoms. This represents the conversion of a lone pair of electrons into a bonding pair, which adds two electrons to the former atom's valence shell while leaving the latter's electron count unchanged.
# In the preceding steps, if there are not enough electrons to fill the valence shells of all atoms, preference is given to those atoms whose electronegativity is higher.

Lewis structures for polyatomic ions may be drawn by the same method. However when counting electrons, negative ions should have extra electrons placed in their Lewis structures; positive ions should have fewer electrons than an uncharged molecule. When the Lewis structure of an ion is written, the entire structure is placed in brackets, and the charge is written as a superscript on the upper right, outside the brackets.

===Miburo method===
A simpler method has been proposed for constructing Lewis structures, eliminating the need for electron counting: the atoms are drawn showing the valence electrons; bonds are then formed by pairing up valence electrons of the atoms involved in the bond-making process, and anions and cations are formed by adding or removing electrons to/from the appropriate atoms.<ref>{{citation |author=Miburo, Barnabe B. |year=1993 |title=Simplified Lewis Structure Drawing for Non-science Majors |journal=[[J. Chem. Educ.]] |volume=75 |issue=3 |page=317 |doi=10.1021/ed075p317|bibcode=1998JChEd..75..317M }}</ref>

A trick is to count up valence electrons, then count up the number of electrons needed to complete the octet rule (or with hydrogen just 2 electrons), then take the difference of these two numbers. The answer is the number of electrons that make up the bonds. The rest of the electrons just go to fill all the other atoms' octets.

===Lever method===
Another simple and general procedure to write Lewis structures and resonance forms has been proposed.<ref>{{citation |author=Lever, A. B. P. |year=1972 |title=Lewis Structures and the Octet Rule |journal=[[J. Chem. Educ.]] |volume=49 |issue=12 |page=819 |doi=10.1021/ed049p819|bibcode=1972JChEd..49..819L }}</ref>{{example needed|date=March 2025}}

This system works in nearly all cases, however there are 3 instances where it will not work{{citation needed|date=October 2024}}. These exceptions are outlined in the table below.
{| class="wikitable"
|+ Exceptions
|-
! The Exception !! How it Breaks the System !! How to Fix the Lewis Structure
|-
| Free Radicals (molecules with unpaired valence electrons) || Sum of TVe will be an odd number. Bond number will not be a whole number.|| Round calculated bond number down to the nearest whole number. (e.g. 4.5 bonds would round down to 4 bonds)
|-
| Valence Shell Deficiency || Does not break the system, must instead memorize when it occurs.|| BeX<sub>2</sub>, BX<sub>3</sub>, and AlX<sub>3</sub>. "X" represents Hydrogen or Halogens. When Be is bonded with 2 other atoms, or when B and Al are bonded with 3 other atoms, they do not form full valence shells. Assume single bonds and use the actual bond number to calculate lone pairs.
|-
| Expanded Octet (only occurs for elements in Groups 3-8)|| Bond calculation will provide too few bonds for the number of atoms in the molecule.|| Assume single bonds, use the minimum number of bonds necessary to create the molecule.
|}