ROCO Lewis: Drawing w Patterns

Most organic chemists cannot tell you exactly how they draw a Lewis structure. This is because they have practiced doing it so much, it no longer requires conscious effort. This essay tries to explain what subconscious thought processes are involved. The essay is divided into three parts: drawing neutral atoms, drawing positively charged atoms, and drawing negatively charged atoms. Problems follow each part.

Drawing rules

A Lewis structure is a drawing that shows all of a molecule's valence electrons and all non-zero formal charges.

Drawing styles vary from chemist to chemist, but most chemists draw covalent bonds as lines, and nonbonding electrons as dots. No symbols are used for ionic bonds (electrostatic attractions and repulsions are implied by the positions of non-zero formal charges).

A pattern-based drawing method

I believe organic chemists draw Lewis structures by reproducing electron patterns that they have learned through frequent drawing practice.

They almost never bother to count electrons, except to check their drawings. Instead, they draw lots of molecules, and electron patterns sink into their minds without any conscious effort to memorize them.

You can imitate (and accelerate) this process by studying the patterns below, and then practicing making (lots of) drawings yourself. You'll find that you quickly get the hang of it.

Electron patterns for neutral atoms

Atoms display characteristic electron patterns, depending on their position in the Periodic Table. The following table displays electron patterns for neutral C, N, O, and F.

# Neighbors	Neutral Atom
# Neighbors	C	N	O	F
1
2
3
4

Don't bother memorizing this table, but study it briefly and see if any patterns leap out at you. If you see some patterns, you can use them to help you draw. Here are some patterns that I see:

The number of lone pairs depends on the atom type and not the number of neighbors. C holds 0 lone pairs, N holds 1, O holds 2, and F holds 3.
The bond pattern depends on the number of neighbors. Fewer neighbors means more multiple bonds.
These patterns apply to other elements that are "vertically" related to C/N/O/F. For example, the same patterns are used by C and Si, by N and P, by O and S, and by F, Cl, Br, and I. This follows from the fact that "vertically" related atoms have the same number of valence electrons.

An example: formic acid, HCO2H

Pattern-based drawings can be created in three steps. First, draw your molecule with single bonds. Second, count the number of neighbors each atom has. Third, draw an appropriate electron pattern for that atom.

For example, the Lewis structure of formic acid, HCO₂H, can be drawn by starting with its single bonds, counting neighbors for C and both O, and completing their electron patterns.

C has three neighbors so it must make one double bond. The double bond must go to a neighbor with a compatible electron pattern and this turns out to be the O with just one neighbor. Neutral O always hold two lone pairs, so we draw these lone pairs around each O.

Review problems I

#1. Use standard electron patterns to add multiple bonds and lone pairs where needed (remember: a complete Lewis structure shows all valence electrons, including lone pairs). Drawing suggestion: Don't do these problems in your head. Draw complete answers on a piece of paper and then check your answers carefully against my drawings.

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#2. If you can read condensed formulas, translate these formulas into complete Lewis structures. Two hints: 1) all of the molecules are neutral and all of their atoms are uncharged, 2) some of these molecules contain multiple bonds; a condensed formula normally shows these bonds, but I have left them out to maximize the mystery.

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Electron patterns for POSITIVELY charged N and O

When an atom holding a lone pair (N, O, F, P, S, etc.) gives away an electron, the remaining electron becomes available for bonding (and an extra bond is needed to achieve a Lewis octet). For example: neutral N (5 valence electrons) can achieve an octet by making only 3 bonds, while N+ (4 electrons, isoelectronic with C) needs to make 4 bonds.

The most important electron patterns for positively charged atoms are based on N+ and O+. Study these drawings so that you understand 1) how each formal charge was arrived at, and 2) similarities between the electron patterns for these charged atoms and closely related neutral atoms.

# Neighbors	Positive Atom
# Neighbors	N⁺ (isoelectronic with neutral C)	O⁺ (isoelectronic with neutral N)
1
2
3
4

An example: methylammonium cation, CH3NH3+

The following drawing shows how these patterns can be used to draw a Lewis structure for H3CNH3+.

We start with the connectivity and then notice that N has an unusual number of neighbors: 4. This is a dead giveaway that N carries a +1 charge (and no lone pairs). C with 4 neighbors is uncharged (and holds no lone pairs). As a final check, we make sure that the overall charge matches the sum of the formal charges.

Review problems II

#3. Every molecule shown below is a CATION. Use standard electron patterns to add multiple bonds, lone pairs, and formal charges where needed. Drawing suggestion: Don't do these problems in your head. Draw complete answers on a piece of paper and then check your answers carefully against my drawings.

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#4. If you can read condensed formulas, translate these formulas into complete Lewis structures. Three hints: 1) all of the molecules are CATIONS and their formal charges must add up to +1, 2) some of these molecules contain multiple bonds; a condensed formula normally shows these bonds, but I have left them out to maximize the mystery, 3) multiple answers may be possible - try to find all of the answers that preserve Lewis octets around every atom.

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Electron patterns for NEGATIVELY charged C, N, and O

Adding an electron to an atom increases the number of lone pairs, and decreases the number of bonds needed to form a Lewis octet. Study these drawings so that you understand 1) how each formal charge was arrived at, and 2) similarities between the electron patterns for these charged atoms and closely related neutral atoms.

# Neighbors	Negative Atom
# Neighbors	C^- (isoelectronic with neutral N)	N^- (isoelectronic with neutral O)	O^- (isoelectronic with neutral F)
1
2
3

Review problems III

#5. Every molecule shown below is an ANION. Use standard electron patterns to add multiple bonds, lone pairs, and formal charges where needed. Drawing suggestion: Don't do these problems in your head. Draw complete answers on a piece of paper and then check your answers carefully against my drawings.

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#6. If you can read condensed formulas, translate these formulas into complete Lewis structures. Three hints: 1) all of the molecules are ANIONS and their formal charges must add up to -1, 2) some of these molecules contain multiple bonds; a condensed formula normally shows these bonds, but I have left them out to maximize the mystery, 3) multiple answers may be possible - try to find all of the answers that preserve Lewis octets around every atom.

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