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Lewis diagrams

A Lewis diagram is just careful bookkeeping of valence electrons. Get the total right, spend every electron on bonds and lone pairs, and the structure that shapes everything downstream falls into place.

§1

Bookkeeping for valence electrons.

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A Lewis diagram shows where all of a molecule's valence electrons go: as bonding pairs between atoms or as lone pairs on single atoms. It is an electron budget, and every electron must be accounted for.

The budget starts with a total: add up the valence electrons each atom contributes (from its group), adjusting for any overall charge — add electrons for a negative ion, subtract for a positive one.

Then you spend that total: connect atoms with bonding pairs, complete each atom's octet (a duet for hydrogen) with lone pairs, and if electrons run short, convert lone pairs into double or triple bonds. The count must come out exactly even.

UNIT 2 TOPIC 2.5 • LEWIS DIAGRAMS LEWIS BUILDER 1 VALENCE ELECTRON LEDGER Count the valence electrons contributed by each atom. ATOM VALENCE # ATOMS TOTAL C 4 1 4 O 6 2 12 TOTAL VALENCE ELECTRONS 16 Formula: CO₂ 4 from C + 2 × 6 from O = 16 valence electrons 2 BUILD THE STRUCTURE: CO₂ 1 CHOOSE A SKELETON C is least electronegative → center. O C O 2 PLACE SINGLE BONDS Connect atoms with 2 e⁻ per bond. O C O 3 COMPLETE OUTER ATOMS Give each outer O an octet. O C O 4 PLACE REMAINING e⁻ ON C 0 e⁻ left — structure unchanged. O C O 0 remaining 5 CONVERT TO MULTIPLE BONDS Shift lone pairs to form octets. O C O 3 FINAL STRUCTURE Carbon dioxide, CO₂ O C O OCTET CHECK C has 8 e⁻ Each O has 8 e⁻ All atoms achieve a full octet. ! MISTAKE TIP Total valence electrons must stay constant. Never add or remove electrons — only move them. CED ANCHOR Lewis diagrams (SAP-4.C) map valence electrons & bonding within a molecule. TOTAL e⁻ START TOTAL e⁻ END 16 = 16 AP Chemistry · Unit 2 · Compound Structure & Properties
Fig. 2.5.1 A Lewis diagram is a valence-electron budget. Total the electrons every atom brings (adjusting for charge), connect the atoms with bonds, then place the rest as lone pairs to complete octets. The count must balance exactly.
§2

Building a Lewis structure.

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A reliable recipe. Most wrong structures come from a miscount in Step 1.

  1. Total the valence electrons. Add each atom's valence count (its group's ones digit). Add one electron per unit of negative charge; subtract one per unit of positive charge.
  2. Pick a central atom. Usually the least electronegative atom (never hydrogen) goes in the center, with others bonded around it.
  3. Connect with single bonds. Draw one bonding pair from the central atom to each surrounding atom; each bond uses two electrons from the total.
  4. Complete octets, then add multiple bonds. Place remaining electrons as lone pairs to fill octets. If the center still lacks an octet, pull a lone pair into a double or triple bond.
§3

The pieces you'll meet.

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A short glossary keeps the bookkeeping honest.

valence e⁻
Valence electrons
The outer electrons each atom brings to the budget; the group number gives the count.
bonding pair
Bonding pair
Two electrons shared between two atoms; drawn as a line.
lone pair
Lone pair
Two electrons on a single atom, not shared.
octet
Octet
Eight electrons around an atom (two for hydrogen); the usual target.
central atom
Central atom
Usually the least electronegative, at the center of the structure.
total
Electron total
The full count that must be spent exactly on bonds and lone pairs.
§4

Worked example: the Lewis structure of CO₂.

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Question. Draw the Lewis structure of carbon dioxide, CO₂.

Step 1 — total. Carbon (group 14) brings 4; each oxygen (group 16) brings 6. Total = 4 + 2×6 = 16 valence electrons.

Steps 2–3 — skeleton. Carbon is the central atom; draw a single bond to each oxygen (4 electrons used, 12 remain).

Step 4 — octets and multiple bonds. Filling the oxygens' octets with lone pairs uses all 12, but carbon is left with only 4 electrons. Convert one lone pair on each oxygen into a second bond, giving two C=O double bonds. Now carbon and both oxygens have octets, and all 16 electrons are placed: O=C=O.

§5

Mistakes that cost real points.

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Pitfall · 01

"Just make the structure look symmetric and it's right."

A Lewis structure is judged by its electron count and octets, not its symmetry. A tidy-looking drawing with the wrong total, or with an atom lacking an octet, is still wrong. Appearance is not the test.

Fix. Verify the count: does the structure use exactly the total valence electrons, with correct octets (duet for H)? That, not symmetry, decides correctness.

Pitfall · 02

"You can forget the charge when counting electrons."

For an ion, the charge changes the electron total. An anion has extra electrons (add one per negative charge); a cation has fewer (subtract one per positive charge). Skipping this gives the wrong total and a wrong structure.

Fix. Always adjust the valence total for charge before spending electrons: +1 electron per negative charge, −1 per positive charge.

Pitfall · 03

"Every atom always ends up with a full octet."

The octet is the usual goal, not an absolute law. Some atoms are exceptions: boron is often content with six electrons, and third-row atoms like sulfur or phosphorus can hold more than eight (expanded octets). Forcing an octet everywhere can produce an impossible structure.

Fix. Aim for octets, but recognize the exceptions (electron-deficient boron; expanded octets for period-3+ centers) when the count demands them.

§6

Skill Check.

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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.

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