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Lewis Diagrams

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A Lewis diagram tracks where a molecule's valence electrons go. The recipe is bookkeeping: total the valence electrons every atom contributes, join the atoms with bonding pairs, and place what remains as lone pairs so each atom reaches an octet (two for hydrogen). Every electron in the total must be accounted for.

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
A Lewis diagram is bookkeeping for valence electrons. Total the valence electrons each atom brings, connect the atoms with bonding pairs, then place the remaining electrons as lone pairs to complete octets. The count must balance.
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The structure stands or falls on the electron count. Getting the total right, then distributing exactly that many electrons as bonds and lone pairs, is what makes a Lewis diagram correct — not just making it look symmetric. From a valid diagram, everything downstream (resonance, formal charge, shape) follows.

The work

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Lesson
Lewis Diagrams ›

A Lewis diagram is a valence-electron budget: count, connect, complete octets. The lesson works the ledger step by step, then closes with a ten-scenario applet that builds structures from the electron count.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items on drawing Lewis diagrams: totaling valence electrons correctly, distributing them as bonds and lone pairs, and completing octets so the electron count balances exactly.

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Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

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