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Valence Electrons and Ionic Compounds

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Valence electrons, the outermost electrons, decide how an atom bonds. Metals tend to lose them and nonmetals to gain them, both moving toward a full outer shell. When electrons transfer, the atoms become ions of opposite charge that attract into an ionic compound. Its formula is fixed by one rule: the positive and negative charge must balance.

UNIT 1 TOPIC 1.8 • VALENCE ELECTRONS AND IONIC COMPOUNDS ION ARCHITECT VALENCE ELECTRONS determine charge. CHARGE BALANCE determines the ionic formula. BEFORE: NEUTRAL ATOMS Mg Cl Cl 2 valence e⁻ (will lose 2) 7 valence e⁻ (needs 1) 7 valence e⁻ (needs 1) = electron from Mg = electron from Cl ELECTRON TRANSFER Mg loses 2 e⁻ (one to each Cl) each Cl gains 1 e⁻ AFTER: IONS FORMED 2+ Mg − Cl − Cl Mg²⁺ lost 2 e⁻ (full shell) Cl⁻ gained 1 e⁻ (full octet) Cl⁻ gained 1 e⁻ (full octet) All ions have full outer shells → more stable CHARGE BALANCE Mg²⁺ + Cl⁻ Cl⁻ = 0 +2 −1 −1 +2 from Mg is balanced by two −1 chloride ions. NET CHARGE = 0 (neutral compound) COUNT THE IONS → FORMULA 1 Mg²⁺ one magnesium ion : 2 Cl⁻ two chloride ions 1 : 2 ratio (lowest whole-number) THE IONIC COMPOUND MgCl2 MAGNESIUM CHLORIDE CED ANCHOR Atoms gain or lose electrons to reach a stable noble-gas electron configuration; opposite charges attract to form the ionic compound. AP Chemistry · Unit 1 · Atomic Structure & Properties
Ionic compounds from electron transfer. Magnesium gives up its two valence electrons, one to each chlorine, becoming Mg²⁺; each chlorine gains one to become Cl⁻. Charge balance (one +2 needs two −1) sets the formula MgCl₂, not the count of atoms you started with.
Ion Architect · Open the sandbox →

The formula of an ionic compound is set by charge balance, not by how many atoms happened to react. Count valence electrons to predict each ion's charge, then combine ions in whatever ratio makes the compound neutral. The subscripts fall out of the charges.

The work

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Lesson
Ionic Compounds ›

Valence electrons set each ion's charge, and charge balance sets the formula. The lesson goes from valence count to ion charge to a neutral formula, then closes with a ten-scenario applet that assembles ionic compounds from their elements.

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

Ten items on valence electrons and ionic bonding: predicting ion charge from valence count, transferring electrons to fill shells, and writing an ionic formula from charge balance rather than the number of atoms reacted.

Not started · 10 items · ~15 min
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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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