AP Chemistry Unit 1 Study Notes
AP Chemistry 1.8: Valence electrons and ionic compounds
Use valence electrons to predict common ions and ionic formulas.
Aligned to Atomic Structure and Properties from the current College Board AP Chemistry course outline. Exam weighting for this unit: 7%-9% of the multiple-choice score range listed by College Board.
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Source note
These detailed Unit 1 notes were organized from the provided study document. For further study, visit Khan Academy. All Khan Academy content is available for free at www.khanacademy.org.
Overview Open
Khan Academy's Unit 1 connects valence electrons and ionization-energy patterns to predictable ionic charges and empirical formulas of ionic compounds.
8.1 Why Valence Electrons Matter Open
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Valence electrons are the electrons most directly involved in chemical reactions.
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Atoms can gain or lose electrons when forming ions.
Main-group elements often form predictable charges based on their valence electrons.
8.2 Common Main-Group Charges Open
| Group | Valence e⁻ | Common ion |
|---|---|---|
| 1 | 1 | +1 |
| 2 | 2 | +2 |
| 13 | 3 | +3 |
| 15 | 5 | −3 |
| 16 | 6 | −2 |
| 17 | 7 | −1 |
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Group 18 elements generally do not readily form simple ions because their valence shells are already filled.
Transition metals are more complicated and can have multiple charges.
8.3 Why Na Forms Na⁺ Open
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Na:
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[Ne]3s¹
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Removing one electron gives:
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Na⁺:
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[Ne]
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Removing a second electron would mean taking an electron from the stable neon-like core, requiring dramatically more energy.
Therefore +1 is favored.
8.4 Why Mg Forms Mg²⁺ Open
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Mg:
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[Ne]3s²
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Lose first electron:
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Mg⁺
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Lose second:
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Mg²⁺ = [Ne]
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The third ionization requires breaking into the core shell and requires far more energy.
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Therefore Mg usually forms:
Mg²⁺
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8.5 Why Cl Forms Cl⁻ Open
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Cl:
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[Ne]3s²3p⁵
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It needs one more electron to fill the 3p subshell:
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Cl⁻:
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[Ne]3s²3p⁶
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So chlorine commonly forms:
Cl⁻
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8.6 Ionic Compounds Must Be Electrically Neutral Open
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The total positive charge must equal the total negative charge.
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Example:
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Na⁺ + Cl⁻
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+1 + −1 = 0
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So formula:
NaCl
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8.7 Example: Mg and Cl Open
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Mg forms:
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Mg²⁺
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Cl forms:
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Cl⁻
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One chloride gives −1.
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We need two:
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Mg²⁺ + 2Cl⁻
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Total:
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+2 − 2 = 0
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Formula:
MgCl₂
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8.8 Example: Al and O Open
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Al³⁺
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O²⁻
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Find the smallest number that balances charges.
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2 Al³⁺:
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+6
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3 O²⁻:
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−6
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Total = 0
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Formula:
Al₂O₃
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8.9 Empirical Formula of an Ionic Compound Open
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Ionic formulas represent the lowest whole-number ratio of ions .
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So if charges hypothetically produced:
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Ca₂O₂
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simplify it to:
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CaO
because the ionic formula should show the simplest ratio.
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8.10 Ionization-Energy Data + Ionic Formula Questions Open
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Suppose element X has:
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IE₁ relatively low
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IE₂ relatively low
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IE₃ extremely high
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This tells you X has two valence electrons and likely forms:
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X²⁺
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Suppose X reacts with oxygen.
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Oxygen commonly forms:
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O²⁻
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Charges balance 1:1:
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X²⁺ + O²⁻
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→ XO
This is exactly the kind of combined reasoning Khan Academy currently tests in Unit 1.
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