AP Chemistry 1.8 Valence Electrons and Ionic Compounds Study Notes - New Syllabus 2024-2025
AP Chemistry 1.8 Valence Electrons and Ionic Compounds Study Notes- New syllabus
AP Chemistry 1.8 Valence Electrons and Ionic Compounds Study Notes – AP Chemistry – per latest AP Chemistry Syllabus.
LEARNING OBJECTIVE
- Explain the relationship between trends in the reactivity of elements and periodicity
Key Concepts:
- Valence Electrons and Ionic Compounds
Valence Electrons and Ionic Compounds
Valence Electrons
Valence electrons are the outermost electrons in an atom. They occupy the highest energy level (shell) and determine how an atom interacts chemically with other atoms.
- Core electrons = inner electrons (not involved in bonding).
- Valence electrons = outer electrons (responsible for bonding and chemical properties).
- The number of valence electrons corresponds to the group number (for main-group elements).
Bond Formation and Likelihood
The likelihood that two elements will form a chemical bond is determined by interactions between:
- The valence electrons of each atom.
- The positive nuclei of each atom.
Atoms combine in ways that minimize potential energy and maximize stability. Strong attraction between a nucleus and another atom’s valence electrons promotes bonding.
Periodic Trends and Analogous Compounds
- Elements in the same column (group) of the periodic table have the same number of valence electrons.
- This leads them to form analogous compounds with similar chemical formulas and properties.
- Example: Lithium forms \( \text{LiCl} \), sodium forms \( \text{NaCl} \), potassium forms \( \text{KCl} \) — all similar structures due to 1 valence electron in Group 1 metals.
Typical Charges in Ionic Compounds
The charges of ions formed by atoms are predicted based on their group (valence electron count) in the periodic table:
| Group | Typical Ion Charge | Examples |
|---|---|---|
| Group 1 | +1 | \( \text{Na}^+ \), \( \text{K}^+ \) |
| Group 2 | +2 | \( \text{Mg}^{2+} \), \( \text{Ca}^{2+} \) |
| Group 13 | +3 | \( \text{Al}^{3+} \) |
| Group 15 | -3 | \( \text{N}^{3-} \), \( \text{P}^{3-} \) |
| Group 16 | -2 | \( \text{O}^{2-} \), \( \text{S}^{2-} \) |
| Group 17 | -1 | \( \text{Cl}^- \), \( \text{Br}^- \) |
| Group 18 | 0 | Noble gases (generally unreactive) |
Ionic Compounds
Ionic compounds are formed when cations and anions combine in fixed ratios to achieve electrical neutrality.
- Held together by strong electrostatic forces (ionic bonds).
- Form giant lattice structures with high melting and boiling points.
- Conduct electricity when molten or dissolved in water (ions are mobile).
Example
Predict the formula of the compound formed between aluminum (Group 13) and oxygen (Group 16).
▶️ Answer/Explanation
Aluminum has 3 valence electrons → forms \( \text{Al}^{3+} \).
Oxygen has 6 valence electrons → forms \( \text{O}^{2-} \).
To balance charges, 2 aluminum ions (total \( +6 \)) combine with 3 oxide ions (total \( -6 \)).
The formula is \( \text{Al}_2\text{O}_3 \).
Example
Why do lithium, sodium, and potassium all form \( +1 \) ions?
▶️ Answer/Explanation
All three elements are in Group 1 of the periodic table and have 1 valence electron in an s orbital.
Losing this electron gives them a stable noble gas configuration.
Thus, Li, Na, and K always form \( +1 \) ions, leading to analogous compounds such as \( \text{LiCl} \), \( \text{NaCl} \), and \( \text{KCl} \).