Overview
This lecture explains the octet rule, how ions form, and the systematic naming and formula writing for ionic compounds using the periodic table.
The Octet Rule & Exceptions
- The octet rule: atoms react to get 8 electrons in their outermost (principal) energy level for stability.
- Hydrogen holds max 2 electrons; beryllium 4; boron and aluminum 6—these never reach an octet.
- Sulfur can have 8, 10, or 12 electrons; phosphorus can have 8 or 10 due to available d orbitals.
Formation of Ions
- Metals lose electrons to form positive ions (cations); non-metals gain electrons to form negative ions (anions).
- The charge on a metal cation equals its group number (e.g., Group 1 = 1+, Group 2 = 2+, etc.).
- Non-metal anion charge = group number minus 8 (e.g., Group 6 = 2-, Group 7 = 1-).
- Atoms lose or gain electrons to achieve a stable octet configuration.
- Cations are named as the element + "ion" (e.g., sodium ion), ignoring the charge in the name.
- Anions change the ending to "-ide" + "ion" (e.g., chloride ion).
Naming and Writing Formulas for Ions
- Name cations: element name + "ion" (e.g., Mg²⁺ → magnesium ion).
- Name anions: element root + "ide ion" (e.g., O²⁻ → oxide ion, N³⁻ → nitride ion).
- Only use the word "ion" if the atom or group has a charge.
Formation and Naming of Ionic Compounds
- Ionic compounds consist of one metal and one non-metal; formula must be electrically neutral.
- Metals transfer electrons to non-metals (e.g., Na + Cl → NaCl).
- To write ionic compound formulas, combine ions in the lowest whole number ratio so charges cancel.
- Crisscross method: use cation charge as subscript for the anion and vice versa, then simplify subscripts.
- Name ionic compounds as "metal name + non-metal root + ide" (e.g., NaBr is sodium bromide).
Key Terms & Definitions
- Octet Rule — Atoms tend to have 8 electrons in their outer shell for stability.
- Cation — Positively charged ion formed by metal atoms losing electrons.
- Anion — Negatively charged ion formed by nonmetals gaining electrons.
- Crisscross Method — A way to write ionic formulas by swapping ion charges as subscripts.
Action Items / Next Steps
- Add common ion charges above each group on your periodic table to assist with writing formulas.
- Practice naming ions and writing formulas for ionic compounds using the crisscross method.