Overview
This lesson explains how electrons arrange in shells around atomic nuclei and why incomplete outer shells make most atoms unstable and reactive.
Electron Shell Fundamentals
- Electrons occupy shells (energy levels) arranged as rings around the nucleus
- First shell holds maximum 2 electrons; second and third shells hold maximum 8 each
- Shells fill from innermost to outermost (lowest to highest energy)
- Draw electrons as crosses (×) or dots (•) in diagrams
Drawing Electron Arrangements
- Atomic number reveals both proton count and electron count for neutral atoms
- Add electrons clockwise in positions: top, right, bottom, left
- Place one electron per position before pairing any electrons
- Continue adding until shell is full or electrons are exhausted
Atomic Stability
- Atoms are stable when their outermost shell is completely full
- Incomplete outer shells make atoms unstable and chemically reactive
- Unstable atoms react with others to gain, lose, or share electrons
Electron Arrangement Examples
| Element | Atomic Number | Electron Arrangement | Outer Shell Status |
|---|
| Sodium | 11 | 2, 8, 1 | Unstable (1 of 8 electrons) |
| Neon | 10 | 2, 8 | Stable (full outer shell) |
| Argon | 18 | 2, 8, 8 | Stable (full outer shell) |
| Calcium | 20 | 2, 8, 8, 2 | Unstable (2 of 8 electrons) |
| Fluorine | 9 | 2, 7 | Unstable (7 of 8 electrons) |
Noble Gases
- Group 0 elements have completely full outer shells (e.g., neon, argon)
- Stable configuration means they do not gain or lose electrons
- Exceptionally unreactive; do not form compounds under normal conditions
Ion Formation
- Ions form when atoms lose or gain electrons to achieve stable outer shells
- Losing electrons creates positive ions (fewer electrons than protons)
- Gaining electrons creates negative ions (more electrons than protons)
- Draw square brackets around ion diagrams with charge in top right
Calcium Ion Example
- Calcium atom (2, 8, 8, 2) loses 2 outer electrons
- Results in Ca²⁺ ion with arrangement 2, 8, 8
- 20 protons but only 18 electrons create 2+ charge
- Third shell becomes full outermost shell, achieving stability
Fluorine Ion Example
- Fluorine atom (2, 7) gains 1 electron to complete outer shell
- Results in F⁻ ion with arrangement 2, 8
- 9 protons but 10 electrons create 1- charge
- Full outer shell provides stability
Key Terms & Definitions
- Electron arrangement: Distribution of electrons across shells around the nucleus
- Outer shell: Furthest shell from nucleus; must be full for stability
- Ion: Charged particle formed when atom gains or loses electrons
- Noble gases: Group 0 elements with full outer shells; unreactive