The Shell Model of the Atom and its Relation to the Periodic Table
The Orderly Atom
Essential Connection: The Bohr model introduced the idea of "shells" or energy levels. This model perfectly explains why the Periodic Table is shaped the way it is. As you move across the table, you fill up these shells in a predictable pattern.
1. Electron Shells and Energy Levels
Electrons do not fly around randomly. They occupy specific regions called shells (or energy levels). Each shell can hold a maximum number of electrons determined by a simple mathematical rule.
Maximum Electrons per Shell
The maximum number of electrons (\(N\)) that can occupy a shell with principal quantum number \(n\) is given by: \[ N = 2n^2 \]Where \(n = 1, 2, 3, 4...\)
🔵 Shell K (n=1)
Closest to the nucleus (lowest energy).
Max Capacity: \(2(1)^2 = 2\) electrons.
🟠 Shell L (n=2)
Second energy level.
Max Capacity: \(2(2)^2 = 8\) electrons.
🟢 Shell M (n=3)
Third energy level.
Max Capacity: \(2(3)^2 = 18\) electrons. (Note: For CSEC, we often focus on filling the first 20 elements, so this acts like a capacity of 8 until Calcium).
2. Interactive: Building the Atom
🔬 Electron Filling Simulator
Use the buttons below to add electrons one by one. Watch how they fill the inner shells first before moving to the outer ones. This simulates how elements are arranged in the Periodic Table (from Hydrogen to Calcium).
3. Relating Shells to the Periodic Table
The Periodic Table is not just a random list; it is a map of electronic structure. As you go from left to right across a row (Period), you are adding one electron at a time.
Periods (Rows)
The Period Number tells you how many electron shells (energy levels) an atom has.
- Period 1: 1 shell (e.g., H, He)
- Period 2: 2 shells (e.g., C, O, Ne)
- Period 3: 3 shells (e.g., Na, Cl, Ar)
Groups (Columns)
The Group Number (for Groups I, II, III... VII) tells you the number of electrons in the outermost shell (Valence Electrons).
- Group I: 1 Valence Electron (Very reactive)
- Group II: 2 Valence Electrons
- Group VIII (0): Full outer shell (Noble Gases, stable)
🎯 The "Inert Pair" & Stability
Atoms "want" to have a full outer shell (like Noble Gases).
- Group I elements (like Sodium) have 1 electron in their outer shell. It is easier to lose this one electron to empty the shell and reveal the full one underneath.
- Group VII elements (like Chlorine) have 7 electrons in their outer shell. It is easier to gain one electron to fill the shell to 8.
4. Valence Electrons and Chemical Properties
The number of electrons in the outermost shell is called the Valency. It determines how an atom reacts.
| Element | Symbol | Atomic No. | Configuration | Valence Electrons | Group |
|---|---|---|---|---|---|
| Lithium | Li | 3 | 2, 1 | 1 | I |
| Neon | Ne | 10 | 2, 8 | 8 (Full) | VIII (0) |
| Aluminum | Al | 13 | 2, 8, 3 | 3 | III |
| Potassium | K | 19 | 2, 8, 8, 1 | 1 | I |
CSEC Practice Questions
Test Your Understanding
1. An element has the electronic configuration 2, 8, 3. Which statement is correct?
2. Which of the following elements is in the same Group as Silicon (Si, atomic number 14)?
(Hint: Si config is 2, 8, 4)
3. The maximum number of electrons that can occupy the second shell (L) is: