IB DP Chemistry - S1.3.3 Main energy levels- Study Notes - New Syllabus - 2026, 2027 & 2028
IB DP Chemistry – S1.3.3 Main energy levels – Study Notes – New Syllabus
IITian Academy excellent Study Notes and effective strategies will help you prepare for your IB DP Chemistry exam.
- IB DP Chemistry 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Chemistry 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
- IB DP Chemistry 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Chemistry 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
Structure 1.3.3 — Main Energy Levels and Electron Capacity
Structure 1.3.3 — Main Energy Levels and Electron Capacity
Main Energy Levels (Shells)
Electrons in an atom are arranged in energy levels or shells. These levels are represented by the principal quantum number \( n \), where:
- \( n = 1 \) is the first (lowest) energy level
- \( n = 2 \) is the second energy level, and so on
Maximum Number of Electrons in an Energy Level:
The number of electrons a main energy level can hold is given by the formula:
\( \text{Maximum electrons} = 2n^2 \)
| Principal Quantum Number (n) | Shell | Maximum Number of Electrons | Electron Capacity Formula |
|---|---|---|---|
| 1 | K | 2 | \( 2 \times 1^2 = 2 \) |
| 2 | L | 8 | \( 2 \times 2^2 = 8 \) |
| 3 | M | 18 | \( 2 \times 3^2 = 18 \) |
| 4 | N | 32 | \( 2 \times 4^2 = 32 \) |
Note:
- While the formula gives the theoretical maximum number of electrons per level, actual electron configurations depend on sublevel energies (s, p, d, f).
- This concept is foundational to the understanding of electron arrangements in atoms and periodicity.
Example
Calculate the maximum number of electrons that can be held in the third main energy level.
▶️Answer/Explanation
The third energy level corresponds to \( n = 3 \).
Use the formula \( 2n^2 \):
\( 2 \times 3^2 = 2 \times 9 = 18 \)
Answer: 18 electrons
Example
An atom has a total of 10 electrons. Deduce the energy levels that are occupied and how many electrons are in each.
▶️Answer/Explanation
Use the capacity of each level in sequence:
- 1st level (n = 1): max 2 electrons → fill this first → 2 electrons
- 2nd level (n = 2): max 8 electrons → remaining 8 electrons
- Total = 2 + 8 = 10 electrons → no need for 3rd level
Answer: 1st level: 2 e⁻, 2nd level: 8 e⁻
Example
What is the minimum number of energy levels needed to accommodate 30 electrons in an atom?
▶️Answer/Explanation
Use energy level capacities cumulatively:
- n = 1 → 2 electrons (total so far: 2)
- n = 2 → 8 electrons (total: 10)
- n = 3 → 18 electrons (total: 28)
- n = 4 → required for remaining 2 electrons → total now 30
Answer: 4 energy levels are needed