Main Energy Levels and Electron Capacity
Quick Notes
- Electrons occupy main energy levels (also called shells), labelled by the integer n = 1, 2, 3, …
- Each energy level can hold a maximum number of electrons given by the formula: Maximum electrons = 2n²
- Energy levels get higher in energy and further from the nucleus as n increases.
- You should be able to deduce the maximum number of electrons in any level using this formula.
Full Notes
What Are Main Energy Levels?
In an atom, electrons are arranged in energy levels (or shells) around the nucleus. These are numbered with the integer n:
- n = 1 is the first (lowest) energy level
- n = 2 is the second level, and so on
Each level can hold a certain maximum number of electrons – not unlimited.
The 2n² Rule
The maximum number of electrons each energy level can hold is given by the formula:
Maximum electrons = 2n²
This formula tells you how many electrons can fit in each main level (shell).
| Energy level (n) | Max electrons (2n²) |
|---|---|
| 1 | 2 |
| 2 | 8 |
| 3 | 18 |
| 4 | 32 |
These are maximum limits — atoms don't always fill all available spots.
In the next section (1.3.4) we explore how electrons fill sub-levels and orbitals (s, p, d, f).
Why This Matters
Knowing the maximum number of electrons in each energy level is very important in chemistry:
- It helps predict how electrons are arranged in atoms (electron configuration).
- Explains patterns in the periodic table.
- Lays the foundation for understanding chemical bonding and reactivity.
Summary
- Electrons occupy shells labelled by n = 1, 2, 3, …
- The maximum number of electrons in a shell is given by 2n².
- Higher values of n mean higher energy and greater distance from the nucleus.
- This knowledge underpins electron configurations and periodic trends.
Linked Questions
How does an element’s highest main energy level relate to its period number in the periodic table?
The period number of an element corresponds to the principal energy level (shell) that contains its highest-energy electrons. For example, elements in Period 2 have electrons occupying the second energy level (n = 2), while those in Period 4 have their outermost electrons in the fourth level (n = 4). This relationship reflects the progressive filling of electron shells as atomic number increases and explains why properties such as atomic radius and ionisation energy show periodic trends across periods.