Electronic structure (AQA GCSE Chemistry): Revision Notes
Electronic structure
Electronic structure shows how electrons are arranged in atoms. You need to know this for the first 20 elements in the periodic table.
What are energy levels?
Electrons move around the nucleus in different energy levels. Each electron stays at one particular energy level at a time. Electrons always fill up the lowest available energy levels first.
Energy levels are also known as shells:
- The innermost shell has the lowest energy
- The outer shell has the highest energy that contains electrons
Think of energy levels like the floors of a building - electrons prefer to live on the ground floor (lowest energy) before moving to higher floors. This is why electrons always fill the inner shells before moving to outer ones.
How many electrons fit in each energy level?
Different energy levels can hold different numbers of electrons. For the first 20 elements, here's what you need to remember:
| Energy level | Maximum electrons |
|---|---|
| First | 2 |
| Second | 8 |
| Third | 8 |
| Fourth | 8 |
This pattern (2, 8, 8, 8) is sometimes called the 2,8,8,8 rule and works perfectly for the first 20 elements. Beyond element 20, the pattern becomes more complex.
Writing electronic structures
You write electronic structures using numbers separated by commas. Each number shows how many electrons are in that energy level, starting from the innermost.
Worked Example: Sodium Electronic Structure
Sodium has 11 electrons (atomic number = 11)
Step 1: Fill the first energy level
- Maximum capacity = 2 electrons
- Electrons used: 2
- Remaining electrons: 11 - 2 = 9
Step 2: Fill the second energy level
- Maximum capacity = 8 electrons
- Electrons used: 8
- Remaining electrons: 9 - 8 = 1
Step 3: Fill the third energy level
- Remaining electrons: 1
- Electronic structure: 2,8,1
Using the periodic table
The periodic table helps you work out electronic structures easily:
- Count the atomic number - this tells you the total number of electrons
- Start from hydrogen and count across to your element
- Fill the energy levels using the 2,8,8,8 pattern
The position in the periodic table also gives you clues:
- Row number = number of energy levels used
- Group number = number of electrons in the outermost shell
The periodic table is organised this way precisely because of electronic structure! Elements in the same group have the same number of outer shell electrons, which is why they have similar chemical properties.
Drawing electronic structure diagrams
You can show electronic structure as a diagram with:
- Nucleus in the centre (small filled circle)
- Energy levels as circles around the nucleus
- Electrons as crosses (×) or dots on the circles
Make sure the electrons are spread out evenly around each circle.
Critical Drawing Rules:
- Always draw electrons evenly spaced around each energy level circle
- Never exceed the maximum number of electrons per energy level (2, 8, 8, 8)
- Start filling from the innermost energy level and work outwards
- Common mistake: Don't cram all electrons into one section of the circle
Key rules to remember
Essential Rules for Electronic Structure:
- Total electrons = atomic number of the element
- Number of energy levels = which row the element is in
- Outer shell electrons = which group the element is in
These rules help you check your answers are correct and provide shortcuts for finding electronic structures quickly.
Key Points to Remember:
- Energy levels fill up in order: first level gets 2, then second gets 8, then third gets 8
- Electronic structure is written with commas between each energy level (like 2,8,1)
- Use the periodic table to count electrons - the atomic number tells you how many
- The row number shows how many energy levels are used
- The group number shows how many electrons are in the outer shell
- Always check: total electrons should equal the atomic number!