Electronic structure (AQA GCSE Chemistry Combined Science): Revision Notes
Electronic structure
Students need to understand how electrons arrange themselves around the nucleus of an atom. This is called electronic structure, and you should know this for the first 20 elements in the periodic table.
What are energy levels and shells?
Electrons don't just float randomly around the nucleus. They sit in specific areas called energy levels or shells.
- The innermost shell has the lowest energy
- The outer shell is the highest occupied energy level
- Electrons always fill up the lowest available energy levels first
Think of it like filling up seats in a cinema - people fill the front rows first before moving to the back.
How many electrons fit in each shell?
Each energy level can only hold a certain number of electrons:
Maximum electron capacity per shell:
- First shell: up to 2 electrons
- Second shell: up to 8 electrons
- Third shell: up to 8 electrons
- Fourth shell: 1 or 2 electrons (for the first 20 elements)
This gives us the pattern: 2, 8, 8, 2
Writing electronic structures
We write electronic structures using numbers separated by commas. Each number shows how many electrons are in each shell, starting from the innermost.
Worked Example: Sodium (atomic number 11)
Step 1: Count total electrons Sodium has 11 electrons total
Step 2: Fill shells in order
- 2 electrons go in the first shell
- 8 electrons go in the second shell
- 1 electron goes in the third shell
Step 3: Write the electronic structure Written as: 2,8,1
Worked Example: Oxygen (atomic number 8)
Step 1: Count total electrons Oxygen has 8 electrons total
Step 2: Fill shells in order
- 2 electrons go in the first shell
- 6 electrons go in the second shell
Step 3: Write the electronic structure Written as: 2,6
Drawing electronic structure diagrams
You can also show electronic structures as diagrams with these steps:
How to draw electronic structure diagrams:
- Draw circles to represent the shells
- Put a dot in the centre for the nucleus
- Use crosses (x) to show where electrons are
- Fill inner shells first
For sodium (2,8,1), you'd draw three circles with the electrons marked as crosses in the right numbers.
Using the periodic table
The periodic table helps you work out electronic structures through these key relationships:
- The atomic number tells you how many electrons the atom has
- The group number tells you how many electrons are in the outer shell
- Count across the periods to see which shells are being filled
Remember that for neutral atoms, the number of electrons equals the atomic number.
Summary
Key Points to Remember:
- Energy levels are also called shells
- Electrons fill the lowest energy levels first
- Maximum electrons: first shell (2), second shell (8), third shell (8), fourth shell (2)
- Electronic structure is written with commas separating each shell (like 2,8,1)
- The atomic number equals the total number of electrons in a neutral atom