Electronic Configuration
Electronic configuration shows how electrons are arranged in the shells (energy levels) around the nucleus of an atom.
Rules for the First 20 Elements
The shells fill as follows:
First shell = 2 electrons
Second shell = 8 electrons
Third shell = 8 electrons
Fourth shell = 2 electrons (for the first 20 elements)
A quick way to remember this is:
2, 8, 8, 2
How to Work Out Electronic Configuration
Step 1
Find the atomic number.
The atomic number tells you the number of electrons in a neutral atom.
Step 2
Fill the shells in order.
Examples
Hydrogen (Atomic Number 1)
1 electron
Electronic configuration:
1
Helium (Atomic Number 2)
2 electrons
Electronic configuration:
2
Lithium (Atomic Number 3)
3 electrons
2 in first shell
1 in second shell
Electronic configuration:
2,1
Carbon (Atomic Number 6)
6 electrons
2 in first shell
4 in second shell
Electronic configuration:
2,4
Oxygen (Atomic Number 8)
8 electrons
2 in first shell
6 in second shell
Electronic configuration:
2,6
Sodium (Atomic Number 11)
11 electrons
2 in first shell
8 in second shell
1 in third shell
Electronic configuration:
2,8,1
Magnesium (Atomic Number 12)
12 electrons
2
8
2
Electronic configuration:
2,8,2
Chlorine (Atomic Number 17)
17 electrons
2
8
7
Electronic configuration:
2,8,7
Calcium (Atomic Number 20)
20 electrons
2
8
8
2
Electronic configuration:
2,8,8,2
Why Is Electronic Configuration Important?
The outer shell electrons determine how reactive an element is.
Group 1 Elements
Have 1 electron in the outer shell.
Very reactive because they want to lose 1 electron.
Examples:
Lithium → 2,1
Sodium → 2,8,1
Potassium → 2,8,8,1
Group 7 Elements
Have 7 electrons in the outer shell.
Reactive because they want to gain 1 electron.
Examples:
Fluorine → 2,7
Chlorine → 2,8,7
Group 0 (Noble Gases)
They have full outer shells, making them very stable and unreactive.
Examples:
Helium → 2
Neon → 2,8
Argon → 2,8,8
Exam Questions
Question 1
Write the electronic configuration of aluminium (atomic number 13).
13 electrons:
2
8
3
✅ Answer: 2,8,3
Question 2
An element has the electronic configuration 2,8,6. What is its atomic number?
Total electrons:
2+8+6=162+8+6=162+8+6=16
✅ Atomic number = 16
Question 3
An atom has the configuration 2,8,8,1.
How many electrons are in its outer shell?
✅ 1 electron
Which group is it in?
✅ Group 1
Quick Summary
Element | Atomic Number | Electronic Configuration |
|---|---|---|
H | 1 | 1 |
He | 2 | 2 |
Li | 3 | 2,1 |
C | 6 | 2,4 |
O | 8 | 2,6 |
Na | 11 | 2,8,1 |
Mg | 12 | 2,8,2 |
Cl | 17 | 2,8,7 |
Ca | 20 | 2,8,8,2 |
Exam Tip 📝
Atomic number = number of electrons.
Fill the shells 2, 8, 8, 2.
The group number equals the number of outer-shell electrons (except Group 0).
The period number equals the number of occupied shells.