Electron Configuration

This lesson covers how electrons are arranged in atoms — known as electron configuration — as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 1: Key Concepts in Chemistry. You need to be able to work out the electron configuration of the first 20 elements and understand how it links to an element's position in the periodic table and its chemical properties.

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Electron Shells

Electrons are arranged in shells (also called energy levels) around the nucleus. Each shell can hold a maximum number of electrons:

Shell Number	Maximum Electrons	Distance from Nucleus
1st shell (innermost)	2	Closest
2nd shell	8	Further out
3rd shell	8 (at GCSE level)	Further still
4th shell	2 (for the first 20 elements at GCSE)	Furthest

Rules for Filling Electron Shells

1. Electrons always fill the lowest energy shell first (the one closest to the nucleus).
2. When a shell is full, electrons begin to fill the next shell out.
3. The first shell holds a maximum of 2 electrons.
4. The second shell holds a maximum of 8 electrons.
5. The third shell holds a maximum of 8 electrons (at GCSE level — in reality it can hold 18, but you only need to know about 8 for Edexcel GCSE).

Exam Tip: The rule for the first 20 elements is often called the 2, 8, 8 rule. After the third shell has 8 electrons, the next 2 electrons go into the fourth shell. This is sufficient for the Edexcel GCSE specification.

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Writing Electron Configurations

An electron configuration is written as a series of numbers separated by commas, showing how many electrons are in each shell from the innermost outward.

Electron Configurations of the First 20 Elements

Element	Symbol	Atomic Number	Electron Configuration
Hydrogen	H	1	1
Helium	He	2	2
Lithium	Li	3	2, 1
Beryllium	Be	4	2, 2
Boron	B	5	2, 3
Carbon	C	6	2, 4
Nitrogen	N	7	2, 5
Oxygen	O	8	2, 6
Fluorine	F	9	2, 7
Neon	Ne	10	2, 8
Sodium	Na	11	2, 8, 1
Magnesium	Mg	12	2, 8, 2
Aluminium	Al	13	2, 8, 3
Silicon	Si	14	2, 8, 4
Phosphorus	P	15	2, 8, 5
Sulfur	S	16	2, 8, 6
Chlorine	Cl	17	2, 8, 7
Argon	Ar	18	2, 8, 8
Potassium	K	19	2, 8, 8, 1
Calcium	Ca	20	2, 8, 8, 2

Exam Tip: You are expected to know the electron configurations of the first 20 elements for the Edexcel GCSE exam. Notice the pattern — once you know the 2, 8, 8 rule and the atomic number, you can work out any configuration quickly.

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How to Determine Electron Configuration from Atomic Number

flowchart TD
    A["Start: Find the atomic number<br/>(= number of electrons in a neutral atom)"] --> B{"Are there more than<br/>2 electrons remaining?"}
    B -- "No" --> C["Place all remaining electrons<br/>in the 1st shell<br/>DONE"]
    B -- "Yes" --> D["Place 2 electrons<br/>in the 1st shell"]
    D --> E{"Are there more than<br/>8 electrons remaining?"}
    E -- "No" --> F["Place all remaining electrons<br/>in the 2nd shell<br/>DONE"]
    E -- "Yes" --> G["Place 8 electrons<br/>in the 2nd shell"]
    G --> H{"Are there more than<br/>8 electrons remaining?"}
    H -- "No" --> I["Place all remaining electrons<br/>in the 3rd shell<br/>DONE"]
    H -- "Yes" --> J["Place 8 electrons<br/>in the 3rd shell"]
    J --> K["Place remaining electrons<br/>in the 4th shell<br/>DONE"]

    style A fill:#2c3e50,color:#fff
    style C fill:#27ae60,color:#fff
    style F fill:#27ae60,color:#fff
    style I fill:#27ae60,color:#fff
    style K fill:#27ae60,color:#fff

Worked Example

What is the electron configuration of phosphorus (atomic number 15)?

1. Phosphorus has 15 electrons (atomic number = 15).
2. First shell: place 2 electrons → 15 − 2 = 13 remaining.
3. Second shell: place 8 electrons → 13 − 8 = 5 remaining.
4. Third shell: place the remaining 5 electrons.
5. Electron configuration: 2, 8, 5

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Link Between Electron Configuration and the Periodic Table

The electron configuration of an element is directly linked to its position in the periodic table:

Feature	Determined By
Group number	Number of electrons in the outer shell
Period number	Number of electron shells occupied

Examples

• Sodium (2, 8, 1): 3 shells → Period 3; 1 outer electron → Group 1
• Chlorine (2, 8, 7): 3 shells → Period 3; 7 outer electrons → Group 7
• Carbon (2, 4): 2 shells → Period 2; 4 outer electrons → Group 4
• Calcium (2, 8, 8, 2): 4 shells → Period 4; 2 outer electrons → Group 2

Exam Tip: The noble gases (Group 0/8) have full outer shells. Helium has 2 electrons (full first shell), and the others (Ne, Ar) have 8 electrons in their outer shell. This full outer shell is what makes noble gases so unreactive.

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Outer Electrons and Chemical Properties

The electrons in the outermost shell (also called valence electrons) determine the chemical properties of an element. This is because chemical reactions involve the transfer or sharing of outer shell electrons.

Why Elements in the Same Group Have Similar Properties

Elements in the same group have the same number of outer shell electrons. This means they react in similar ways: