Electron Configuration

This lesson covers how electrons are arranged in atoms — known as electron configuration — as required by the Edexcel GCSE Combined Science specification (1SC0). You need to be able to work out the electron configuration of the first 20 elements and understand how electron configuration links to an element's position in the periodic table.

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Electron Shells (Energy Levels)

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)	Furthest

Rules for Filling Electron Shells

1. Electrons always fill the lowest energy shell first (closest to the nucleus).
2. When a shell is full, electrons begin to fill the next shell out.
3. The 1st shell holds a maximum of 2 electrons.
4. The 2nd and 3rd shells each hold a maximum of 8 electrons (at GCSE level).

This is known as the 2, 8, 8 rule. After the third shell has 8 electrons, the next 2 electrons go into the fourth shell (for the first 20 elements).

Exam Tip: At GCSE level, you only need to know the 2, 8, 8 rule. In reality, the third shell can hold up to 18 electrons, but you will not be asked about this in the Combined Science specification.

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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

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Drawing Electron Configuration Diagrams

When drawing an electron configuration diagram:

1. Draw the nucleus in the centre (you can label it with the element symbol or the number of protons and neutrons).
2. Draw concentric circles (rings) around the nucleus for each occupied shell.
3. Mark electrons as dots or crosses on each ring.
4. Fill shells from the inside out, following the 2, 8, 8 rule.

Worked Example: Sodium (Na, atomic number 11)

• Total electrons: 11
• 1st shell: 2 electrons (full)
• 2nd shell: 8 electrons (full)
• 3rd shell: 1 electron
• Electron configuration: 2, 8, 1

Worked Example: Chlorine (Cl, atomic number 17)

• Total electrons: 17
• 1st shell: 2 electrons (full)
• 2nd shell: 8 electrons (full)
• 3rd shell: 7 electrons
• Electron configuration: 2, 8, 7

Exam Tip: When drawing electron diagrams, make sure your dots or crosses are clearly positioned on the shells, not floating between them. Examiners will mark you wrong if it is unclear which shell an electron belongs to.

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Linking Electron Configuration to the Periodic Table

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

Periodic Table Feature	Electron Configuration Link
Group number	= number of electrons in the outer shell
Period number	= number of electron shells occupied

Examples

Element	Electron Configuration	Period	Group
Lithium (Li)	2, 1	2 (two shells)	1 (one outer electron)
Carbon (C)	2, 4	2 (two shells)	4 (four outer electrons)
Sodium (Na)	2, 8, 1	3 (three shells)	1 (one outer electron)
Chlorine (Cl)	2, 8, 7	3 (three shells)	7 (seven outer electrons)
Calcium (Ca)	2, 8, 8, 2	4 (four shells)	2 (two outer electrons)

Why This Matters

• Elements in the same group have the same number of outer electrons — this is why they have similar chemical properties.
• Elements in the same period have the same number of electron shells.

graph TD
    A["Electron Configuration<br/>e.g. 2, 8, 1"] --> B["Number of shells = 3<br/>→ Period 3"]
    A --> C["Outer electrons = 1<br/>→ Group 1"]
    B --> D["Element is in<br/>Period 3, Group 1<br/>= Sodium"]
    C --> D

    style A fill:#2c3e50,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#27ae60,color:#fff

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Predicting Properties from Electron Configuration

Because elements in the same group share the same number of outer electrons:

• Group 1 elements (1 outer electron) are reactive metals that lose one electron easily.
• Group 7 elements (7 outer electrons) are reactive non-metals that gain one electron easily.
• Group 0 elements (full outer shell — 2 for He, 8 for the others) are very unreactive noble gases.

Exam Tip: If asked to predict the properties of an element from its electron configuration, first identify its group (from outer electrons) and period (from number of shells), then compare it to known elements in the same group.

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Summary

• Electrons are arranged in shells (energy levels) around the nucleus.
• The maximum electrons per shell follow the 2, 8, 8 rule (at GCSE level).
• Shells fill from the innermost outward.
• Group number = number of outer-shell electrons.
• Period number = number of occupied electron shells.
• Elements in the same group have similar chemical properties because they have the same number of outer electrons.
• You must be able to write the electron configuration and draw the electron diagram for the first 20 elements.

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Worked Examples — Writing Electron Configurations