What Is Electronic Configuration?

Electronic configuration describes how electrons are arranged in the shells (energy levels) around the nucleus of an atom.

Electrons do not move randomly around the nucleus. Instead, they occupy specific energy levels called shells.

Understanding electronic configuration helps explain:

• The structure of atoms
• The periodic table
• Chemical bonding
• The formation of ions

Electron Shells

Electrons move around the nucleus in different shells or energy levels.

Each shell can hold a limited number of electrons.

For the first 20 elements, the electron arrangement usually follows the 2,8,8 rule.

ShellMaximum ElectronsFirst Shell2Second Shell8Third Shell8

These shells fill from the inside outward, starting with the shell closest to the nucleus.

The 2,8,8 Rule

The 2,8,8 rule means:

• The first shell holds up to 2 electrons
• The second shell holds up to 8 electrons
• The third shell holds up to 8 electrons

Electrons fill shells in order, starting from the first shell.

Example:

An atom with 11 electrons will have the arrangement:

2,8,1

This means:

• 2 electrons in the first shell
• 8 electrons in the second shell
• 1 electron in the third shell

Examples of Electronic Configuration

Hydrogen (Atomic Number 1)

Electrons = 1

Electronic configuration:

1

Carbon (Atomic Number 6)

Electrons = 6

Electronic configuration:

2,4

Oxygen (Atomic Number 8)

Electrons = 8

Electronic configuration:

2,6

Sodium (Atomic Number 11)

Electrons = 11

Electronic configuration:

2,8,1

Calcium (Atomic Number 20)

Electrons = 20

Electronic configuration:

2,8,8,2

Why Electronic Configuration Is Important

Electronic configuration determines how atoms react chemically.

The outermost shell is called the valence shell, and the electrons in this shell are known as valence electrons.

Atoms tend to react in ways that allow them to achieve a full outer shell, which makes them more stable.

This explains why atoms form chemical bonds.

Electronic Configuration and the Periodic Table

Electronic configuration also explains the arrangement of elements in the periodic table.

• The period number shows the number of electron shells
• The group number shows the number of electrons in the outer shell

Example:

Sodium has electronic configuration 2,8,1, so it belongs to:

• Period 3 (three shells)
• Group 1 (one outer electron)

Why Atoms Form Ions

Atoms often gain or lose electrons to achieve a full outer shell.

Examples:

Sodium (2,8,1) loses one electron to become:

Na⁺ → 2,8

Chlorine (2,8,7) gains one electron to become:

Cl⁻ → 2,8,8

These ions are more stable because they have a complete outer shell.

Exam Tip (5070)

Students are commonly asked to:

• Write the electronic configuration of elements
• Identify the number of outer shell electrons
• Explain the relationship between electronic configuration and the periodic table

Example exam question:

Write the electronic configuration of magnesium (atomic number 12).

Answer:

2,8,2

Practice Question

Write the electronic configuration of chlorine (atomic number 17).

Answer

Chlorine has 17 electrons.

Electronic configuration:

2,8,7

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