3.7 Electronic Configuration

This topic explains how electrons are arranged in an atom — in shells and sub-shells — and how to write the electronic configuration of an element.

Definition

The arrangement of electrons in sub-shells is called the electronic configuration.

Simple explanation: Electronic configuration tells you where the electrons are located in an atom — which shell and which sub-shell.

Main Idea

To understand electronic configuration, you should know about shells and sub-shells.

• Shells are the main energy levels (K, L, M, N, ...).
• Sub-shells are subdivisions of shells (s, p, d, f).
• Electrons fill the lowest energy sub-shell first (Aufbau Principle).

Core Concept

Shells

According to Bohr's atomic theory, the electron in an atom revolves around the nucleus in one of the circular paths called shells or orbits. Each shell has a fixed energy. So each shell is also called an energy level.

Each shell is described by an n value. n can have values 1, 2, 3, ...

As the value of n increases, the distance of the electron from the nucleus and the energy of the shell increases.

Simple explanation: Shells are like layers of an onion around the nucleus. The K shell is closest; the N shell is farthest.

Sub-Shells

A shell or energy level is subdivided into sub-shells or sub-energy levels.

The n value of a shell is placed before the symbol for a sub-shell.

Simple explanation: Each shell is divided into sub-shells, like rooms inside a floor of a building.

Capacity of Sub-Shells

Order of Energy of Sub-Shells

The increasing order of energy of the sub-shells belonging to different shells is:

1s < 2s < 2p < 3s < 3p < 4s < 3d < …

Simple explanation: Electrons fill the lowest-energy sub-shell first. Notice that 4s fills before 3d — that's because 4s has lower energy than 3d.

Aufbau Principle

The arrangement of electrons in sub-shells is called the electronic configuration. We can fill the electrons present in various elements by using the Aufbau Principle.

According to this principle: Electrons fill the lowest energy sub-shell that is available first.

This means electrons will fill first 1s, then 2s, then 2p, and so on.

Simple explanation: "Aufbau" means "building up" in German. Electrons build up from the lowest energy level to the highest.

Detailed Explanation

Filling Order of Sub-Shells

The order in which sub-shells fill is:

1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s …

Simple explanation: This order is based on the energy of each sub-shell. Lower energy fills first.

How to Write Electronic Configuration

Step 1: Determine the number of electrons in the atom (atomic number for a neutral atom).

Step 2: Fill electrons into sub-shells in the order given above.

Step 3: Respect the capacity of each sub-shell (s = 2, p = 6, d = 10, f = 14).

Example: Sodium (Na), atomic number = 11

Electrons = 11

1s² 2s² 2p⁶ 3s¹

Total = 2 + 2 + 6 + 1 = 11 electrons.

Example: Chlorine (Cl), atomic number = 17

Electrons = 17

1s² 2s² 2p⁶ 3s² 3p⁵

Total = 2 + 2 + 6 + 2 + 5 = 17 electrons.

Example: Calcium (Ca), atomic number = 20

Electrons = 20

1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

Total = 2 + 2 + 6 + 2 + 6 + 2 = 20 electrons.

Note: 4s fills before 3d because it has lower energy.

Electron Configuration and the Periodic Table

The electronic configuration determines:

• Block of the element (s, p, d, f) — based on the sub-shell where the last electron goes.
• Period of the element — based on the highest n value.
• Group of the element — based on the number of valence electrons.

These are covered in detail in Unit 4 (Periodic Table).

Intermediate Level

Why 4s Fills Before 3d

The 4s sub-shell has lower energy than the 3d sub-shell. So according to the Aufbau Principle, 4s fills first.

• Potassium (K, Z = 19): 1s² 2s² 2p⁶ 3s² 3p⁶ **4s¹**
• Calcium (Ca, Z = 20): 1s² 2s² 2p⁶ 3s² 3p⁶ **4s²**
• Scandium (Sc, Z = 21): 1s² 2s² 2p⁶ 3s² 3p⁶ **3d¹ 4s²**

The n + l Rule

A useful rule for predicting the order of sub-shell energies:

• Calculate n + l for each sub-shell:
• n = principal quantum number
• l = azimuthal quantum number (s = 0, p = 1, d = 2, f = 3)
• Lower n + l → lower energy → fills first.
• If n + l is the same, lower n fills first.

• 4s: n + l = 4 + 0 = 4
• 3d: n + l = 3 + 2 = 5

Since 4 < 5, 4s fills before 3d.

Sub-Shell Capacity

Electronic Configuration and Chemical Properties

The valence shell electronic configuration determines the chemical properties of an element.

• Elements with the same valence configuration have similar chemical properties.
• This is why elements in the same group of the periodic table behave similarly.

Important Points

• - Electronic configuration: arrangement of electrons in sub-shells.
• - Shells: K, L, M, N, ... (n = 1, 2, 3, 4, ...).
• - Sub-shells: s, p, d, f.
• - **Capacities:** s = 2, p = 6, d = 10, f = 14.
• - Filling order: 1s < 2s < 2p < 3s < 3p < 4s < 3d < ...
• - Aufbau Principle: electrons fill the lowest energy sub-shell first.
• - 4s fills before 3d because 4s has lower energy.
• - Examples:
• - Na (Z = 11): 1s² 2s² 2p⁶ 3s¹
• - Cl (Z = 17): 1s² 2s² 2p⁶ 3s² 3p⁵
• - Ca (Z = 20): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

Remember

• - **Shells:** K, L, M, N (n = 1, 2, 3, 4).
• - Sub-shells: s (2e⁻), p (6e⁻), d (10e⁻), f (14e⁻).
• - Filling order: 1s < 2s < 2p < 3s < 3p < 4s < 3d < ...
• - Aufbau Principle: lowest energy fills first.
• - 4s fills before 3d.
• - Na: 1s² 2s² 2p⁶ 3s¹
• - Cl: 1s² 2s² 2p⁶ 3s² 3p⁵
• - Ca: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

Common Mistakes

• 1. Filling 3d before 4s. 4s fills first because it has lower energy.
• 2. Exceeding sub-shell capacity. s = 2, p = 6, d = 10, f = 14.
• 3. Forgetting the Aufbau Principle. Always fill the lowest energy sub-shell first.
• 4. Confusing shells with sub-shells. Shells are K, L, M, N; sub-shells are s, p, d, f.
• 5. Thinking the n value alone determines filling order. The n + l rule (or the standard filling order) determines the actual order.
• 6. Writing configurations for ions incorrectly. For cations, remove electrons from the highest n sub-shell first.

Quick Revision

• Electronic configuration: arrangement of electrons in sub-shells.
• Shells are described by n values: K (n=1), L (n=2), M (n=3), N (n=4).
• Sub-shells: s, p, d, f.
• **Maximum electrons:** s = 2, p = 6, d = 10, f = 14.
• Filling order: 1s < 2s < 2p < 3s < 3p < 4s < 3d < ...
• Aufbau Principle: electrons fill the lowest energy sub-shell first.
• 4s fills before 3d.
• Examples: Na (1s² 2s² 2p⁶ 3s¹), Cl (1s² 2s² 2p⁶ 3s² 3p⁵), Ca (1s² 2s² 2p⁶ 3s² 3p⁶ 4s²).

60-Second Revision

• Electronic configuration = electrons in sub-shells.
• **Shells:** K, L, M, N (n = 1, 2, 3, 4).
• Sub-shells: s (2), p (6), d (10), f (14).
• Filling order: 1s < 2s < 2p < 3s < 3p < 4s < 3d < ...
• Aufbau: lowest energy first.
• 4s before 3d.
• Warning: don't fill 3d before 4s.