Electron Configuration
Understand how electrons are arranged in atoms using shells, subshells, and orbitals, and learn to write electron configurations using the Aufbau principle.
Shells, Subshells and Orbitals
Electrons in an atom are arranged in energy levels (shells), each of which contains one or more subshells (s, p, d, f), and each subshell contains orbitals -- regions of space where electrons are most likely to be found.
Energy Level Structure
Shell 1 (n=1)
1s
Max 2 electrons
Shell 2 (n=2)
2s, 2p
Max 8 electrons
Shell 3 (n=3)
3s, 3p, 3d
Max 18 electrons
Shell 4 (n=4)
4s, 4p, 4d, 4f
Max 32 electrons
Orbital Types
s orbital: Spherical shape, 1 orbital per subshell, holds 2 electrons.
p orbitals: Dumbbell shape, 3 orbitals per subshell, holds 6 electrons.
d orbitals: Complex shapes, 5 orbitals per subshell, holds 10 electrons.
Maximum Electrons
Each orbital holds a maximum of 2 electrons with opposite spins (Pauli exclusion principle).
Maximum per shell = 2n² where n is the shell number.
The Aufbau Principle and Filling Order
The Aufbau principle (German for "building up") states that electrons fill orbitals starting from the lowest energy level and moving to higher levels. The filling order does not always follow shell numbers because some subshells overlap in energy.
Orbital Filling Order
1s
2s
2p
3s
3p
4s
3d
4p
Note: 4s fills before 3d because 4s is lower in energy for most elements.
Hund's rule: When filling orbitals of equal energy (e.g., the three 2p orbitals), electrons occupy them singly first with parallel spins before pairing up. This minimises electron-electron repulsion.
Writing Electron Configurations
An electron configuration lists the occupied subshells with the number of electrons in each as a superscript. This notation tells us exactly where every electron in an atom is located.
Example Configurations
Carbon (Z = 6)
1s² 2s² 2p²
6 electrons total
Sodium (Z = 11)
1s² 2s² 2p⁶ 3s¹
11 electrons total
Chlorine (Z = 17)
1s² 2s² 2p⁶ 3s² 3p⁵
17 electrons total
Iron (Z = 26)
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶
26 electrons total
Linking Configuration to the Periodic Table
Group number relates to the number of valence (outermost) electrons for main group elements.
Period number indicates the outermost shell being filled.
Block: s-block (Groups 1-2), p-block (Groups 13-18), d-block (transition metals), f-block (lanthanides/actinides).
Key Vocabulary
Orbital
A region of space around the nucleus where there is a high probability of finding an electron. Each orbital holds a maximum of 2 electrons.
Aufbau Principle
Electrons fill orbitals in order of increasing energy, starting from the lowest available energy level.
Valence Electrons
The electrons in the outermost shell of an atom. They determine an element's chemical properties and bonding behaviour.
Pauli Exclusion Principle
No two electrons in an atom can have the same set of four quantum numbers -- each orbital holds at most 2 electrons with opposite spins.
Worked Examples
Write the electron configuration of phosphorus (Z = 15).
Step 1: Phosphorus has 15 electrons. Fill orbitals in order: 1s, 2s, 2p, 3s, 3p.
Step 2: 1s² (2), 2s² (4), 2p⁶ (10), 3s² (12), 3p³ (15).
Answer: 1s² 2s² 2p⁶ 3s² 3p³. Phosphorus has 5 valence electrons (in shell 3).
Write the electron configuration of calcium (Z = 20).
Step 1: Calcium has 20 electrons. Fill: 1s², 2s², 2p⁶, 3s², 3p⁶, 4s².
Step 2: Check: 2+2+6+2+6+2 = 20. Correct.
Answer: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s². Note that 4s fills before 3d.
Identify the element with configuration 1s² 2s² 2p⁶ 3s² 3p⁴.
Step 1: Count total electrons: 2+2+6+2+4 = 16.
Step 2: For a neutral atom, number of electrons = atomic number = 16.
Answer: The element is sulfur (S), with 6 valence electrons in the third shell.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
How many electrons can the 2p subshell hold?
Question 2
What is the electron configuration of neon (Z = 10)?
Question 3
According to the Aufbau principle, which subshell fills immediately after 3p?
Question 4
How many valence electrons does aluminium (Z = 13) have?
Question 5
What is the maximum number of electrons in shell 3 (n = 3)?
Key Concepts Summary
- ●Electrons occupy shells (n = 1, 2, 3...), subshells (s, p, d, f), and orbitals (max 2 electrons each).
- ●The Aufbau principle fills orbitals from lowest to highest energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p...
- ●Hund's rule: electrons fill degenerate orbitals singly before pairing.
- ●Maximum electrons per shell = 2n².
- ●Valence electrons in the outermost shell determine chemical behaviour and bonding.