Electron Configuration

Electrons fill orbitals by the Aufbau principle, the Pauli exclusion principle, and Hund’s rule. They enter lower-energy orbitals first, at most two with opposite spins per orbital, and unpaired electrons first among equal-energy orbitals. Changing the atomic number changes the configuration, and the number of unpaired electrons links to magnetism and chemical behavior. Here you change Z and watch the orbital energy diagram fill according to those three rules.

Where Do Electrons Go?

💡 Analogy: Moving into an Apartment

①Fill from floor 1 (1s) up → Aufbau principle

②Max 2 per room with opposite spins → Pauli exclusion

③On the same floor, fill each room singly first → Hund rule

④These three rules determine electron configuration

Configuration by Atomic Number

Atomic Number Z6

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①Z=1 (H): 1s¹ — one electron

②Z=6 (C): 1s²2s²2p² — Hund rule at 2p!

③Z=10 (Ne): 2p completely filled → noble gas

④Z=19 (K): 4s fills before 3d (energy crossover)

Three Rules

Aufbau Principle

1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p

Fill orbitals from lowest energy first

Pauli Exclusion Principle

Max 2 per orbital (↑↓)

No two electrons can have the same quantum numbers

Hund's Rule

Same-energy orbitals → singly fill first

Minimizes electron-electron repulsion

💡 Unpaired Electrons and Chemistry

①More unpaired electrons → stronger paramagnetism

②All paired → diamagnetic

③Variable oxidation states of transition metals (3d) ← unpaired electrons

④Same group = same valence electrons → similar chemistry

Work It Out

Example 1

Write the ground-state electron configuration of sodium (Na, atomic number 11).

By the Aufbau principle, fill the 11 electrons starting from the lowest-energy orbitals.

1s² 2s² 2p⁶ 3s¹

Add up the electrons in each orbital to confirm it matches the atomic number.

sum of electrons = 2+2+6+1 = 11

▸ 1s² 2s² 2p⁶ 3s¹

The last electron sits alone in 3s, so sodium is a group-1 metal with one valence electron.

Example 2

Find the electron configuration and number of unpaired electrons of oxygen (O, atomic number 8).

Fill the 8 electrons by the Aufbau principle.

1s² 2s² 2p⁴

Place the four 2p electrons according to Hund’s rule.

2p orbitals: ↑↓ ↑ ↑ → 2 unpaired electrons

▸ 1s² 2s² 2p⁴, 2 unpaired electrons

Hund’s rule fills each box singly first, so 2p⁴ leaves two unpaired electrons.

Summary

Max Electrons per Shell

Shell n: max 2n²

n=1→2, n=2→8, n=3→18, n=4→32

Max per Subshell

s=2, p=6, d=10, f=14

Each subshell l holds 2(2l+1)

2022 KICE mock Chemistry I type, adapted

Which element has the ground-state configuration 1s² 2s² 2p⁶ 3s² 3p⁵? (Decide by atomic number.)

①F

②Ne

③Na

④Cl

⑤Ar

▸ ④ Cl

sum of electrons = 2+2+6+2+5 = 17

Identify the element with atomic number 17.

atomic number 17 → Cl (chlorine)

🎯 Exam Points

①Write configurations: 1s²2s²2p⁶3s²3p⁶...

②4s fills before 3d (energy inversion)

③Noble gases (He, Ne, Ar) = full shell = stable

④Counting unpaired electrons — magnetism and bonding

⑤Ion configurations: cations remove from outermost first

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Atomic Model Evolution

Periodic Table

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