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Grade 10-11 (age 15-17)

high school Electron Configuration

Electrons do not drop into orbitals at random; they take their places by three rules applied in order. The Aufbau principle fills the lowest-energy orbitals first, the Pauli exclusion principle allows at most two electrons of opposite spin per orbital, and Hund's rule spreads electrons one per orbital across orbitals of equal energy before any pair up. With those three rules you can write a configuration from the atomic number alone, and the unpaired electrons left over hint at magnetism and bonding. In the diagram each box is one orbital and the up and down arrows are two electrons of opposite spin, so raise Z and follow the order in which the boxes fill.

Which Box Fills First?

💡 Think of Tenants Moving into a Building
①Fill the lowest floor (1s) first. Nobody moves upstairs while a lower floor has room; that is the Aufbau principle
②Each room takes at most two tenants, facing opposite ways. The ↑ and ↓ sharing a box in the diagram is the Pauli exclusion principle
③When a floor has several rooms, each gets one tenant before anyone doubles up. Oxygen puts one electron in each of the three 2p boxes, then the fourth pairs up in the first box, leaving 2 unpaired electrons
④Applying the three rules in order gives exactly one ground-state configuration for any atom

Filling Up as Z Grows

6
🔍 Elements to Stop At
①Z = 1 (H): 1s¹. A single electron takes the lowest box
②Z = 6 (C): 1s² 2s² 2p². The two 2p electrons go into separate boxes instead of sharing one, so there are 2 unpaired electrons
③Z = 10 (Ne): 2p is full and nothing is unpaired, which is why neon is an unreactive noble gas
④Z = 19 (K): the 3d boxes are empty, yet the 19th electron goes to 4s, because 4s lies lower in energy than 3d
⑤Z = 24 (Cr) and 29 (Cu) are exceptions: one 4s electron moves to 3d, leaving 3d half full or completely full

The Three Rules

Aufbau Principle
1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p
Orbitals fill in order of increasing energy
Pauli Exclusion Principle
At most 2 electrons per orbital (↑↓)
No two electrons in an atom can share all four quantum numbers
Hund's Rule
Equal-energy orbitals → one each first, spins parallel
Electrons kept apart repel less, which lowers the energy
💡 Unpaired Electrons Change Behavior
①Atoms with unpaired electrons are paramagnetic, drawn into a magnetic field, and more unpaired electrons make the pull stronger
②When every electron is paired, the atom is diamagnetic and weakly pushed out of the field
③Elements in the same group share a valence configuration (every group-1 element ends in ns¹), so they react alike
④In transition metals 4s and 3d sit close in energy, so they can give up electrons from both and show several oxidation states

Work It Out

Example 1
Write the ground-state electron configuration of sodium (Na, atomic number 11).
1
Place 11 electrons starting from the lowest-energy orbital: 2 in 1s, 2 in 2s, 6 in 2p, and the last one goes to 3s.
1s² 2s² 2p⁶ 3s¹
2
Add the superscripts to confirm they match the atomic number.
2 + 2 + 6 + 1 = 11
▸ 1s² 2s² 2p⁶ 3s¹
The last electron sits alone in 3s at the start of a new shell. Sodium gives up that one electron easily, which makes it a reactive group-1 metal.
Example 2
Find the ground-state electron configuration of oxygen (O, atomic number 8) and its number of unpaired electrons.
1
Fill 8 electrons by the Aufbau principle.
1s² 2s² 2p⁴
2
By Hund's rule, put one of the four 2p electrons in each of the three boxes, then pair the last one in the first box.
2p: ↑↓ ↑ ↑ → 2 unpaired electrons
▸ 1s² 2s² 2p⁴, 2 unpaired electrons
Four 2p electrons do not pack into two boxes as two pairs. The oxygen in the first diagram shows exactly this arrangement.

Summary

Maximum Electrons per Shell
Shell with principal quantum number n: at most 2n²
n = 1 → 2, n = 2 → 8, n = 3 → 18, n = 4 → 32
Maximum Electrons per Subshell
s = 2, p = 6, d = 10, f = 14
A subshell with azimuthal quantum number l has 2l + 1 orbitals and holds 2(2l + 1) electrons
2022 KICE mock Chemistry I type, adapted
Which element has the ground-state configuration 1s² 2s² 2p⁶ 3s² 3p⁵?
①F
②Ne
③Na
④Cl
⑤Ar
▸ ④ Cl
1
The superscripts add up to the number of electrons, which equals the atomic number.
2
2 + 2 + 6 + 2 + 5 = 17, so pick the element with atomic number 17.
atomic number 17 → Cl (chlorine)
🎯 Exam Points
①Write configurations in filling order and check that the superscripts add up to the atomic number
②4s fills before 3d
③Full shells or subshells, as in He, Ne and Ar, are stable
④Count unpaired electrons by drawing boxes with Hund's rule: p³ has 3, p⁴ has 2
⑤Cations lose electrons from the outermost shell first: Na⁺ loses 3s¹ and becomes 1s² 2s² 2p⁶
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Evolution of Atomic Models
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Periodic Table & Properties

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