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Every element’s chemistry — whether it’s a reactive metal or an inert gas, whether it forms +1 ions or −2 ions — comes down to how its electrons are arranged. That arrangement is the electron configuration, and three rules are enough to work it out for almost every element.
Shells, subshells and orbitals
Electrons around a nucleus are organized in levels:
- Shells (n = 1, 2, 3…) — broadly, how far from the nucleus the electrons are.
- Subshells within each shell, labelled s, p, d, f.
- Orbitals within each subshell. Each orbital holds a maximum of two electrons.
| Subshell | Orbitals | Max electrons |
|---|---|---|
| s | 1 | 2 |
| p | 3 | 6 |
| d | 5 | 10 |
| f | 7 | 14 |
Shell 1 has only an s subshell, shell 2 has s and p, shell 3 has s, p and d, and shell 4 onward has s, p, d and f.
Rule 1: the Aufbau principle (lowest energy first)
Aufbau is German for “building up”. Electrons fill the lowest-energy subshells first. The energy order isn’t simply shell by shell, because subshells from different shells overlap. The order is:
1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p
Notice that 4s fills before 3d, and 6s before 4f. A common way to remember this is the diagonal rule: write the subshells in rows (1s; 2s 2p; 3s 3p 3d; 4s 4p 4d 4f…) and read along diagonals from top right to bottom left.
The easiest way of all is to use the periodic table itself, which is laid out in exactly this order — see s, p, d and f blocks.
Rule 2: the Pauli exclusion principle (two per orbital, opposite spins)
Wolfgang Pauli stated in 1925 that no two electrons in an atom can have the same set of four quantum numbers. The practical meaning: an orbital holds at most two electrons, and they must have opposite spins — drawn as one arrow up (↑) and one down (↓).
Rule 3: Hund’s rule (spread out before pairing)
When electrons enter a subshell with several orbitals of equal energy (p, d or f), they occupy each orbital singly first, with parallel spins, before any pairing up. Electrons repel each other, so they prefer separate orbitals.
Nitrogen’s 2p subshell, with three electrons:
- Correct: [↑] [↑] [↑]
- Wrong: [↑↓] [↑] [ ]
This is why a nitrogen atom has three unpaired electrons, and an oxygen atom (2p⁴: [↑↓] [↑] [↑]) has two. Atoms and ions with unpaired electrons are attracted into magnetic fields (paramagnetism), so Hund’s rule has measurable consequences.
Writing configurations
The notation lists each subshell with its electron count as a superscript.
- Hydrogen (1): 1s¹
- Carbon (6): 1s² 2s² 2p²
- Sodium (11): 1s² 2s² 2p⁶ 3s¹
- Iron (26): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶
Check: the superscripts always add up to the atomic number.
Noble-gas shorthand
Long configurations get tedious, so the inner electrons are abbreviated with the symbol of the previous noble gas in square brackets:
- Sodium: [Ne] 3s¹
- Iron: [Ar] 4s² 3d⁶
- Lead: [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p²
The part after the bracket shows the outer electrons, which are the ones that matter for chemistry.
Configurations of ions
- Negative ions gain electrons into the next available subshell. Cl: [Ne] 3s² 3p⁵ → Cl⁻: [Ne] 3s² 3p⁶ (the same as argon).
- Positive ions lose electrons from the highest shell number first — and for transition metals that means the 4s electrons go before the 3d ones, even though 4s filled first. Fe: [Ar] 4s² 3d⁶ → Fe²⁺: [Ar] 3d⁶ → Fe³⁺: [Ar] 3d⁵.
That last point catches out a lot of students. “Last in, first out” does not apply to transition metals.
When the rules bend
A handful of elements don’t follow the simple filling order. Chromium is [Ar] 4s¹ 3d⁵ rather than 4s² 3d⁴, and copper is [Ar] 4s¹ 3d¹⁰ rather than 4s² 3d⁹. We explain why in electron configuration exceptions.
Quick answers
Why does 4s fill before 3d? For the neutral atoms where they start filling, 4s is slightly lower in energy. Once 3d electrons are present, the balance shifts, which is why 4s electrons are also the first removed.
What are valence electrons? The electrons in the outermost shell (plus, for transition metals, often the d electrons). See how to find valence electrons.
Do heavy elements follow these rules? Mostly, with more exceptions among the d- and f-block elements, and predicted rather than measured configurations for the heaviest synthetic ones.
Try it
The electron configuration generator gives the full and noble-gas notation for any element, draws the orbital box diagram following Hund’s rule, and handles the exceptions.
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