Electron Configuration Calculator
Find the full electron configuration for any element (Z = 1–118), following the Aufbau filling order, plus its noble-gas shorthand notation and valence electron count. Two 3D diagrams show electron shells filling up and compare the Aufbau exception where 4s fills before 3d, with charts showing subshell capacities and filling order.
Reviewed by the ToolNestr Editorial Team — July 2026
Pick a preset, or enter an atomic number directly.
Filling order in 3D
1. Electron shells filling outward
Nested shells around a central nucleus, each holding progressively more electrons as atomic number increases.
2. The 4s-before-3d exception
The 4s subshell (outer, amber) sits at slightly lower energy than the 3d subshell (inner, indigo) — so it fills first.
Electron configuration graphs
How it works
The core idea in one line: electrons fill available orbitals from lowest energy to highest, and because orbital energies don't line up neatly with shell number, the real filling order (1s,2s,2p,3s,3p,4s,3d,...) zig-zags in a way that surprises students expecting strict numerical order.
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p
the Aufbau filling order — not simple shell-by-shell order
s = 2, p = 6, d = 10, f = 14
maximum electrons per subshell type
Each subshell type holds a fixed number of electrons based on its orbital count (s:1, p:3, d:5, f:7 orbitals, each holding 2 electrons), giving maximum capacities of 2, 6, 10, and 14 respectively. Filling proceeds strictly by increasing energy level along the Aufbau order, adding electrons subshell by subshell until the atom's full electron count is reached. Noble-gas shorthand simply replaces everything up through the most recent completed noble-gas configuration with that gas's symbol in brackets, since that inner 'core' never participates in ordinary bonding.
Worked example 1 — oxygen (Z = 8)
Given: Oxygen has atomic number Z = 8. Find its full electron configuration and valence electron count.
Oxygen needs 2 more electrons to fill its outer shell to a stable 8 — exactly why it so readily forms 2⁻ ions or two covalent bonds.
Worked example 2 — iron (Z = 26, an Aufbau-order example)
Given: Iron has atomic number Z = 26. Find its full electron configuration, noting the 4s-before-3d filling order.
This is the classic Aufbau exception: 4s fills before 3d during buildup, even though 3d is conventionally written first when listing the final configuration by shell number.
Subshell capacities and filling order
Each subshell type holds a fixed maximum number of electrons, based on how many orbitals it contains.
| Subshell | Orbitals | Max electrons |
|---|---|---|
| s | 1 | 2 |
| p | 3 | 6 |
| d ★ | 5 | 10 |
| f | 7 | 14 |
★ Each orbital holds a maximum of 2 electrons (opposite spins), so capacity = orbitals × 2 — this is where every subshell's maximum electron count comes from.
Where electron configuration actually matters
📊 Predicting chemical reactivity
An element's valence electron configuration predicts how many bonds it tends to form and how reactive it is — elements with nearly full or nearly empty outer shells (like halogens and alkali metals) are typically the most reactive.
🧲 Magnetic properties of materials
Unpaired electrons in partially filled d or f subshells (common in transition metals and lanthanides) give materials like iron and neodymium their magnetic properties — a direct consequence of their electron configuration.
🔬 Spectroscopy and atomic emission
When electrons absorb energy and jump between subshells, then fall back down, they emit light at specific wavelengths — the basis of atomic emission spectroscopy, flame tests, and neon signage.
📐 Understanding periodic table blocks
The periodic table's s-block, p-block, d-block, and f-block regions directly correspond to which subshell type is being filled last for elements in that region — electron configuration is the reason the table has the shape it does.
Common misconceptions
"Electrons fill shell 3 completely before any electrons enter shell 4."
This is one of the most common electron-configuration misconceptions — the 4s subshell actually fills before 3d, due to its slightly lower energy. Filling follows the Aufbau order (1s,2s,2p,3s,3p,4s,3d,4p,...), not strict numerical shell order.
"Valence electrons always equal the total electron count."
Only the electrons in the outermost occupied shell count as valence electrons — inner, already-filled shells (called the core) do not participate in ordinary bonding, even though they are still part of the atom's total electron count.
"Every element's configuration follows the Aufbau order exactly, with no exceptions."
A handful of elements (notably Chromium and Copper) deviate from the predicted Aufbau order because a half-filled or fully-filled d subshell provides extra stability — Chromium is [Ar]4s¹3d⁵ rather than the naively predicted [Ar]4s²3d⁴.
"Noble-gas shorthand changes an element's actual electron count."
Shorthand notation is purely a writing convenience — [Ne]3s¹ for sodium represents exactly the same 11 electrons as the full 1s²2s²2p⁶3s¹, just with the noble-gas core abbreviated.
Formula sources & further reading
The formulas here are standard, traceable to:
- • OpenStax, Chemistry 2e — Chapter 6, "Electronic Structure and Periodic Properties of Elements" (free, peer-reviewed). openstax.org
- • Brown, LeMay & Bursten, Chemistry: The Central Science — Chapter 6, Electronic Structure of Atoms.
- • Zumdahl & Zumdahl, Chemistry — Electron configurations and the Aufbau principle.
Configurations follow the standard Aufbau filling order. A small number of known exceptions (e.g. Cr, Cu, and some heavier elements) are not special-cased in this calculator's default output — check a reference table for those specific elements.
How to use this calculator
Enter the atomic number
Type Z from 1 to 118, or pick a preset element.
Read the configuration
The full configuration, shorthand, and valence count appear instantly.
Compare on the chart
See subshell capacities and how the filling order departs from simple shell order.
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Frequently asked questions
What is the Aufbau principle?
The Aufbau principle states that electrons fill the lowest-energy orbitals available first, following the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p — not simply shell-by-shell in numerical order.
Why does 4s fill before 3d?
Even though shell 3 is "lower" than shell 4, the 4s orbital happens to sit at a slightly lower energy level than 3d for the elements where this matters, so electrons occupy 4s first. This is one of several well-known Aufbau ordering exceptions.
How many electrons fit in each subshell?
s subshells hold up to 2 electrons, p subshells hold up to 6, d subshells hold up to 10, and f subshells hold up to 14 — these capacities come directly from the number of orbitals available in each subshell type (1, 3, 5, and 7 orbitals respectively, times 2 electrons each).
What is noble-gas shorthand notation?
Instead of writing out every filled subshell, noble-gas shorthand replaces the electron configuration of the nearest preceding noble gas with its symbol in brackets — for example, Sodium (Z=11) is written [Ne]3s¹ instead of 1s²2s²2p⁶3s¹.
What are valence electrons?
Valence electrons are the electrons in an atom's outermost occupied shell, which largely determine its chemical bonding behavior. For main-group elements, this is simply the electrons in the highest-numbered s and p subshells; transition metals have some added nuance because their (n−1)d electrons can also participate in bonding.