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What does this calculator estimate?
An electron configuration calculator writes the orbital filling for any element. Enter an atomic number (1–118) to see the full configuration, the element name, and electron count.
- Aufbau order: 1s 2s 2p 3s 3p 4s 3d 4p…
- Capacity: s=2, p=6, d=10, f=14
- Iron (Z=26): …4s² 3d⁶
How electron configurations work
Electrons fill orbitals from lowest to highest energy (the Aufbau principle), with the periodic table's blocks reflecting the s, p, d, and f orbitals being filled. The configuration predicts an element's chemistry — valence electrons determine bonding.
Limitations to watch for
Chromium and copper are exceptions (half-filled d orbitals are more stable), which the tool follows standard tables for. Configurations for heavy elements involve relativistic effects. This is the ground state — excited states differ.
How to use it in practice
Use the configuration to identify valence electrons and predict oxidation states and bonding. Read the last occupied subshell for the element's block and family. Compare with the periodic table to see the pattern.
['Enter the atomic number (1–118).', 'The tool builds the configuration in Aufbau order.', "Use the valence shell to predict the element's chemistry."]
How this calculator works
Formula
Electrons fill orbitals in energy order (Aufbau): 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s… with shell capacities 2 (s), 6 (p), 10 (d), 14 (f). The tool writes the configuration for any atomic number 1–118.
Worked example
Iron (Z = 26): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶. Chlorine (Z = 17): 1s² 2s² 2p⁶ 3s² 3p⁵.
Assumptions to verify
- The Aufbau filling order applies.
- Standard ground-state configurations (including Cr/Cu exceptions).
- Atomic numbers 1–118 are supported.
Frequently asked questions
What is an electron configuration?
The arrangement of an atom's electrons among its orbitals — written as subshells with superscript counts (e.g., iron: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶).
What is the Aufbau principle?
Electrons fill the lowest-energy orbitals first: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on.
How many electrons fit in each subshell?
s holds 2, p holds 6, d holds 10, f holds 14. The superscripts in a configuration must sum to the atomic number.
Why is chromium's configuration unusual?
Cr (Z=24) is [Ar] 4s¹ 3d⁵ instead of 4s² 3d⁴ — a half-filled d subshell is more stable. Copper similarly: [Ar] 4s¹ 3d¹⁰.
How do I find valence electrons?
Look at the highest principal energy level (n): for iron, 4s² gives 2 valence electrons. Groups in the periodic table reflect this.
What does the configuration predict?
Valence electrons determine how an element bonds, its oxidation states, and its position in the periodic table.
What does the tool support?
Atomic numbers 1–118 — hydrogen through oganesson — with standard ground-state configurations.
Cite this tool
BoringToolsKit. “Electron Configuration Calculator.” boringtoolskit.com/electron-configuration-calculator/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
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