Electron Configuration Calculator
Pick an element and, if you like, an ion charge. You get the full configuration, noble-gas shorthand and the orbital diagram.
Orbital diagram
- Valence electrons (neutral atom)
- 8
- Block
- d-block
- Electrons per shell (neutral)
- 2, 8, 14, 2
- Unpaired electrons
- 4
Watch the subshells fill: the Aufbau order
Add electrons one at a time. Each goes into the lowest-energy subshell with space: follow the diagonal arrows, not the shell numbers. Notice 4s filling before 3d at potassium (19).
Start with a bare nucleus.
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AI answers can be wrong. The calculators on this page are deterministic; trust them for numbers.
Reading the notation
In 3d⁶, 3 is the shell (principal quantum number n), d is the subshell type (l = 2), and the superscript 6 is the number of electrons in it. Noble-gas notation swaps the inner electrons for the previous noble gas in brackets: iron’s 1s² 2s² 2p⁶ 3s² 3p⁶ is exactly argon, so iron is [Ar] 3d⁶ 4s².
Three rules decide the ground state: the Aufbau principle (lowest energy first), the Pauli exclusion principle (at most two electrons per orbital, opposite spins) and Hund’s rule (fill orbitals singly before pairing). The orbital diagram shows all three at once.
Need configurations for the whole table at once? See the periodic table with electron configurations, or count outer electrons with the valence electron calculator.
Questions students ask
How do you write an electron configuration?
Fill subshells in order of increasing energy (the Aufbau principle): 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p. An s subshell holds 2 electrons, p holds 6, d 10 and f 14. Stop when you have placed as many electrons as the atomic number. Write each subshell with its electron count as a superscript, e.g. oxygen 1s² 2s² 2p⁴.
Why does 4s fill before 3d?
In a neutral potassium or calcium atom, the 4s orbital is lower in energy because it penetrates closer to the nucleus. The n + l rule (Madelung rule) captures this: 4s has n + l = 4, 3d has 5, so 4s fills first. Once 3d starts to fill, it drops below 4s, which is why transition metals lose their 4s electrons first when they form ions.
How do I write the configuration of an ion?
For a negative ion, add electrons to the next available subshell (Cl⁻: [Ne] 3s² 3p⁶). For a positive ion, remove electrons from the highest shell number first, not the last one filled. Fe²⁺ is [Ar] 3d⁶, not [Ar] 3d⁴ 4s².
What are the exceptions to the Aufbau principle?
Chromium ([Ar] 3d⁵ 4s¹) and copper ([Ar] 3d¹⁰ 4s¹) are the ones every course tests. A half-filled or completely filled d subshell has extra stability from exchange energy, enough to pull one electron out of 4s. The calculator flags every exception using measured ground-state configurations.
What is Hund’s rule?
Within a subshell, electrons occupy empty orbitals singly with parallel spins before any orbital gets a second electron. That is why nitrogen’s three 2p electrons sit in three separate boxes in the orbital diagram.