Electron Configuration Calculator
Enter an element's atomic number and this tool builds the predicted ground-state electron configuration. Electrons are placed into atomic orbitals from lowest energy upward, following the Aufbau principle with Madelung's n+l ordering. Each subshell is filled to its capacity (2 for s, 6 for p, 10 for d, 14 for f) before the next begins. The output shows the full configuration string, the total electrons placed, the number of occupied subshells, and the electrons in the highest principal level. This is the standard textbook prediction; a handful of elements deviate experimentally.
Aufbau filling method
Subshell capacity = 2 * (2l + 1)
s=2, p=6, d=10, f=14
Madelung order: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p
Fill each subshell to capacity in order until Z electrons are placed
The configuration is assembled by walking the Madelung sequence and depositing electrons into each subshell until the atomic number is reached. The highest n level total is read from the largest principal quantum number that received electrons.
Quantum-filling facts
- The Aufbau principle places electrons in the lowest-energy orbital available first.
- Madelung's rule orders subshells by increasing n+l, then by increasing n for ties.
- The Pauli exclusion principle limits each orbital to two electrons of opposite spin.
- Subshell capacities are 2, 6, 10 and 14 for s, p, d and f respectively.
- Predicted configurations match experiment for most elements; chromium and copper are well-known exceptions.
Electron configuration: frequently asked questions
How is an electron configuration determined?
Electrons fill orbitals from lowest to highest energy following the Aufbau principle. The ordering is given by Madelung's rule: orbitals fill in order of increasing n+l, and for equal n+l, increasing n. Each subshell holds a maximum of 2(2l+1) electrons: 2 in s, 6 in p, 10 in d, and 14 in f.
Does this calculator show real exceptions like chromium and copper?
No. It applies the strict Aufbau and Madelung ordering, which is the standard predicted ground-state configuration taught in chemistry. A small number of elements (such as chromium, copper, and several heavier atoms) deviate experimentally due to subshell stability. The predicted filling is shown for clarity and consistency.
What is Madelung's rule?
Madelung's rule, also called the Klechkowski rule, states that subshells fill in order of increasing n+l value. When two subshells have the same n+l, the one with the lower principal quantum number n fills first. This gives the familiar sequence 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p and so on.
How many electrons fit in each subshell?
The capacity of a subshell is 2(2l+1). An s subshell (l=0) holds 2, a p subshell (l=1) holds 6, a d subshell (l=2) holds 10, and an f subshell (l=3) holds 14. The calculator fills each subshell to capacity in order until all electrons are placed.
What atomic numbers does this support?
Enter any whole atomic number from 1 upward. The Madelung sequence used here covers through the 7p subshell, which is sufficient for all currently named elements up to atomic number 118 (oganesson).
Official sources
- NIST: NIST Atomic Spectra Database.
- IUPAC: IUPAC Periodic Table of the Elements.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.