ELECTRON STRUCTURE
& ORBITAL VIEWER
Aufbau Exceptions
Our database includes exact configurations for transition metals like Cr, Cu, Ag, and Au that deviate from standard subshell filling.
Hund's Rule & Pauli Exclusion Principle
Principal (n)
3
Energy Level
Azimuthal (l)
2
Subshell (d)
Magnetic (ml)
-2
Orientation
Spin (ms)
-1/2
Rotation
QUANTUM MECHANICS
IN THE BROWSER
"If you think you understand quantum mechanics, you don't understand quantum mechanics." — Richard Feynman.
Our tool simplifies the math so you can focus on the physics.
1Aufbau Principle
Electrons always occupy the lowest available energy orbital first (1s < 2s < 2p).
2Pauli Exclusion
No two electrons can have identical sets of quantum numbers; orbitals hold max 2 electrons with opposite spins.
3Hund's Rule
Orbitals of the same subshell are first occupied singly by electrons with fixed spins.
EXPLAINER & FAQ
Q. How are quantum numbers derived?
Each electron is a wave function solution. We solve for the 'n', 'l', 'ml', and 'ms' integers that define that electron's specific probability cloud.
Q. What about Cr and Cu exceptions?
To minimize energy, atoms like Chromium prefer half-filled subshells (4s1 3d5) instead of standard filling (4s2 3d4).
Q. What is a differentiating electron?
It is the last electron added to an atom to complete its configuration relative to the element with one fewer proton.
Jaconir Scientific Tool • 2026 Edition
How to read an electron configuration
Five steps from an element symbol to the four quantum numbers an exam question asks for.
Find the element by symbol, name or atomic number
Search accepts any of the three, so "Fe", "iron" and "26" all land on the same element. The viewer loads its accepted ground-state configuration rather than deriving one on the fly, which matters for the twenty elements where the derived answer would be wrong.
Read the noble-gas shorthand first, then expand it
Iron shows as [Ar] 4s² 3d⁶. The bracket stands for the full configuration of the previous noble gas — argon, eighteen electrons — so you only read the part that distinguishes this element. Coursework usually wants the shorthand; expand to the full string when you need to count electrons in a specific shell.
Use the box diagram to check Hund and Pauli
The orbital box diagram draws one box per orbital in the subshell — one for s, three for p, five for d, seven for f — with arrows for electron spin. Hund's rule says every box in a subshell takes one electron before any takes a second; Pauli says the two in a box must have opposite spin. Half-filled and filled subshells are visible immediately in that layout, which is the whole reason the diagram is worth drawing.
Read the four quantum numbers off the differentiating electron
The differentiating electron is the last one added relative to the element with one fewer proton, and the panel gives its n, l, mₗ and mₛ. Principal n is the shell, azimuthal l is the subshell shape (0 = s, 1 = p, 2 = d, 3 = f), mₗ is which orbital within the subshell, mₛ is the spin. This is the set most exam questions actually ask for.
Check the shell counts when the question is about valence
The principal shell breakdown — how many electrons sit in n = 1, 2, 3 and so on — answers valence and period questions faster than counting through the configuration string. For main-group elements the outermost shell count is the valence electron count; for transition metals it is not, which is exactly where the exceptions below start to matter.
The 20 elements that break Aufbau order
Computed by comparing each accepted configuration against strict n + l filling: 13 in the d block, 7 in the f block, and none anywhere else. Every one is an electron promoted to reach a half-filled or filled subshell.
| Element | Z | Aufbau predicts | Accepted |
|---|---|---|---|
| Chromium (Cr) | 24 | …3p6 4s2 3d4 | …3p6 4s1 3d5 |
| Copper (Cu) | 29 | …3p6 4s2 3d9 | …3p6 4s1 3d10 |
| Niobium (Nb) | 41 | …4p6 5s2 4d3 | …4p6 5s1 4d4 |
| Molybdenum (Mo) | 42 | …4p6 5s2 4d4 | …4p6 5s1 4d5 |
| Ruthenium (Ru) | 44 | …4p6 5s2 4d6 | …4p6 5s1 4d7 |
| Rhodium (Rh) | 45 | …4p6 5s2 4d7 | …4p6 5s1 4d8 |
| Palladium (Pd) | 46 | …4p6 5s2 4d8 | …3d10 4p6 4d10 |
| Silver (Ag) | 47 | …4p6 5s2 4d9 | …4p6 5s1 4d10 |
| Lanthanum (La) | 57 | …5p6 6s2 4f1 | …5p6 6s2 5d1 |
| Cerium (Ce) | 58 | …5p6 6s2 4f2 | …6s2 4f1 5d1 |
| Gadolinium (Gd) | 64 | …5p6 6s2 4f8 | …6s2 4f7 5d1 |
| Platinum (Pt) | 78 | …6s2 4f14 5d8 | …6s1 4f14 5d9 |
| Gold (Au) | 79 | …6s2 4f14 5d9 | …6s1 4f14 5d10 |
| Actinium (Ac) | 89 | …6p6 7s2 5f1 | …6p6 7s2 6d1 |
| Thorium (Th) | 90 | …6p6 7s2 5f2 | …6p6 7s2 6d2 |
| Protactinium (Pa) | 91 | …6p6 7s2 5f3 | …7s2 5f2 6d1 |
| Uranium (U) | 92 | …6p6 7s2 5f4 | …7s2 5f3 6d1 |
| Neptunium (Np) | 93 | …6p6 7s2 5f5 | …7s2 5f4 6d1 |
| Curium (Cm) | 96 | …6p6 7s2 5f8 | …7s2 5f7 6d1 |
| Lawrencium (Lr) | 103 | …7s2 5f14 6d1 | …7s2 5f14 7p1 |
Configurations shown from the differentiating subshells only; the noble-gas core is identical in both columns. Derived from the same 118-element database the viewer above reads.
The rules the viewer applies
Four rules decide every configuration on this page, and one of them explains almost all the exceptions.
- Aufbau order is n + l, not n1s 2s 2p 3s 3p 4s 3d 4p 5s 4d …
- Subshells fill in order of increasing n + l, and where two subshells tie, the lower n goes first. That is why 4s (n + l = 4) fills before 3d (n + l = 5) despite the lower principal number, and it is the single most common source of confusion when writing configurations by hand. The viewer applies this ordering everywhere except where an accepted configuration overrides it.
- Hund's rule: spread before you pair
- Within a subshell, electrons occupy separate orbitals with parallel spin until every orbital holds one. Pairing two electrons in one orbital costs energy through electrostatic repulsion, so nature avoids it while empty orbitals remain. Nitrogen's three 2p electrons sit in three separate boxes, all spin-up — not one pair and a single.
- Pauli exclusion: two per orbital, opposite spin
- No two electrons in an atom share all four quantum numbers, which caps every orbital at two electrons of opposite spin. That is where the subshell capacities come from: one s orbital holds 2, three p orbitals hold 6, five d hold 10, seven f hold 14.
- Why half-filled and filled subshells win
- A half-filled or completely filled d subshell is more stable than the strict filling order predicts, through a combination of exchange energy between parallel spins and reduced repulsion. When promoting one s electron buys a d⁵ or d¹⁰ arrangement, the atom takes it. That single mechanism explains most of the exceptions listed below, and it is why they cluster in the d block rather than appearing at random.
Frequently asked questions
How do I write the electron configuration of an element?
Fill subshells in order of increasing n + l — 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d and so on — respecting the capacities two, six, ten and fourteen for s, p, d and f. Stop when you have placed as many electrons as the atomic number. Then optionally replace the leading part with the previous noble gas in brackets to get the shorthand. That procedure is right for ninety-eight of the hundred and eighteen elements; the rest are listed on this page.
Which elements are exceptions to the Aufbau principle?
Twenty, all in the d and f blocks: chromium, copper, niobium, molybdenum, ruthenium, rhodium, palladium, silver, lanthanum, cerium, gadolinium, platinum, gold, actinium, thorium, protactinium, uranium, neptunium, curium and lawrencium. The table on this page shows what strict Aufbau order predicts alongside the accepted configuration for each, so you can see exactly which electron moved.
Why are chromium and copper exceptions?
Both gain stability by promoting one 4s electron into 3d. Chromium becomes 4s¹ 3d⁵ instead of 4s² 3d⁴, trading a filled s subshell for a half-filled d subshell; copper becomes 4s¹ 3d¹⁰ instead of 4s² 3d⁹, buying a completely filled d subshell. Half-filled and filled d configurations are stabilised by exchange energy between parallel spins and by lower electron–electron repulsion, and that gain outweighs the cost of the promotion.
What is an orbital diagram and how do I draw one?
One box per orbital — one for s, three for p, five for d, seven for f — with an arrow per electron, up for one spin and down for the other. Fill left to right, one electron per box before doubling up (Hund), and never more than two per box, always opposed (Pauli). The viewer draws this for the differentiating subshell so you can check your own by eye.
What are the four quantum numbers?
n is the principal quantum number, the shell, counting from 1. l is azimuthal and describes the subshell shape: 0 is s, 1 is p, 2 is d, 3 is f. mₗ names which orbital within that subshell, running from −l to +l. mₛ is spin, either +½ or −½. Together they identify one electron uniquely, which is precisely what the Pauli exclusion principle requires.
Does it cover ions, or only neutral atoms?
Neutral ground-state atoms only. For a cation, remove electrons from the highest principal shell first rather than reversing the filling order — iron loses its 4s electrons before its 3d, giving Fe²⁺ as [Ar] 3d⁶, not [Ar] 4s² 3d⁴. That ordering catches people out because it is not simply the Aufbau sequence played backwards.
How many elements are covered?
All 118, hydrogen through oganesson, with the accepted ground-state configuration for each. Configurations for the heaviest synthetic elements are predicted rather than spectroscopically measured, since only a handful of atoms of each have ever existed; treat anything above roughly element 100 as the accepted theoretical value.
Is anything sent to a server?
No. The element data ships with the page and every lookup, diagram and quantum-number derivation runs in your browser. There is no account and nothing is uploaded.