How this instrument works
The Aufbau principle (German for 'building up') describes how electrons fill an atom's orbitals in its lowest-energy, ground state: lowest-energy orbitals fill first, before any electron occupies a higher-energy one. That gives a standard filling order — 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and onward — that lets you predict an atom's full electron configuration just from its atomic number, without needing a table of measured data for every single element.
Most elements follow that order exactly. But a handful of transition metals don't, because nature rewards certain especially stable electron arrangements enough to break the 'always fill the next lowest slot' rule. Chromium (Z=24) is expected to be [Ar]3d⁴4s², but its real ground state is [Ar]3d⁵4s¹ — a half-filled 3d subshell turns out to be more stable than the naive prediction. Copper (Z=29) shows the same pattern one better: expected [Ar]3d⁹4s², actual [Ar]3d¹⁰4s¹, because a completely filled 3d subshell wins out. These two are the textbook examples every general chemistry course covers, and this instrument reports the real, exception-corrected configuration for both.
This instrument's lookup covers hydrogen through krypton (Z=1–36) — every element in the first four periods. Configuration exceptions become considerably more numerous and harder to state as simple rules starting in period five and beyond (lanthanide and actinide behavior in particular gets genuinely irregular), which is why this tool's scope stops at krypton rather than extending further: every configuration it returns has been individually verified rather than generated from a rule that starts breaking down.
- Select an element from the dropdown — hydrogen (Z=1) through krypton (Z=36) are available.
- Read the atomic number shown alongside your selection, for reference.
- Read the ground-state electron configuration, written in standard subshell notation (1s², 2s², 2p⁶, and so on).
Worked example — copper's textbook Aufbau exception
Copper's atomic number is 29, so a strict, unmodified Aufbau filling order would predict [Ar]3d⁹4s² — filling 4s before 3d, as the general rule says, then putting the remaining electrons into 3d up to nine. But copper's actual, experimentally measured ground state is [Ar]3d¹⁰4s¹: one electron is promoted from 4s into 3d, completing the 3d subshell at a full ten electrons and leaving 4s with only one.
The reason is subtle but real: a completely filled d subshell (3d¹⁰) turns out to be lower in overall energy than the naive prediction, enough to outweigh the small energy cost of half-emptying the 4s orbital. Chromium shows the same phenomenon one shell earlier, promoting an electron to reach a half-filled 3d⁵ rather than the 'expected' 3d⁴4s² — both are standard, well-documented exceptions covered in essentially every general chemistry textbook, not calculation errors.
Questions
What is the Aufbau principle?
It's the rule that, in an atom's lowest-energy ground state, electrons fill available orbitals starting from the lowest energy level and moving upward, one at a time, without skipping ahead to a higher-energy orbital while a lower one remains available. It gives a standard, predictable order for building up an atom's full electron configuration directly from its atomic number.
Why are chromium and copper exceptions to the Aufbau principle?
Both gain extra stability from having a particularly favorable d-subshell arrangement — copper's 3d becomes completely filled (3d¹⁰), and chromium's 3d becomes exactly half filled (3d⁵) — by promoting one electron out of the 4s orbital that the strict filling order would otherwise fill first. That gained stability from the special d-subshell arrangement outweighs the small energy cost of the promotion, so the atom's true, experimentally measured ground state configuration differs from what the naive Aufbau order alone would predict.
Why does this calculator stop at krypton (Z=36)?
Because configuration exceptions become significantly more common and harder to state as simple, reliable rules starting in period five and continuing through the lanthanides and actinides — several additional exceptions appear that require case-by-case verification rather than a clean general pattern. This instrument's lookup table covers hydrogen through krypton, where every configuration has been individually checked, rather than extending further on an unverified rule.
What does the superscript number in an electron configuration mean?
It's the number of electrons occupying that particular subshell. In oxygen's configuration 1s²2s²2p⁴, for example, the superscripts mean two electrons in 1s, two in 2s, and four in 2p — eight electrons total, matching oxygen's atomic number of 8. Every subshell has a maximum capacity (2 for s, 6 for p, 10 for d, 14 for f), and the superscript can never exceed that maximum.
How is electron configuration related to an element's position on the periodic table?
They're built from the same underlying pattern — the periodic table's rows (periods) and blocks (s, p, d, f) directly track which shell and subshell an element's outermost, or valence, electrons are filling. Elements in the same column (group) typically share the same valence electron configuration, which is exactly why elements in a group tend to share similar chemical behavior: valence electron arrangement, more than atomic number alone, drives how an element reacts.