Drag or click electrons into subshells with Aufbau, Pauli, and Hund's rule enforced live.
Building electron configurations correctly, with the three governing rules enforced simultaneously, is a foundational AP Chemistry Unit 1 skill — and the handful of elements that violate the naive pattern are exactly where exams concentrate their hardest questions.
The Aufbau principle states that electrons fill available orbitals starting from the lowest energy level upward, following the (n+l) ordering that explains why 4s fills before 3d despite 3 being a smaller principal quantum number — a frequent point of confusion since it seems to violate simple shell-by-shell filling. The Pauli exclusion principle limits any single orbital to a maximum of two electrons, and only if they have opposite spin — no two electrons in the same atom can share an identical set of all four quantum numbers.
Hund's rule governs how electrons fill a set of degenerate (equal-energy) orbitals, such as the three p orbitals or five d orbitals within the same subshell: electrons occupy separate orbitals singly, with parallel spin, before any pairing occurs. This is why nitrogen's three 2p electrons occupy px, py, and pz individually rather than doubling up in just two of the three — minimizing electron-electron repulsion and maximizing total spin, which is genuinely lower in energy.
Chromium and copper are the two most commonly tested Aufbau exceptions in period 4: chromium's actual ground-state configuration is [Ar]3d⁵4s¹ rather than the "expected" [Ar]3d⁴4s², and copper's is [Ar]3d¹⁰4s¹ rather than [Ar]3d⁹4s² — both violations exist because a half-filled (d⁵) or completely filled (d¹⁰) d subshell carries extra stability from favorable exchange energy, enough to make promoting one 4s electron into the d subshell energetically worthwhile.
Whether an atom or ion is diamagnetic (all electrons paired, weakly repelled by a magnetic field) or paramagnetic (one or more unpaired electrons, attracted to a magnetic field) follows directly and unambiguously from its completed electron configuration.
This builder lets you fill electrons into subshells with Aufbau, Pauli, and Hund's rule enforced live for any element, correctly handles the Cr/Cu exceptions, and displays magnetism directly from the resulting configuration.