Article — Electron Configuration Quiz
How the electron configuration quiz works
The electron configuration quiz draws random elements and ions from a 35-question bank and accepts both full and noble-gas shorthand answers. Scoring is live, exceptions for chromium (4s1 3d5) and copper (4s1 3d10) are baked in, and ion answers follow the rule of stripping highest-n electrons first.
Use the quiz as flashcard practice before a chemistry exam, or as a sanity check after reading the periodic table. The answer box parses whitespace and brackets loosely, so [Ar]4s2 3d6, [Ar] 4s2 3d6, and 1s2 2s2 2p6 3s2 3p6 4s2 3d6 are all marked correct for iron.
The Aufbau order behind every configuration
Every neutral atom in the ground state fills orbitals in a fixed sequence: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p. The mnemonic is the Madelung diagonal — read top-left to bottom-right and you cover the whole periodic table.
Subshell capacities are 2, 6, 10, 14 for s, p, d, f respectively. A complete configuration adds up to the atomic number Z. Vanadium with Z = 23 reaches [Ar] 4s2 3d3, which sums to 18 + 2 + 3 = 23. Get the sum wrong and the quiz will reject the answer.
The Aufbau order is not predicted from first principles. It is observed from spectroscopy and reproduced by careful Hartree-Fock calculations, but small reorderings happen for heavier elements where relativistic effects start to matter.
Noble-gas shorthand in the quiz
The shorthand replaces inner-shell electrons with the symbol of the previous noble gas in square brackets. Sulfur is [Ne] 3s2 3p4, vanadium is [Ar] 4s2 3d3, antimony is [Kr] 5s2 4d10 5p3. The quiz accepts both forms because both are correct and both are taught.
Why use shorthand at all? It highlights the valence electrons, which drive chemistry. The argon core of vanadium contributes nothing to its bonding behavior, so writing it out adds clutter without information. For period-3 and period-4 atoms the saving is modest. For uranium (Z = 92) the full form fills three lines while the shorthand fits on one.
Hund's rule and parallel spins
Within a degenerate subshell, electrons spread out across separate orbitals with parallel spins before pairing up. Nitrogen at Z = 7 has 2s2 2p3: each p orbital carries one electron, all spins parallel. The quiz does not test orbital diagrams directly, but Hund's rule is why nitrogen is paramagnetic and why oxygen's two unpaired 2p electrons make O2 magnetic.
Half-filled and fully-filled subshells are extra stable because of exchange energy. That stabilization is small for s and p but large for d — large enough to bend the Aufbau order for two famous d-block atoms.
Cr and Cu electron configuration exceptions
Chromium at Z = 24 has [Ar] 4s1 3d5, not the expected [Ar] 4s2 3d4. Half-filled 3d gains enough exchange stabilization to overcome the cost of moving one 4s electron up. Copper at Z = 29 follows the same logic for fully-filled 3d: [Ar] 4s1 3d10, not [Ar] 4s2 3d9.
The other d-block exceptions live in periods 5 and 6 — Mo, Pd, Ag, Pt, Au — but the quiz focuses on the two examples every general chemistry course covers.
Cr (Z=24) [Ar] 4s1 3d5Cu (Z=29) [Ar] 4s1 3d10Mo (Z=42) [Kr] 5s1 4d5Ag (Z=47) [Kr] 5s1 4d10Ion electron configuration rules
To find a cation's configuration, write the neutral atom and remove electrons starting from the highest principal quantum number n. Iron neutral is [Ar] 4s2 3d6. Fe2+ drops the two 4s electrons (higher n than 3d once 3d is occupied), giving [Ar] 3d6. Fe3+ drops one more from 3d, giving the half-filled and unusually stable [Ar] 3d5.
For anions, add electrons to the next available subshell in Aufbau order. Cl− is [Ar], adopting the noble-gas configuration of argon. Most common anions in general chemistry — O2−, F−, S2−, N3− — all reach a noble-gas configuration.
The half-filled stability that bends Cr and Cu in their neutral form vanishes once you ionize. Cr3+ is [Ar] 3d3, derived by removing two 4s electrons and one 3d electron from neutral [Ar] 4s1 3d5. Do not start from the "wrong" [Ar] 4s2 3d4 — you will get the right answer by luck but the reasoning is broken.
Quiz scoring and feedback
The score line shows correct answers, total attempts, and accuracy percent. Correct answers auto-advance after about a second. Wrong answers show the expected configuration and a short note (e.g. "half-filled 2p, Hund") so you can review the reasoning, then stay on the question so you can study it.
Use the Skip button when a question feels too easy to bother answering, or to reshuffle if you want a fresh prompt. Reset wipes the running score without reloading the page. Aim for 80% accuracy on the full bank before exam day — the bank covers every pattern that general chemistry tests are known to ask about.
Periodic table electron configuration blocks
The periodic table is partitioned into four blocks based on which subshell is being filled in the outermost electron. Groups 1 and 2 (plus helium) make up the s-block, where the ns subshell fills. Groups 13 to 18 make up the p-block, filling the np subshell. The d-block holds the transition metals where (n−1)d fills, and the f-block at the bottom holds the lanthanides and actinides where (n−2)f fills.
This partitioning is why the periodic table has its familiar shape. The d-block has 10 columns because d holds 10 electrons; the f-block has 14 columns because f holds 14. Once you recognize the block of any element you can write its valence electron configuration almost without thinking — the column tells you the count and the row tells you the principal quantum number.
Common electron configuration mistakes
- 4s before 3d in neutral atoms (correct), but the reverse when removing electrons from cations.
- Forgetting Cr and Cu — the two d-block exceptions every general chemistry course tests.
- Subscript arithmetic — the sum of all superscripts must equal Z for neutral atoms.
- Hund's rule violations — pairing up before half-filling a degenerate subshell is wrong.
- Wrong noble-gas core — use the noble gas immediately before the element, not two periods back.
- Ion direction — cations lose electrons, anions gain them; always count Z ± charge for the final total.