Writing an electron configuration should take under a minute, but most students slow themselves down by memorizing a zigzag diagram they half-remember on exam day. There is a faster, more reliable route: read the order straight off the periodic table itself. This guide gives you the rules, the shortcut, and worked examples — so electron configurations stop being a memory test.
The Three Rules You're Actually Using
Every electron configuration obeys three principles. Know what each does and the process is no longer mechanical guessing.
- The Aufbau principle: electrons fill the lowest-energy orbital available before moving to a higher one. Build the atom from the ground up.
- The Pauli exclusion principle: an orbital holds at most 2 electrons, and they must have opposite spins. This sets capacities: an s subshell (1 orbital) holds 2, p (3 orbitals) holds 6, d (5 orbitals) holds 10, f (7 orbitals) holds 14.
- Hund's rule: within a subshell, electrons spread out one per orbital before any orbital gets a second. It matters for orbital diagrams and unpaired-electron counts.
For writing the configuration itself, Aufbau plus the capacities do most of the work.
The Filling Order
Electrons fill subshells in this energy order:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d
The non-obvious part is that 4s fills before 3d, and 5s before 4d — energy levels overlap once you reach the fourth row. You can reproduce this with the diagonal-rule diagram, but memorizing a diagram is exactly the fragile step we want to skip.
The Periodic Table Shortcut
The periodic table is the filling order — it was built that way. Read left to right, top to bottom, and the blocks hand you each subshell:
- Groups 1–2 are the s-block. The row number is the s level.
- Groups 13–18 are the p-block. The p level is the row number.
- The transition metals are the d-block. The d level is the row number minus 1 — that is the 4s-before-3d overlap, shown geometrically.
- The lanthanides and actinides are the f-block, level = row minus 2.
To write any element's configuration, start at hydrogen and sweep across each period to your element, naming the block and counting boxes. You never recall a diagram — you just read the map in front of you.
Worked Examples
Iron (Z = 26). Sweep the table: 1s² (row 1) → 2s² 2p⁶ (row 2) → 3s² 3p⁶ (row 3) → 4s² (start of row 4) → then into the d-block, where iron is the 6th element, giving 3d⁶. Total: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶. Count the superscripts: 2+2+6+2+6+2+6 = 26. ✓
Noble-gas shorthand. Find the noble gas just before your element and put it in brackets, then continue. Iron becomes [Ar] 4s² 3d⁶ — argon covers the first 18 electrons. This is faster to write and what most exams expect.
The two famous exceptions
Chromium and copper break the pattern. Chromium is [Ar] 4s¹ 3d⁵, not 4s² 3d⁴, and copper is [Ar] 4s¹ 3d¹⁰, not 4s² 3d⁹. A half-filled (d⁵) or fully filled (d¹⁰) d subshell is extra stable, so an atom will borrow one electron from 4s to reach it. Memorize these two; they are the exceptions general chemistry tests most.
Configurations for Ions
Ions need one extra rule, and it is the rule students get wrong most often.
For an anion (negative ion), just add electrons in the normal filling order. Chloride, Cl⁻, has one more electron than chlorine, so it becomes [Ne] 3s² 3p⁶ — the configuration of argon.
For a cation (positive ion), you remove electrons — but not in the reverse of the filling order. You remove from the highest principal quantum number (n) first. For main-group metals this rarely matters, but for transition metals it is decisive. Iron is [Ar] 4s² 3d⁶. To form Fe²⁺ you remove the two electrons with the highest n, which are the 4s electrons, not the 3d ones — even though 4s filled first. Fe²⁺ is therefore [Ar] 3d⁶, and Fe³⁺ is [Ar] 3d⁵. The rule "filled last is not removed first; highest n is removed first" reverses what most students expect.
Getting Help
Electron configuration is the structure behind the patterns you read off the table. To see how filling order produces atomic size and reactivity, work through periodic table trends, or browse the General Chemistry study guides for related topics.
Conclusion
You can write electron configurations fast by treating the periodic table as the filling order it already is: sweep period by period, name each block (s, p, d minus one, f minus two), and count boxes to your element. Use noble-gas shorthand to shorten the write-up, and memorize only chromium and copper as exceptions. No fragile diagram required — just read the map.
