Study Guide
How to Write Electron Configuration: Step-by-Step Guide
Electron configuration describes how electrons are distributed among the orbitals of an atom. Writing it correctly requires understanding three rules: the Aufbau principle (fill lowest energy orbitals first), the Pauli exclusion principle (maximum two electrons per orbital with opposite spins), and Hund's rule (spread electrons across orbitals before pairing).
This guide walks through every step of writing electron configurations, including full notation, noble-gas shorthand, ion configurations, and the common exceptions you will encounter in class.
The quick method in 5 steps
Step 1: Find the atomic number
The atomic number equals the number of protons and, for a neutral atom, the number of electrons. Write it down before filling anything.
Oxygen has atomic number 8, so a neutral oxygen atom has 8 electrons. Chlorine has atomic number 17, so a neutral chlorine atom has 17 electrons.
Step 2: Know the Aufbau filling order
Electrons fill orbitals from lowest to highest energy. The sequence is:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p
Orbital capacities: s holds 2, p holds 6, d holds 10, f holds 14.
The 4s orbital fills before 3d because it has slightly lower energy for neutral atoms. This is why potassium is [Ar] 4s1, not [Ar] 3d1.
Step 3: Fill the orbitals in order
Place electrons one at a time following the filling order. Apply Hund's rule: when filling a p, d, or f subshell, put one electron in each orbital before pairing any.
Oxygen (8 electrons): 1s2 2s2 2p4 -- the 2p subshell gets one electron in each of its three orbitals first, then the fourth electron pairs in the first orbital.
Chlorine (17 electrons): 1s2 2s2 2p6 3s2 3p5 -- fill through 3s completely, then place the remaining 5 electrons in 3p.
Step 4: Write the full configuration
Write subshells in order of increasing principal quantum number (n) for final display, not filling order. This means 3d comes before 4p, even though 4s filled before 3d.
Full notation for iron (26 electrons): 1s2 2s2 2p6 3s2 3p6 3d6 4s2 Filling order: 1s 2s 2p 3s 3p 4s 3d Written order: 1s 2s 2p 3s 3p 3d 4s
Step 5: Use noble-gas shorthand (optional)
Find the noble gas that comes before your element in the periodic table. Replace its full configuration with the noble gas symbol in brackets.
Iron: [Ar] 3d6 4s2 (argon is the noble gas before iron, and [Ar] represents 1s2 2s2 2p6 3s2 3p6)
Gold: [Xe] 4f14 5d10 6s1 (xenon is the noble gas before gold)
Noble-gas shorthand is the standard for elements beyond period 3 because writing the full configuration becomes very long.
Worked examples: full notation
Example 1: Silicon (Si, atomic number 14)
Step 1: 14 electrons. Step 2: Follow the filling order: 1s2 2s2 2p6 3s2 3p2. Step 3: Assign superscripts: 1s2 (2 e-), 2s2 (2 e-), 2p6 (6 e-), 3s2 (2 e-), 3p2 (2 e-). Total: 14. Correct. Full notation: 1s2 2s2 2p6 3s2 3p2 Shorthand: [Ne] 3s2 3p2
Example 2: Calcium (Ca, atomic number 20)
Step 1: 20 electrons. Step 2: Fill through 4s: 1s2 2s2 2p6 3s2 3p6 4s2. Full notation: 1s2 2s2 2p6 3s2 3p6 4s2 Shorthand: [Ar] 4s2
Example 3: Bromine (Br, atomic number 35)
Step 1: 35 electrons. Step 2: Fill: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5. Full notation: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p5 (reordered by n) Shorthand: [Ar] 3d10 4s2 4p5
Example 4: Tin (Sn, atomic number 50)
Step 1: 50 electrons. Step 2: Fill: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p2. Full notation: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p2 Shorthand: [Kr] 4d10 5s2 5p2
Example 5: Uranium (U, atomic number 92)
Step 1: 92 electrons. Step 2: Fill: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f4. Full notation: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f4 6s2 6p6 7s2 Shorthand: [Rn] 5f4 7s2
Common mistakes and how to avoid them
Mistake 1: Confusing filling order with written order
The filling order (Aufbau) is not the same as the final written order. The written order groups subshells by principal quantum number n. Filling: 1s 2s 2p 3s 3p 4s 3d 4p Written: 1s 2s 2p 3s 3p 3d 4s 4p
Fix: Use filling order to place electrons, then reorder by n for the final answer.
Mistake 2: Forgetting 4s fills before 3d
Many students write 3d before 4s in the filling sequence. For neutral atoms, 4s has slightly lower energy than 3d. Correct: 1s 2s 2p 3s 3p 4s 3d 4p Incorrect: 1s 2s 2p 3s 3p 3d 4s 4p (wrong filling order, though this is the correct written order)
Fix: Memorize the energy-order sequence or use the periodic table as a guide.
Mistake 3: Miscounting electrons in ions
Neutral atom configurations do not apply directly to ions. A Fe2+ ion has lost 2 electrons, giving it 24 electrons instead of 26.
Fe neutral: [Ar] 3d6 4s2 Fe2+: [Ar] 3d6 (the 4s electrons are removed first for transition-metal cations)
Fix: For cations, subtract the charge from the atomic number first. For anions, add the charge.
Mistake 4: Misapplying Hund's rule
When filling a p subshell with 4 electrons (like oxygen), do not pair two orbitals before filling all three.
Correct 2p4: ↑↓ ↑ ↑ (one pair, two singles) Incorrect 2p4: ↑↓ ↑↓ □ (one empty orbital)
Fix: Draw orbital boxes and fill each orbital with one electron before pairing.
Mistake 5: Not checking total electron count
After writing a configuration, always sum the superscripts. If the total does not match the atomic number (or adjusted count for ions), the configuration is wrong.
Fix: Add the superscripts: 1s2 + 2s2 + 2p6 + ... = total electrons.
Mistake 6: Writing the noble-gas core incorrectly
The noble gas bracket represents the FULL configuration of that noble gas, including all inner shells.
[Ar] = 1s2 2s2 2p6 3s2 3p6 (18 electrons) [Ne] = 1s2 2s2 2p6 (10 electrons)
Do not leave out the inner d or f electrons when using shorthand.
Mistake 7: Ignoring the exceptions
Chromium (Cr, Z=24) and copper (Cu, Z=29) have configurations that differ from the Aufbau prediction.
Cr predicted: [Ar] 3d4 4s2 Cr actual: [Ar] 3d5 4s1 (half-filled d subshell is extra stable)
Cu predicted: [Ar] 3d9 4s2 Cu actual: [Ar] 3d10 4s1 (filled d subshell is extra stable)
Other exceptions include molybdenum, silver, and gold. Your textbook may list more.
Ion electron configurations
Writing configurations for cations (positive ions)
When an atom loses electrons to form a positive ion, remove electrons starting from the highest n value, not necessarily the last subshell that was filled.
For main-group elements: remove from the outermost s and p subshells. For transition metals: remove from the 4s subshell before the 3d subshell.
Example: Fe (26 e-) → Fe2+ (24 e-) Neutral Fe: [Ar] 3d6 4s2 Remove 4s electrons first: [Ar] 3d6
Example: Al (13 e-) → Al3+ (10 e-) Neutral Al: [Ne] 3s2 3p1 Remove 3s and 3p electrons: [Ne] (same as Ne)
Writing configurations for anions (negative ions)
When an atom gains electrons, add the extra electrons to the outermost partially filled subshell.
Example: Cl (17 e-) → Cl- (18 e-) Neutral Cl: [Ne] 3s2 3p5 Add one electron: [Ne] 3s2 3p6 = [Ar]
Example: O (8 e-) → O2- (10 e-) Neutral O: 1s2 2s2 2p4 Add two electrons: 1s2 2s2 2p6 = [Ne]
Isoelectronic series
Ions with the same electron configuration are called isoelectronic. For example, O2-, F-, Na+, Mg2+, and Al3+ all have the configuration [Ne] (10 electrons). Recognizing isoelectronic series helps predict relative sizes and properties.
Try it
Use the interactive calculator
Quick verification checklist
- Count electrons: does the total match the atomic number (adjusted for ions)?
- Follow Aufbau order: did you fill 4s before 3d?
- Apply Hund's rule: did you spread electrons before pairing in p, d, or f subshells?
- Check the written order: subshells grouped by n, not filling order.
- Verify shorthand: does the noble-gas core represent the correct number of inner electrons?
- Handle exceptions: is this chromium, copper, or another element with a known exception?
Common questions about writing electron configurations
What is the easiest way to remember the Aufbau filling order?
Use the diagonal rule: draw arrows from 1s to 7p following the diagonals. Or use the periodic table: period 1 fills 1s, period 2 fills 2s 2p, period 3 fills 3s 3p, period 4 fills 4s 3d 4p, and so on.
Why does 4s fill before 3d?
For neutral atoms, the 4s orbital has slightly lower energy than 3d because of electron shielding and penetration effects. This changes for ions, where 3d often becomes lower than 4s.
Do I always need to write the full configuration?
No. For elements beyond period 3, noble-gas shorthand is standard and usually preferred in textbooks and exams. Check your instructor's preference.
How do I know which electrons to remove for transition-metal ions?
Remove the 4s electrons first, then 3d electrons if needed. Even though 4s filled first, it is the outermost shell and the first to be lost during ionization.
What are the most common electron configuration exceptions?
Chromium ([Ar] 3d5 4s1), copper ([Ar] 3d10 4s1), molybdenum ([Kr] 4d5 5s1), silver ([Kr] 4d10 5s1), and gold ([Xe] 4f14 5d10 6s1). Exceptions occur when a half-filled or completely filled d subshell provides extra stability.
Can I always trust the Aufbau principle?
The Aufbau principle works for most elements on the periodic table, but exceptions exist for roughly 20 elements where electron-electron interactions override the simple energy order. Use a reliable periodic table or configuration chart for confirmation.
Continue learning
Study notes and practice checks
How to use this guide effectively
Do not just read this guide. After each section, write the configuration for a different element on paper. Start with simple period-2 elements (Li, Be, B, C, N, O, F, Ne) and work up to period 4 transition metals.
For each element you try: 1. Write the atomic number. 2. Predict the configuration using the filling order. 3. Write the full notation. 4. Write the noble-gas shorthand. 5. Check with the electron configuration calculator. 6. If you made a mistake, identify which step was wrong.
This active recall method is far more effective than passive reading.
Edge cases to practice
- Elements at the boundary between s and d blocks (K, Ca, Sc, Ti)
- Elements with f electrons (lanthanides and actinides)
- Ions of transition metals (Fe2+, Fe3+, Cu+, Cu2+)
- Anions of nonmetals (N3-, O2-, F-, S2-)
- Exceptions (Cr, Cu, Mo, Ag, Au)
Practicing these edge cases will prepare you for exam questions that go beyond memorization.
Extra practice prompt
Practice prompt
Write the electron configuration for titanium (Ti, Z=22) without looking at any reference. Use the 5-step method: count electrons, follow Aufbau order (1s 2s 2p 3s 3p 4s 3d), apply Hund's rule for 3d, write the full notation in order of n, then write the noble-gas shorthand. Check your total electron count and verify with the calculator.
Now try a harder one: write the configuration for Tc3+ (technetium ion, Z=43, charge +3). Remember to account for the charge before filling and remember that 4s electrons are removed before 4d for transition-metal cations.
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