Electron Configuration Calculator

Determine electron configuration for any element using aufbau principle. Enter atomic number or element symbol to get ground state electron configuration in subshell notation.

⚗️ Atomic Structure📐 1s 2s 2p 3s 3p...🧪 Chemistry
Atomic number Z (1-118)
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Formula & Reference

VariableSymbolFormulaUnits
Electron Configuration Calculator—1s 2s 2p 3s 3p...subshell notation

Step-by-Step Examples

Example 1
Iron (Z=26)

Fe: atomic number 26.

  • Filling order: 1s2 2s2 2p6 3s2 3p6 4s2 3d6
  • Total: 2+2+6+2+6+2+6 = 26
  • Abbreviated: [Ar] 3d6 4s2
✓ Fe: 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^6
Example 2
Sodium (Z=11)

Na: atomic number 11.

  • 1s2 2s2 2p6 3s1
  • Total: 2+2+6+1 = 11
  • Abbreviated: [Ne] 3s1
✓ Na: 1s^2 2s^2 2p^6 3s^1
Example 3
Oxygen (Z=8)

O: atomic number 8.

  • 1s2 2s2 2p4
  • Total: 2+2+4 = 8
✓ O: 1s^2 2s^2 2p^4

Real-World Applications

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Periodic Table Patterns
Electron configurations explain the block structure of the periodic table: s, p, d, f blocks.
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Chemical Bonding
Valence electrons from configuration determine bonding capacity and molecular geometry.
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Spectroscopy
Atomic emission spectra result from electrons changing configurations. Finger-print of each element.
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Magnetism
d-electron configurations determine magnetic properties: Fe, Co, Ni are ferromagnetic from unpaired d electrons.

Common Mistakes to Avoid

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4s fills before 3d (filling order)

Aufbau principle: fill in energy order, not principal quantum number order. 4s fills before 3d.

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Exceptions: Cr and Cu

Cr: [Ar] 3d5 4s1 (not 3d4 4s2). Cu: [Ar] 3d10 4s1 (not 3d9 4s2). Half-filled and filled d subshells have extra stability.

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3d empties before 4s for ions

When forming Fe^2+: remove 4s electrons first. Fe: [Ar]3d6 4s2. Fe^2+: [Ar]3d6. Not [Ar]3d4.

Frequently Asked Questions

What is the aufbau principle? ▾
Fill orbitals in order of increasing energy. Order: 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d... Use the diagonal rule.
What is Hund rule? ▾
Within a subshell, electrons fill each orbital singly (with same spin) before pairing. 2p^4 in oxygen: up up up down (not two pairs).
What is the Pauli exclusion principle? ▾
No two electrons in an atom can have identical quantum numbers. Each orbital holds max 2 electrons with opposite spins.
Why are Cr and Cu exceptions? ▾
Half-filled (d5) and completely filled (d10) d subshells are especially stable due to exchange energy. One electron moves from 4s to complete the d subshell.
What does the valence configuration tell you? ▾
Valence electrons (outermost) determine reactivity and bonding. Na: [Ne]3s1 (1 valence e). Cl: [Ne]3s2 3p5 (7 valence e). Noble gases: full valence shells.

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Formula Explorer connections

Interpretation: This relationship uses electronic structure, bonding or molecular geometry to predict a chemical property or structural descriptor. Assumption: The model may be an approximation; resonance, solvent, coordination environment, conformation and experimental conditions can affect real molecules.

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