# Electron Configuration Calculator

Find the electron configuration of any element (1-118). Shows full configuration and noble gas shorthand using the aufbau principle.

## What this calculates

Enter an atomic number (1 through 118) to get the full electron configuration and noble gas shorthand notation. The calculator follows the aufbau principle, Hund's rule, and the Pauli exclusion principle, with corrections for known exceptions like chromium and copper.

## Inputs

- **Atomic Number** — min 1, max 118 — The atomic number (Z) equals the number of protons and, for a neutral atom, the number of electrons.

## Outputs

- **Element** — formatted as text — Element name and symbol.
- **Full Electron Configuration** — formatted as text — Complete electron configuration using the aufbau principle.
- **Noble Gas Shorthand** — formatted as text — Abbreviated configuration using the nearest noble gas core.
- **Valence Electrons** — formatted as text — Number of electrons in the outermost shell.

## Details

Electron configuration describes how electrons are distributed among an atom's orbitals. It determines chemical behavior, bonding patterns, and placement on the periodic table.

**The Aufbau Filling Order**

Electrons fill orbitals from lowest to highest energy:

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p

Each subshell holds a fixed number of electrons: s = 2, p = 6, d = 10, f = 14.

**Three Key Rules**

1. **Aufbau Principle** -- electrons fill the lowest-energy orbital first
2. **Pauli Exclusion Principle** -- each orbital holds at most 2 electrons with opposite spins
3. **Hund's Rule** -- electrons occupy empty orbitals in a subshell before pairing up

**Worked Example: Iron (Z = 26)**

Full configuration: 1s2 2s2 2p6 3s2 3p6 4s2 3d6

Noble gas shorthand: [Ar] 4s2 3d6

Iron has 8 valence-region electrons (2 in 4s and 6 in 3d), though only the 2 in 4s are considered "valence" for main-group counting.

**Common Exceptions**

Some elements break the pattern because half-filled or fully filled d subshells are extra stable:

| Element | Expected | Actual |
|---------|----------|--------|
| Cr (24) | [Ar] 4s2 3d4 | [Ar] 4s1 3d5 |
| Cu (29) | [Ar] 4s2 3d9 | [Ar] 4s1 3d10 |
| Mo (42) | [Kr] 5s2 4d4 | [Kr] 5s1 4d5 |
| Ag (47) | [Kr] 5s2 4d9 | [Kr] 5s1 4d10 |
| Au (79) | [Xe] 6s2 4f14 5d9 | [Xe] 6s1 4f14 5d10 |

## Frequently Asked Questions

**Q: What is the aufbau principle?**

A: The aufbau principle states that electrons fill orbitals starting with the lowest energy level and working upward. The filling order is determined by the (n + l) rule: subshells with a lower sum of the principal quantum number (n) and angular momentum quantum number (l) fill first. When two subshells have the same sum, the one with the lower n fills first.

**Q: Why do chromium and copper break the expected pattern?**

A: Half-filled and fully filled d subshells have extra stability due to electron exchange energy. Chromium prefers [Ar] 4s1 3d5 (half-filled d) over [Ar] 4s2 3d4, and copper prefers [Ar] 4s1 3d10 (full d) over [Ar] 4s2 3d9. The energy gain from the symmetric d arrangement outweighs having a full 4s orbital.

**Q: What is noble gas shorthand notation?**

A: Noble gas shorthand replaces the inner-shell electrons with the symbol of the preceding noble gas in brackets. For example, sodium (Z = 11) has full configuration 1s2 2s2 2p6 3s1. The first 10 electrons match neon, so the shorthand is [Ne] 3s1. This highlights only the valence electrons.

**Q: How do I count valence electrons from a configuration?**

A: For main-group elements, valence electrons are in the outermost shell (highest n value). Count the electrons in the s and p subshells of that shell. For example, oxygen is [He] 2s2 2p4, so it has 6 valence electrons. For transition metals, the counting is more nuanced because d electrons also participate in bonding.

---

Source: https://vastcalc.com/calculators/chemistry/electron-configuration
Category: Chemistry
Last updated: 2026-04-08