Electron Configuration Calculator | All Elements 1-118

What Is an Electron Configuration Calculator?

An electron configuration calculator finds how the electrons in an atom are arranged among its orbitals. An orbital is a region around the nucleus where an electron is likely to be found. Enter an atomic number from 1 to 118, and the calculator returns the electron configuration in full notation and in noble gas (shorthand) notation, along with a diagram of how the orbitals fill.

What Is Electron Configuration?

Electron configuration is a short code that shows how many electrons sit in each subshell of an atom. It is written as a series of labels like 1s, 2s, and 2p, each followed by a small superscript number. The label names the energy level and the shape of the subshell. The superscript counts the electrons in it. For example, carbon's configuration is 1s² 2s² 2p², meaning two electrons in the 1s subshell, two in 2s, and two in 2p.

Three rules govern how electrons fill these subshells:

• Aufbau principle: electrons fill the lowest-energy subshells first, before moving to higher ones.
• Pauli exclusion principle: each orbital holds at most two electrons, and the two must spin in opposite directions.
• Hund's rule: when several orbitals in a subshell have equal energy, such as the three p orbitals, electrons fill each one singly before any orbital gets a second electron.

The order in which subshells fill, from lowest to highest energy, is:

11s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p
2

Note that 4s fills before 3d, even though the "3" is lower than "4". This is one of the trickier parts of the pattern, and it is why the order above, not the raw shell numbers, decides the sequence.

Electron Configuration Formula: How to Calculate It

To build a configuration by hand, work through the filling order above and place electrons into each subshell until they run out. Each type of subshell holds a fixed maximum number of electrons:

Steps:

1. Find the atomic number of the element. This equals the number of electrons in a neutral atom.
2. Go through the filling order (1s, 2s, 2p, 3s, ...) and fill each subshell to its maximum before moving to the next.
3. Stop once every electron has been placed. The last subshell may be only partly full.

Worked example: sodium (Na, atomic number 11)

Sodium has 11 electrons. Filling in order: 1s holds 2, 2s holds 2, 2p holds 6 (running total: 10), leaving 1 electron for 3s.

Full notation: 1s² 2s² 2p⁶ 3s¹

Noble gas notation

Noble gas notation shortens the configuration by replacing the completed inner shells with the symbol of the nearest lighter noble gas in brackets. The six noble gases used as reference points are helium (2), neon (10), argon (18), krypton (36), xenon (54), and radon (86).

Sodium's first ten electrons, 1s² 2s² 2p⁶, match neon's full configuration exactly. So sodium can be written as:

[Ne] 3s¹

This shorthand is common for larger atoms, where the full notation would run to a dozen or more terms.

More worked examples

Exceptions to the Aufbau Principle

A handful of elements do not follow the filling order exactly. This happens because a subshell that is exactly half full or completely full is slightly more stable than the arrangement the Aufbau principle predicts, so one electron shifts from the s subshell to the d or f subshell nearby.

A few dozen other elements, mostly among the transition metals, lanthanides, and actinides, have similar irregularities. The calculator applies the experimentally observed configuration for each of these elements rather than the one the plain Aufbau order would predict.

How to Use the Electron Configuration Calculator

1. Enter an atomic number between 1 and 118.
2. Choose noble gas notation or full notation.
3. Read the element name, symbol, electron configuration, and orbital filling diagram.
4. Copy the result with the copy button.

Why Electron Configuration Matters

The electron configuration of an atom, especially the arrangement of its outermost (valence) electrons, largely decides how it bonds with other atoms. Elements in the same column of the periodic table share a similar valence configuration, which is why they behave alike chemically. Chemists use electron configuration to predict oxidation states, bond types, and reactivity. Physicists use it to explain atomic spectra, the specific colors of light an element absorbs or emits. Electron configuration also underlies quantum number notation, where four numbers (n, l, mₗ, mₛ) describe an electron's energy level, subshell shape, orbital orientation, and spin.

Frequently Asked Questions

What is electron configuration? Electron configuration is the arrangement of electrons among an atom's subshells, written as a sequence like 1s² 2s² 2p⁶. It follows the Aufbau principle, the Pauli exclusion principle, and Hund's rule.

How do you write an electron configuration? List each occupied subshell in order of filling, with a superscript showing its electron count. Carbon (atomic number 6) is written 1s² 2s² 2p².

What is noble gas notation? It is a shorthand that replaces the electrons matching a full noble gas configuration with that gas's symbol in brackets, followed by the remaining electrons. Sodium is [Ne] 3s¹ instead of 1s² 2s² 2p⁶ 3s¹.

Why do chromium and copper break the normal filling pattern? A half-filled or fully filled d subshell is more stable than the configuration Aufbau order predicts, so one 4s electron moves into the 3d subshell.

How many electrons can a subshell hold? An s subshell holds up to 2, p up to 6, d up to 10, and f up to 14.

How do valence electrons relate to the periodic table? Valence electrons are those in the outermost shell. For main-group elements, the number of valence electrons usually matches the element's group number, which is why elements in the same group react in similar ways.