Electron Configuration
Electron configuration is the distribution of electrons of an atom or molecule among its atomic orbitals, described by the principal quantum number (n), sublevel (s, p, d, f), and the number of electrons in each sublevel. Electron configuration determines how an element behaves chemically — its valence electrons (outermost shell) govern reactivity, oxidation states, and bonding capacity. The Aufbau principle, Pauli exclusion principle, and Hund's rule govern how electrons fill orbitals.
Key Formula
General notation: [sublevel][superscript number of electrons], e.g. 1s² 2s² 2p⁶ 3s¹
LaTeX: 1s^2\, 2s^2\, 2p^6\, 3s^2\, 3p^6\, \ldots
| Symbol | Meaning | Unit |
|---|---|---|
| n | Principal quantum number (shell number) | dimensionless integer |
| s, p, d, f | Sublevel/orbital type (shape) | dimensionless |
| superscript | Number of electrons in that sublevel | dimensionless |
Worked Example
Problem
Write the full electron configuration of Silicon (Si, Z = 14).
Solution
Step 1: Silicon has 14 electrons to fill. Step 2: Fill in order using Aufbau principle: 1s² → 2 electrons (total: 2) 2s² → 2 electrons (total: 4) 2p⁶ → 6 electrons (total: 10) 3s² → 2 electrons (total: 12) 3p² → 2 electrons (total: 14) Step 3: Full configuration: 1s² 2s² 2p⁶ 3s² 3p².
Answer
Electron configuration of Silicon: 1s² 2s² 2p⁶ 3s² 3p². It has 4 valence electrons (in 3s² 3p²), explaining its Group 14 position.
Electron Configurations of the First 10 Elements
| Element | Z | Configuration | Valence Electrons | Group |
|---|---|---|---|---|
| Hydrogen (H) | 1 | 1s¹ | 1 | 1 |
| Helium (He) | 2 | 1s² | 2 | 18 |
| Lithium (Li) | 3 | 1s² 2s¹ | 1 | 1 |
| Beryllium (Be) | 4 | 1s² 2s² | 2 | 2 |
| Boron (B) | 5 | 1s² 2s² 2p¹ | 3 | 13 |
| Carbon (C) | 6 | 1s² 2s² 2p² | 4 | 14 |
| Nitrogen (N) | 7 | 1s² 2s² 2p³ | 5 | 15 |
| Oxygen (O) | 8 | 1s² 2s² 2p⁴ | 6 | 16 |
| Fluorine (F) | 9 | 1s² 2s² 2p⁵ | 7 | 17 |
| Neon (Ne) | 10 | 1s² 2s² 2p⁶ | 8 | 18 |
Interactive Tools
Ptable – Electron Configuration
View and explore electron configurations for all 118 elements
Open ToolKhan Academy – Electron Configuration
Step-by-step guide to writing electron configurations using the Aufbau principle
Open ToolWolfram Alpha – Electron Configuration
Compute electron configurations and orbital diagrams for any element
Open Tool
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Related Terms
Electron
An electron is a negatively charged subatomic particle that occupies the space around an atom's nucleus in regions called orbitals or electron shells, with a charge of -1 elementary charge and a mass of 9.109 × 10⁻³¹ kg (about 1/1836 the mass of a proton). Electrons govern chemical bonding, electrical conductivity, and the optical properties of matter by determining how atoms interact with one another. In a neutral atom, the number of electrons equals the number of protons.
Atomic Number
The atomic number (symbol Z) is the number of protons in the nucleus of an atom and uniquely identifies the chemical element — all atoms of the same element have the same atomic number. In a neutral atom, the atomic number also equals the number of electrons, which determines the element's chemical behavior, reactivity, and position on the periodic table. The atomic number ranges from 1 (hydrogen) to 118 (oganesson) for currently known elements.
Ion
An ion is an atom or molecule that has gained or lost one or more electrons, resulting in a net electric charge — positive ions (cations) form when electrons are lost, and negative ions (anions) form when electrons are gained. Ions play a fundamental role in chemical bonding (ionic compounds), electrochemistry, biological processes (nerve impulses rely on Na⁺ and K⁺ ions), and analytical chemistry. The charge of an ion is written as a superscript, such as Ca²⁺ or Cl⁻.
The term combines "electron" (from Greek "elektron", amber) and "configuration" (from Latin "configuratio", form or shape). The theoretical framework was developed in the 1920s by Niels Bohr, Wolfgang Pauli (exclusion principle, 1925), and Friedrich Hund (Hund's rule, 1925), with the Aufbau principle formalised by Bohr.