Electron configuration describes how electrons are distributed among the atomic orbitals of an element. It is one of those topics that connects the abstract world of quantum mechanics to the observable properties of elements — why sodium is reactive, why noble gases are inert, and why transition metals have multiple oxidation states.
The Basics: Shells, Subshells, and Orbitals
Electrons occupy shells (numbered 1, 2, 3, ...), which contain subshells (s, p, d, f), which contain orbitals. Each orbital can hold a maximum of two electrons with opposite spins. An s subshell has one orbital (2 electrons max), p has three (6 max), d has five (10 max), and f has seven (14 max). The filling order follows a specific pattern: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, and so on. Note that 4s fills before 3d — this is a common source of confusion.
Aufbau Principle and Exceptions
The Aufbau principle states that electrons fill orbitals starting with the lowest energy level first. For the most part, this works perfectly. But there are exceptions. Chromium (Z=24) has the configuration [Ar] 3d5 4s1 instead of [Ar] 3d4 4s2, and copper (Z=29) is [Ar] 3d10 4s1 instead of [Ar] 3d9 4s2. These exceptions occur because half-filled and fully filled d subshells are slightly more stable than nearly filled ones. You will encounter a few more exceptions in the heavier elements, but chromium and copper are the two you need to know for general chemistry.
Our Electron Configuration tool generates the full configuration for any element by atomic number.
Noble Gas Notation
Writing out the full electron configuration for heavy elements gets tedious. Noble gas notation abbreviates the configuration by starting with the previous noble gas in brackets. For iron (Z=26), instead of writing 1s2 2s2 2p6 3s2 3p6 3d6 4s2, you write [Ar] 3d6 4s2. The [Ar] part represents the electron configuration of argon (Z=18), which is the noble gas just before iron on the periodic table. This notation is more compact and highlights the valence electrons, which are the ones involved in chemical bonding.
Why Electron Configuration Matters
Electron configuration determines an element's position on the periodic table and explains many of its chemical properties. The number of valence electrons (electrons in the outermost shell) predicts how an element will react. Group 1 elements have one valence electron and readily lose it to form +1 ions. Group 17 elements have seven valence electrons and readily gain one to form -1 ions. Transition metals, with electrons in d orbitals, often form multiple ions (like Fe2+ and Fe3+). Understanding electron configuration is essential for predicting chemical behavior, writing formulas for ionic compounds, and understanding periodic trends.