Electron Configuration
Definition and meaning of Electron Configuration in chemistry.
Electron configuration is the arrangement of electrons in an atom's orbitals. Chemists write it in shorthand, such as 1s² 2s² 2p⁶, where each term lists an energy level, an orbital type, and the number of electrons it holds. This arrangement determines how many valence electrons an atom has, which controls how it bonds with other atoms.
In more detail
Electrons fill orbitals according to three simple rules. The Aufbau principle says electrons fill the lowest-energy orbitals first. The Pauli exclusion principle says each orbital can hold at most two electrons, and those two must spin in opposite directions.
Hund's rule says electrons entering orbitals of equal energy spread out singly before any orbital gets a second electron. Following these rules lets chemists predict an atom's configuration just from its atomic number. Valence electrons, the ones in the outermost shell, largely control an element's chemical behavior and its position on the periodic table.
A few transition metals break the expected pattern. Chromium and copper, for example, end up with only one electron in their 4s orbital instead of two, because a half-filled or completely filled d subshell is extra stable. Students often assume every element follows the predicted order exactly, but these exceptions are well documented and worth learning separately.
Each type of subshell holds a set maximum: an s subshell holds up to 2 electrons, a p subshell up to 6, a d subshell up to 10, and an f subshell up to 14.
Key facts
| Field | General Chemistry |
|---|---|
| Governing rules | Aufbau principle, Pauli exclusion principle, Hund's rule |
| Common exception | Chromium: [Ar] 3d⁵ 4s¹ instead of [Ar] 3d⁴ 4s² |
| Shorthand form | Noble-gas core notation, such as [Ne] 3s¹ |
| Valence electrons | Outermost electrons that control bonding behavior |
Sodium, atomic number 11, has the electron configuration 1s² 2s² 2p⁶ 3s¹. Chemists often abbreviate this as [Ne] 3s¹, using the symbol for neon to represent the first ten electrons. The single electron in the 3s orbital is sodium's only valence electron, which explains why sodium readily loses one electron to form a Na+ ion.
Frequently asked questions
Why do chromium and copper not follow the expected filling order?
Chromium, [Ar] 3d⁵ 4s¹, and copper, [Ar] 3d¹⁰ 4s¹, gain extra stability from a half-filled or fully filled 3d subshell. One electron shifts from the 4s orbital into 3d compared to the order predicted by strict Aufbau filling.
How does electron configuration relate to the periodic table?
An element's position in the periodic table lines up with its highest-energy subshell being filled. Elements in the same group share the same valence-shell configuration, which explains their similar chemical properties.
What are valence electrons?
Valence electrons are the electrons in an atom's outermost occupied energy level. They are the electrons involved in forming chemical bonds with other atoms.