Electron configurations describe where the electrons in an atom are most likely to be found. The Aufbau principle, Pauli exclusion principle, and Hund's rule give a reliable set of rules for building these configurations from the lowest-energy orbitals upward. These rules matter because electron arrangement explains patterns in the periodic table, chemical bonding, ion formation, and reactivity.
Learning them also helps connect atomic structure to observable chemical behavior.
Key Facts
- Aufbau principle: electrons fill lower-energy orbitals before higher-energy orbitals.
- Pauli exclusion principle: one orbital can hold at most 2 electrons, and they must have opposite spins.
- Hund's rule: electrons occupy equal-energy orbitals singly with parallel spins before pairing.
- Common filling order begins 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p.
- Orbital capacities are s = 2, p = 6, d = 10, f = 14 electrons.
- Example: oxygen has 8 electrons, so its configuration is 1s2 2s2 2p4.
Vocabulary
- Electron configuration
- An electron configuration is a notation that shows how electrons are arranged among the orbitals of an atom.
- Orbital
- An orbital is a region around the nucleus where an electron is likely to be found.
- Aufbau principle
- The Aufbau principle states that electrons fill the lowest available energy orbitals before filling higher-energy orbitals.
- Pauli exclusion principle
- The Pauli exclusion principle states that two electrons in the same orbital must have opposite spins.
- Hund's rule
- Hund's rule states that electrons fill orbitals of the same energy one at a time with the same spin before any pairing occurs.
Common Mistakes to Avoid
- Filling 3d before 4s in neutral atoms is wrong because the usual Aufbau order places 4s lower in energy during filling for many neutral atoms.
- Pairing electrons in p orbitals too early is wrong because Hund's rule requires one electron in each equal-energy p orbital before pairing begins.
- Putting two electrons with the same spin in one orbital is wrong because the Pauli exclusion principle requires opposite spins in the same orbital.
- Adding the wrong total number of electrons is wrong because a neutral atom must have the same number of electrons as its atomic number.
Practice Questions
- 1 Write the full electron configuration for a neutral phosphorus atom with atomic number 15.
- 2 Draw the orbital diagram for nitrogen, atomic number 7, showing arrows for electron spins in the 1s, 2s, and 2p orbitals.
- 3 Explain why the three 2p electrons in nitrogen occupy separate p orbitals before any two of them pair up.