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.
Understanding Chemistry: Aufbau, Pauli, and Hund's Rules
An orbital is not a tiny circular path around the nucleus. It is a region described by probability. A drawing of an orbital shows where an electron is likely to be detected after many measurements.
The s orbitals are roughly spherical. The p orbitals have two lobes and come in three orientations. The d and f orbitals have more complex shapes.
These shapes affect how atoms bond because overlapping orbitals can place electron density between nuclei. In class, box diagrams are a useful bridge between orbital shapes and electron configurations. One box represents one orbital.
Arrows represent electrons. The arrow direction represents spin, which is an intrinsic quantum property rather than a literal spinning motion.
Energy order is not determined by distance from the nucleus alone. For example, an s orbital can penetrate closer to the nucleus than a p orbital in the same main energy level. This gives s electrons a lower energy in many atoms.
As atoms gain more electrons, electron repulsion and shielding change the energy of orbitals. That is why the order includes some surprising steps, such as the filling of the four s orbital before the three d orbitals in neutral atoms.
The pattern is a strong working rule, but it is not a complete explanation of every atom. A few transition metals have arrangements that differ because nearly filled or half filled d groups can be especially stable.
The rules become more meaningful when drawing a set of p boxes. Place one electron in each empty box before adding a second electron to any box. This arrangement reduces repulsion because electrons stay farther apart.
The unpaired electrons left in partially filled orbitals have observable effects. Substances with unpaired electrons are often attracted to a magnetic field. This property is called paramagnetism.
Atoms or ions in which all electrons are paired are usually weakly repelled by a magnetic field. These effects help chemists test ideas about electron arrangements using real measurements rather than relying only on diagrams.
Electron configurations must often be adjusted when an atom becomes an ion. For main group atoms, electrons are generally removed from the outermost occupied energy level first. Transition metals need extra care.
Although the four s orbital fills before the three d orbitals for many neutral atoms, electrons are removed from the four s orbital first when many transition metal ions form. This point causes common mistakes on school problems. Count the total electrons carefully, including the charge.
Then write the configuration or box diagram, check each orbital capacity, and inspect equal energy boxes for correct single occupancy before pairing. The final electrons, called valence electrons, are especially important because they largely control bonding and common ion charges.
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.