Aufbau principle

Definition

The Aufbau principle states that electrons fill atomic orbitals of the lowest available energy levels before occupying higher levels. This helps determine electron configurations for atoms.

5 Must Know Facts For Your Next Test

1. According to the Aufbau principle, electrons fill orbitals starting from the lowest energy level (1s) and move to higher ones sequentially.
2. The order of orbital filling is determined by the increasing values of $n + l$, where $n$ is the principal quantum number and $l$ is the azimuthal quantum number.
3. If two orbitals have the same value of $n + l$, the one with a lower $n$ value fills first.
4. Exceptions to this principle can occur in transition metals and heavier elements due to electron-electron interactions and relativistic effects.
5. The rule is essential in predicting the ground state electron configurations of elements on the periodic table.

Review Questions

• What does the Aufbau principle state about how electrons fill orbitals?
• How do you determine which orbital will fill first using the Aufbau principle?
• Can you name an exception to the Aufbau principle?

Related terms

Pauli Exclusion Principle:

States that no two electrons in an atom can have identical sets of four quantum numbers.

Hund's Rule:

Electrons will occupy degenerate orbitals singly as far as possible before pairing up.

Electron Configuration:

The distribution of electrons of an atom or molecule in atomic or molecular orbitals.