Aufbau principle
States that electrons fill atomic orbitals of the lowest available energy levels before occupying higher energy levels.
What is Aufbau principle?
A principle stating that in the ground state of an atom or ion, electrons occupy orbitals in order of increasing energy.
Key formula / rule: (n+l) Rule for Orbital Energy
Key points
- Understand the concept of orbital energy levels.
- Predict the order of filling of atomic orbitals.
- Determine the ground-state electron configuration of atoms and ions.
- Recognize exceptions to the Aufbau principle.
Common exam trap
Assuming orbitals fill strictly in order of 'n' (e.g., filling 3d before 4s).
Definitions
- Term
Aufbau Principle
- Meaning
A principle stating that in the ground state of an atom or ion, electrons occupy orbitals in order of increasing energy.
- Term
Ground State
- Meaning
The lowest possible energy state of an atom or molecule.
- Term
Electronic Configuration
- Meaning
The arrangement of electrons in the atomic orbitals of an atom or molecule.
Learning objectives
Understand the concept of orbital energy levels.
Predict the order of filling of atomic orbitals.
Determine the ground-state electron configuration of atoms and ions.
Recognize exceptions to the Aufbau principle.
Formulae
- Name
(n+l) Rule for Orbital Energy
- Note
This rule helps determine the order of filling for orbitals.
- Expression
Orbital energy increases with increasing (n+l). If (n+l) is the same, energy increases with increasing n.
Prerequisites
Atomic structure
Quantum numbers (n, l, ml, ms)
Orbital shapes (s, p, d, f)
Energy levels of orbitals
Common mistakes
Assuming orbitals fill strictly in order of 'n' (e.g., filling 3d before 4s).
Forgetting the (n+l) rule or its exceptions.
Confusing Aufbau principle with Hund's rule or Pauli exclusion principle.
Keywords
Aufbau principle
Electron configuration
Orbital filling
Energy levels
Quantum numbers
(n+l) rule
Ground state
Atomic structure
Hund's rule
Pauli exclusion principle
Practice preview
According to the Aufbau principle, which of the following orbitals will be filled after 3p?…
easy
Which of the following statements about the Aufbau principle is INCORRECT?…
hard
Which of the following electronic configurations violates the Aufbau principle?…
medium