Electronic Configuration Rules
Aufbau principle, Pauli exclusion, Hund's rule and stability of half-filled sets. (Chemistry › Structure of Atom, NEET UG syllabus.)
What is Electronic Configuration Rules?
The distribution of electrons of an atom or molecule in atomic or molecular orbitals.
Key formula / rule: (n+l) Rule for Orbital Energy
Key points
- State and explain the Aufbau principle, Pauli exclusion principle, and Hund's rule.
- Write the electronic configuration for elements up to Z=36 using these rules.
- Identify and explain exceptions to the Aufbau principle (e.g., Cr, Cu).
- Relate electronic configuration to the stability of atoms.
Common exam trap
Incorrectly applying the (n+l) rule for orbital filling order.
Definitions
- Term
Electronic Configuration
- Meaning
The distribution of electrons of an atom or molecule in atomic or molecular orbitals.
- Term
Atomic Orbital
- Meaning
A region around the nucleus where the probability of finding an electron is maximum.
- Term
Degenerate Orbitals
- Meaning
Orbitals within the same subshell that have the same energy (e.g., 2px, 2py, 2pz).
- Term
Aufbau Principle
- Meaning
States that electrons fill atomic orbitals of the lowest available energy levels before occupying higher levels.
- Term
Pauli Exclusion Principle
- Meaning
States that no two electrons in the same atom can have identical values for all four quantum numbers.
- Term
Hund's Rule of Maximum Multiplicity
- Meaning
States that every orbital in a subshell is singly occupied with one electron before any one orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin.
Learning objectives
State and explain the Aufbau principle, Pauli exclusion principle, and Hund's rule.
Write the electronic configuration for elements up to Z=36 using these rules.
Identify and explain exceptions to the Aufbau principle (e.g., Cr, Cu).
Relate electronic configuration to the stability of atoms.
Draw orbital diagrams for elements.
Formulae
- Name
(n+l) Rule for Orbital Energy
- Note
'n' is principal quantum number, 'l' is azimuthal quantum number.
- Expression
Orbitals with lower (n+l) values fill first. If (n+l) is same, orbital with lower 'n' fills first.
Prerequisites
Knowledge of atomic structure (nucleus, electrons).
Understanding of quantum numbers (principal, azimuthal, magnetic, spin).
Familiarity with atomic orbitals (s, p, d, f shapes and capacities).
Common mistakes
Incorrectly applying the (n+l) rule for orbital filling order.
Violating Pauli's principle by placing more than two electrons in an orbital or giving them the same spin.
Violating Hund's rule by pairing electrons in degenerate orbitals before all are singly occupied.
Forgetting or misapplying the exceptions for Cr and Cu.
Confusing subshells with individual orbitals.
Keywords
Electronic configuration
Aufbau principle
Pauli exclusion principle
Hund's rule
orbital
subshell
quantum numbers
stability
half-filled
fully-filled
exchange energy
chromium
copper
Practice preview
How many unpaired electrons are present in the ground state of a Cobalt(II) ion (Co2+, atomic number of Co is 27)?…
hard
What is the correct ground state electronic configuration for Nitrogen (N, atomic number 7)?…
medium
The extra stability of half-filled and fully-filled subshells is primarily due to which two factors?…
medium