3.2Electronic Configuration

3.2    Electronic Configuration

Ways to represent electronic configuration


Electronic configuration describes how the electrons in an atom/ion are arranged in their shells, sub-shells and orbitals.


  • Using ‘electrons-in-boxes’:


  • Using energy levels:

  • Using s, p, d and f notation:

  • Using the noble gas ‘core’:

Filling in the orbitals


  • Three general rules of filling the orbitals are:
    • Aufbau Principle states that in the ground state of an atom, the electrons must occupy the orbitals in the order of increasing energy.

[ Note: 4s has a slightly lower energy than 3d, therefore electrons are filled in 4s orbital first before the 3d orbitals. ]

  • Pauli’s Exclusion Principle states that an orbital can only accommodate a maximum of two electrons The two electrons must have opposite spins.


  • Hund’s Rule states that in a set of degenerate orbitals, electrons must occupy the orbital singly ftrst before pairing. The reason is because, two electrons occupying the same orbital will experience repulsion.

  • Electronic configuration of the elements (up to Z =  38)

  • The odd ones (K, Sc, Cr and Cu):
  1. For potassium, 4s is filled before 3d because 4s has a lower energy level than 3d.
  2. For scandium (to zinc, the d-block elements), the energy level of 3d and 4s are reversed. 4s is at a higher energy level This is because once the 3d orbital(s) is/are filled, the 3d electrons repel the 4s electrons to a higher energy level.
  3. For chromium, the electronic configuration is [Ar]3d⁵4s¹ instead of [Ar]3d⁴4s². This is because orbitals that are fully ftlled or half ftlled have extra stability due to their symmetrical charge distribution
  4. For copper, the electronic configuration is [Ar]3d¹⁰4s¹ instead of [Ar]3d⁹4s². The reason is the same as stated in (iii).


Electronic configuration of  ions


  • In the formation of cation, the electrons are removed in the order of decreasing energy (the reverse of filling in).


  • For the d-block elements, the electrons in 4s is removed first before This is because once the 3d orbital is filled, the 4s electrons are repelled to a higher energy level than 3d.


  • In the formation of anion, the electrons are added in the same manner as filling the electrons


Orbitals and the Periodic Table

  • The elements in the Periodic Table can be divided into four blocks according to their valence shell electronic configuration


  • The s-block elements have their valence electron(s) in the s orbital


  • The p-block elements have their valence electrons in the p orbital


  • The d-block elements have d orbitals filling


  • The f-block elements have f orbitals filling


Some useful facts

  • The valence electrons always appear at the end of the electronic configuration. (but not necessary the last one).


  • The valence electrons are large responsible for the chemical properties of an element.


  • The number of valence electron will indicate the group number of that element in the Periodic Table


  • The outermost quantum shell number will indicate the period of that element in the Periodic Table