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