Determining Electronic Configuration (Cambridge International A-Level Chemistry)

Determining Electronic Configurations

• Electron configuration shows how electrons are arranged in shells, subshells, and orbitals.

• There are two formats:

  • Full configuration; lists all electrons from 1s onward
  • Shorthand configuration; uses the symbol of the nearest noble gas in brackets to represent inner electrons (e.g. [Ar])

• Ions form when atoms gain or lose electrons:

  • Anions (negative) form by adding electrons to the outer shell
  • Cations (positive) form by removing electrons from the outer shell

• Transition metals:

  • Fill the 4s before 3d when neutral
  • Lose electrons from 4s first, not 3d, when forming ions

• In the Periodic Table the elements are grouped into blocks based on their valence subshell:

  • s-block: valence electrons in an s orbital
  • p-block: valence electrons in a p orbital
  • d-block: valence electrons in a d orbital
  • f-block: valence electrons in an f orbital

The blocks of the Periodic Table

The elements can be divided into the s, p, d or f block, according to their outer shell electron configuration

Examples

• Electronic configuration of Fe

  • Atomic number = 26 so there are 26 electrons
  • Full configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶
  • Shorthand: [Ar] 4s² 3d⁶

• Electronic configuration of Fe²⁺

  • Atomic number = 26 so there are 26 electrons, but the Fe²⁺ ion only has 24 electrons
  • Electrons are removed from the 4s orbital before the 3d
  • Full configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶
  • Shorthand: [Ar] 3d⁶

Exceptions to the Aufbau principle

• Chromium and copper have the following electron configurations:

  • Cr is [Ar] 3d⁵ 4s¹ not [Ar] 3d⁴ 4s²
  • Cu is [Ar] 3d¹⁰ 4s¹ not [Ar] 3d⁹ 4s²

• This is because the [Ar] 3d⁵ 4s¹ and [Ar] 3d¹⁰ 4s¹ configurations are energetically favourable

• By promoting an electron from 4s to 3d, these atoms achieve a half full or full d-subshell, respectively

Worked Example

• Write down the full and shorthand electron configuration of the following elements: Answer:

  1. Potassium

  2. Calcium

  3. Gallium

  4. Ca²⁺

  • Potassium

    • Electrons: 19
    • Full configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
    • Shorthand: [Ar] 4s¹
    • Note: Argon (Ar) has 18 electrons and is the previous noble gas

  • Calcium

    • Electrons: 20
    • Full configuration: 1s² 2s² 2p⁶3s² 3p⁶ 4s²
    • Shorthand: [Ar] 4s²
    • Note: 4s is filled before 3d because it is lower in energy

  • Gallium

    • Electrons: 31
    • Full configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p¹
    • Shorthand: [Ar] 3d¹⁰ 4s² 4p¹
    • Note: Includes filled 3d subshell after argon

  • Calcium 2+ ion

    • Electrons: 18 (after losing two 4s electrons)
    • Full configuration: 1s² 2s² 2p⁶ 3s² 3p⁶
    • Shorthand: [Ar]
    • Note: Ca²⁺ has the same configuration as argon