Determining Electronic Configurations (CIE A Level Chemistry)

• Writing out the electronic configuration tells us how the electrons in an atom or ion are arranged in their shells, subshells and orbitals
• This can be done using the full electron configuration or the shorthand version
  • The full electron configuration describes the arrangement of all electrons from the 1s subshell up
  • The shorthand electron configuration includes using the symbol of the nearest preceding noble gas to account for however many electrons are in that noble gas

• Ions are formed when atoms lose or gain electrons
  • Negative ions are formed by adding electrons to the outer subshell
  • Positive ions are formed by removing electrons from the outer subshell
  • The transition metals fill the 4s subshell before the 3d subshell but lose electrons from the 4s first and not from the 3d subshell (the 4s subshell is lower in energy)

• The Periodic Table is split up into four main blocks depending on their electronic configuration:
  • s block elements
    • Have their valence electron(s) in an s orbital

  • p block elements
    • Have their valence electron(s) in a p orbital

  • d block elements
    • Have their valence electron(s) in a d orbital

  • f block elements
    • Have their valence electron(s) in an f orbital

The elements can be divided into four blocks according to their outer shell electron configuration

Exceptions

• Chromium and copper have the following electron configurations, which are different to what you may expect:
  • 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 stable

Worked example: Electron configuration

Answer

Answer 1: Potassium has 19 electrons so the full electronic configuration is:

1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹

The 4s orbital is lower in energy than the 3d subshell and is therefore filled first

The nearest preceding noble gas to potassium is argon which accounts for 18 electrons so the shorthand electron configuration is:

[Ar] 4s¹

Answer 2: Calcium has 20 electrons so the full electronic configuration is:

1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

The 4s orbital is lower in energy than the 3d subshell and is therefore filled first

The shorthand version is [Ar] 4s² since argon is the nearest preceding noble gas to calcium which accounts for 18 electrons

Answer 3: Gallium has 31 electrons so the full electronic configuration is:

   1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p¹

The shorthand version is [Ar] 3d¹⁰ 4s² 4p¹ since argon is the nearest preceding noble gas to calcium which accounts for 18 electrons

Answer 4: What this means is that if you ionise calcium and remove two of its outer electrons, the electronic configuration of the Ca²⁺ ion is identical to that of argon

Ca²⁺ is 1s² 2s² 2p⁶ 3s² 3p⁶                                                           

Ar is also 1s² 2s² 2p⁶ 3s² 3p⁶