• 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?3s²3p⁶?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:

[Ar] 3d¹⁰?4s²?4p¹

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 1s²2s²?2p⁶?3s²?3p⁶

Ar?is also 1s²?2s²?2p⁶?3s²3p⁶