Determining Electronic Configurations
- Writing out the electronic configuration tells us how the electrons in an atom or ion are arranged in their shells, sub-shells 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 sub-shell 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 sub-shell
- Positive ions are formed by removing electrons from the outer sub-shell
- The transition metals fill the 4s sub-shell before the 3d sub-shell but lose electrons from the 4s first and not from the 3d sub-shell
- Remember: The 4s sub-shell 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
- s block elements
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
Exceptions
- Chromium and copper have the following electron configurations, which are different to what you may expect:
- Cr is [Ar] 4s1 3d5 not [Ar] 4s2 3d4
- Cu is [Ar] 4s1 3d10 not [Ar] 4s2 3d9
- This is because the [Ar] 4s1 3d5 and [Ar] 4s1 3d10 configurations are energetically stable
Worked example
Write down the full and shorthand electron configuration of the following:
- Potassium
- Calcium
- Gallium
- Ca2+
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 nearest preceding noble gas to potassium is argon which accounts for 18 electrons so the shorthand electron configuration is:
- [Ar] 4s2
Answer 3:
- Gallium has 31 electrons so the full electronic configuration is:
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p1
- Again, the nearest preceding noble gas to potassium is argon which accounts for 18 electrons so the shorthand electron configuration is:
- [Ar] 4s2 3d10 4p1
Answer 4:
- If you ionise calcium and remove its two outer electrons, the electronic configuration of the Ca2+ ion is identical to that of argon.
- So, the full electronic configuration is:
- 1s2 2s2 2p6 3s2 3p6
- Since this is the electronic configuration of an argon atom, the shorthand electron configuration of Ca2+ is:
- [Ar]
