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Electron-configurations are a condensed way of representing the subshells and orbitals of the electrons in an atom.
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๐“—๐“ธ๐”€ ๐“ฌ๐“ช๐“ท ๐“˜ ๐”€๐“ป๐“ฒ๐“ฝ๐“ฎ ๐“ช๐“ท ๐“ฎ๐“ต๐“ฎ๐“ฌ๐“ฝ๐“ป๐“ธ๐“ท ๐“ฌ๐“ธ๐“ท๐“ฏ๐“ฒ๐“ฐ๐“พ๐“ป๐“ช๐“ฝ๐“ฒ๐“ธ๐“ท?
An electron-configuration involves simply the number of electrons in the atom. The atom fills orbitals with electrons following this following diagram:

Each s level holds 2 electrons, each p level holds 6 electrons, each d level holds 10 electrons, and each f level holds 14 electrons.
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๐“”๐”๐“ช๐“ถ๐“น๐“ต๐“ฎ
I will now show an example of how to write an electron-configuration for the atom lithium in its neutral state (unionized)
A neutral lithium atom has 3 electrons.
We will start with the 1s orbital, giving it 2 electrons.
The electron-configuration then becomes 1sยฒ.
However, there is still 1 electron unaccounted for. This will reside in the next orbital: 2s.
The final electron-configuration, thus, for lithium becomes:
1sยฒ2sยน.
Notice how the sum of the superscripts gives you the number of electrons.
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๐“’๐“ช๐“ฝ๐“ฒ๐“ธ๐“ท๐“ผ ๐“ช๐“ท๐“ญ ๐“ช๐“ท๐“ฒ๐“ธ๐“ท๐“ผ
Cations and anions can also be expressed through electron-configurations. To do so, simply add or subtract the number of electrons that were added or removed from the neutral atom.
For example, if atomic number 8 loses 1 electron to become a cation, the electron configuration will reduce by one, and will increase by one if it gains one electron.
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๐“๐“ธ๐“ซ๐“ต๐“ฎ ๐“ฐ๐“ช๐“ผ ๐“ผ๐“ฑ๐“ธ๐“ป๐“ฝ๐“ฌ๐“พ๐“ฝ๐“ผ
To further condense things, when the number of electrons gets tedious to write out, it is handy to refer to the nearest noble gas and go from there.
I will use the example of pottassium.
Potassium has an electron-configuration of 1sยฒ2sยฒ2pโถ3sยฒ3pโถ4sยน
The nearest noble gas is Argon, with electron-configuration 1sยฒ2sยฒ2pโถ3sยฒ3pโถ.
Thus, the electron-configuration of potassium can be shortened to
[Ar]4sยน
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๐“›๐“ฎ๐“ฝ'๐“ผ ๐“น๐“ป๐“ช๐“ฌ๐“ฝ๐“ฒ๐“ฌ๐“ฎ! (๐“ผ๐“ฎ๐“ฎ ๐“ฎ๐“ท๐“ญ ๐“ฏ๐“ธ๐“ป ๐“ช๐“ท๐“ผ๐”€๐“ฎ๐“ป๐“ผ)

1) If you are given helium in a neutral state, what would the electron configuration be?

2) If the electron-configuration was
1sยฒ2sยฒ2pโถ
what would the atom be?

3) If you are given sodium as a cation with charge +1, what would the electron configuration be?

4) What atom has the electron configuration
[Kr] 5sยน?
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๐“๐“ท๐“ผ๐”€๐“ฎ๐“ป๐“ผ:

  1. 1sยฒ
  2. Atomic # 10: Neon
  3. Loses 1 electron, becomes 1sยฒ2sยฒ2pโถ, or just [Ne}
  4. Atomic #37: Rubidium
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