Steps for determining electron configurations:
1. Identify how many electrons are in the atom.
Example: Carbon has 6 electrons
2. Determine the principal energy level (n) of the atom
n = 2 for carbon (it is in the second period)
3. Determine the number of sublevels.
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In principal energy level 1, there is 1 (1s).
In principal energy level 2, there are 4 (2s, 2p, 2p, 2p)
So for carbon we are working with 5 potential sublevels (they might
not all be used).
4. Assign electrons to the sublevels following the three rules (Aufbau, Pauli
Exclusion, and Hund’s).
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1s will be filled first, with the maximum of 2 electrons. You still have
four electrons left.
2s will be filled next, with the maximum of 2 electrons. You still have
two electrons left.
2p will be filled next, with the maximum of 2 electrons. You don’t
have any electrons left now.
5. Write the complete electron configuration.
1s22s22p2
Notice that the superscripts (the electrons in each sublevel) add up to the
total number of electrons in the atom. 2 + 2 + 2 = 6
6. To write the abbreviated electron configuration, determine what the previous
noble gas is from looking at the periodic table.
The noble gas which comes before carbon is helium.
7. Identify the portion of carbon’s electron configuration that is the same as helium’s
electron configuration.
Helium, which has 2 electrons, has an electron configuration of 1s2.
8. Substitute the chemical symbol of the noble gas surrounded by brackets into the
original electron configuration to make the abbreviated electron configuration.
[He]2s22p2