As mentioned in “Basics,” electron configuration follows a pattern. This allows us to use the periodic table to our advantage. As you progress element by element, you are progressing through electron configuration, adding one electron each time.
You can use the periodic table to determine the ending of an atom’s electron configuration.
Ex. Oxygen has an electron configuration of 1s² 2s² 2p⁴. It’s “ending” is 2p⁴.
We can classify each area of the periodic table into “blocks” -> s-block, p-block, d-block, and f-block. The diagram below shows the different blocks ↴
Elements in the s-block have an ending in the s-subshell.
p-block = ending in the p-subshell
d-block = ending in the d-subshell
f-block = ending in the f-subshell
To determine the principal energy level (n), just look at which row the element is in.
Note that the s-block starts at n=1, the p-block starts at n=2, d-block starts at n=3, and the f-block starts at n=4.
Finally, to determine the superscript, start at the left side of the element’s “block” and count over to the right.
Note that there are lots of exceptions in the f-block. However, you probably won’t have to write electron configurations for those elements.
Here’s an example of determining the “ending” of oxygens electron configuration ↴
This is helpful because now we don’t have to count the electrons to determine where our electron configuration stops. We just need to keep “filling” the subshells of oxygen until we get to 2p⁴. (Although, feel free to count, it isn’t a bad thing to be thorough.)
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