Electron configuration looks like a memorization exercise, but it's really just following one consistent fill order. Once that order is automatic, writing a configuration for any element becomes a quick, mechanical process.
The fill order
Electrons fill orbitals from lowest to highest energy, following this order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, and so on. Notice that 4s fills before 3d, even though 3 is a smaller number, because 4s is actually lower in energy at that point on the periodic table.
The noble gas shortcut
Instead of writing every single orbital out, you can start from the nearest noble gas that comes before the element and continue from there. For example, sulfur's full configuration is 1s² 2s² 2p⁶ 3s² 3p⁴, and its noble gas shortcut is [Ne] 3s² 3p⁴, using neon's configuration as a stand in for everything before it.
The two famous exceptions
Chromium and copper don't follow the expected pattern. Both shift one electron from the 4s orbital into 3d to reach a more stable half-filled or fully-filled d subshell. Chromium is [Ar] 4s¹ 3d⁵ instead of the expected 4s² 3d⁴, and copper is [Ar] 4s¹ 3d¹⁰ instead of 4s² 3d⁹.
Writing configuration for an ion
For a cation, remove electrons from the highest principal quantum number first, not necessarily the last subshell that was filled. This matters specifically for transition metals: electrons come out of 4s before 3d, even though 3d was filled after 4s going in.
Where students actually lose points here
Filling 3d before 4s
The fill order isn't strictly by principal quantum number. 4s fills before 3d despite the smaller number, and getting this backward is the most common configuration mistake.
Removing electrons from the wrong orbital for a cation
Electrons leave the highest n value first, meaning 4s empties before 3d for a transition metal ion, which is the opposite of the order they were added.
Forgetting the chromium and copper exceptions
These get memorized as exceptions but rarely understood. Both shift an electron to reach a more stable half-filled or fully-filled d subshell, and that reasoning is exactly what a free response question is likely to ask for.
The foundation for periodic trends
Electron configuration explains almost every periodic trend that comes right after it, from atomic radius to ionization energy, so getting it solid here pays off immediately.
Common questions
What's the noble gas shortcut for electron configuration?
Start from the nearest noble gas that comes before the element in question, write its symbol in brackets, and continue the configuration from there instead of writing every orbital out from the beginning.
How do you write electron configuration for an ion?
For a cation, remove electrons starting from the highest principal quantum number, not necessarily the last subshell filled. For transition metals, that means 4s electrons come out before 3d electrons.
Why does chromium break the expected pattern?
Chromium shifts one electron from 4s into 3d to reach a half-filled d subshell, which is more stable than the pattern would otherwise predict. Copper does the same thing to reach a fully-filled d subshell.