What a learner can do afterwards
- Writes the full electron configuration of any element up to krypton
- Explains why the 4s sub-shell fills first and why its electrons also leave first when a transition metal forms an ion
- Draws the arrows-in-boxes picture for a named element, including the half-filled cases
- Links the s, p and d blocks of the table to the sub-shell being filled across them
1 · Read
Electrons live in orbitals, and each orbital holds at most two. Orbitals group into sub-shells: s with one orbital, p with three, d with five. Filling follows the Aufbau order, lowest energy first: 1s, then 2s, 2p, 3s, 3p, and onward. Iron, with 26 electrons, ends as 1s2 2s2 2p6 3s2 3p6 4s2 3d6.
The 4s sub-shell fills before 3d because at that point it sits lower in energy. Potassium and calcium place outer electrons in 4s, and the 3d row only begins at scandium. Ions flip the story: a transition metal losing electrons drops its 4s first, even though they arrived first. Iron(II) is therefore 3d6 with an empty 4s. Chromium (4s1 3d5) and copper (4s1 3d10) break ranks for the extra stability of a half-full or full d set.
The arrows-in-boxes picture shows what the writing hides. Each box is one orbital and each arrow one electron, with arrows pairing up only after every box in the set holds one. Oxygen 2p, for example, is three boxes holding four arrows: one pair plus two singles.
The periodic table is this picture laid flat. The two left columns are the s block, the six right columns the p block, and the middle ten the d block. Read across a row and you are filling the sub-shell the block is named for.
Fill lowest first, park 4s before 3d, remove 4s first, and read the blocks as the filling order.
2 · Watch
Take it off screen
Where it sits
Learn first
This opens up
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.