- s-block·2 electronsHydrogen, helium, groups 1–2
- p-block·6 electronsGroups 13–18
- d-block·10 electronsTransition metals
- f-block·14 electronsLanthanides and actinides
The periodic table is not a rectangle that happens to have two leftover rows. It is a map of which subshell the table assigns as atomic number increases. s, p, d, and f are the same letters as in the hydrogen clouds: ℓ = 0, 1, 2, 3. Each letter holds 2(2ℓ + 1) electrons, so the blocks are 2, 6, 10, and 14 columns wide. Ground-state exceptions (Cr, Cu, La, Ac, Lr) still sit in those columns. The map is therefore the classroom layout, not a spectroscopist’s last-electron list.
Capacity is geometry plus spin
One orbital holds two electrons (spin up and spin down). An s subshell is one orbital. A p subshell is three. d is five. f is seven. That is the entire reason the table’s columns come in those widths:
| Block | ℓ | Orbitals | Electrons | Elements here |
|---|---|---|---|---|
| s-block | 0 | 1 | 2 | 14 |
| p-block | 1 | 3 | 6 | 36 |
| d-block | 2 | 5 | 10 | 38 |
| f-block | 3 | 7 | 14 | 30 |
Helium is s-block and still a noble gas
Helium’s configuration is 1s², so it belongs with the s-block on the left. We draw it above neon because a full shell — not the letter of the last orbital — is what makes it unreactive. The table is doing two jobs at once: filling order (blocks) and valence behavior (families). When they disagree, helium is the exhibit.
Why the f-block is parked underneath
Periods 6 and 7 would be 32 elements wide if lanthanides and actinides stayed in line. Pulling the 15-element f rows out keeps the main grid at 18 columns — the s + d + p width of period 4 onward. The dashed 57–71 and 89–103 markers on the interactive table are the hitching points, not a third kind of element. Lanthanum and actinium are drawn in those rows even though their ground states are 5d¹ 6s² and 6d¹ 7s². Lutetium and lawrencium then close the rows with a full f¹⁴ shell (Lr actually ends 7p¹). The f block here is therefore the 15-column classroom strip, not “atoms currently filling f.”
Read a configuration off the map
Scanning the page left to right, top to bottom traces 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p… That is close to the Aufbau sequence, but it is not identical to it. 4s appears before 3d because the table is ordered by atomic number and group. Actual orbital energies, however, sit close and swap as soon as 3d starts to fill. Click any tile on the map for the element’s page, or pick a worked example:
- s-block — Sodium ends in 3s¹ — one electron in an s cloud, group 1.
- p-block — Carbon ends in 2p². The p-block is where most of organic chemistry lives.
- d-block — Iron is 3d⁶ 4s². Colored complexes and variable charges come from the open d shell.
- f-block — Holmium is 4f¹¹ 6s². The f-block is pulled out so the table stays 18 columns wide.
Once the letters mean shapes, not decorations, the quantum numbers n, ℓ, m are just the address of each tile’s last electron.