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The periodic table’s strange shape — two tall columns on the left, six on the right, a sunken block of ten in the middle and two loose rows floating underneath — isn’t a design choice. It’s a map of how electrons fill orbitals. Once you see the four blocks, the shape makes complete sense.
The four blocks
Each block is named after the type of subshell that the element’s last-added (highest-energy) electron goes into:
| Block | Groups | Subshell being filled | Width |
|---|---|---|---|
| s-block | 1–2 (plus helium) | s | 2 columns |
| p-block | 13–18 | p | 6 columns |
| d-block | 3–12 | d | 10 columns |
| f-block | lanthanides and actinides | f | 14 columns |
Why those widths?
The width of each block is exactly the number of electrons its subshell can hold:
- An s subshell has 1 orbital → 2 electrons → 2 columns
- A p subshell has 3 orbitals → 6 electrons → 6 columns
- A d subshell has 5 orbitals → 10 electrons → 10 columns
- An f subshell has 7 orbitals → 14 electrons → 14 columns
Add them up for period 6 or 7: 2 + 14 + 10 + 6 = 32 elements per row. That’s exactly how many elements there are in those periods.
What each block contains
- s-block: the reactive metals — alkali metals (group 1) and alkaline earth metals (group 2) — plus hydrogen and helium.
- p-block: the most varied block — metals, all the metalloids, all the nonmetals except hydrogen, the halogens and the noble gases.
- d-block: the transition metals.
- f-block: the lanthanides (rare earth elements) and the actinides.
Reading electron configurations off the table
Because the table is laid out in filling order, you can read any element’s configuration by walking along the rows from hydrogen to the element:
- Each row is a new shell (for s and p) — period 3 fills 3s then 3p.
- The d-block in period n fills the (n−1)d subshell — period 4’s d-block is 3d.
- The f-block in period n fills (n−2)f — period 6’s lanthanides fill 4f.
Example: bromine (period 4, group 17). Walk through: [Ar] then across period 4 — 4s² (s-block), 3d¹⁰ (d-block), 4p⁵ (five steps into the p-block). Bromine: [Ar] 4s² 3d¹⁰ 4p⁵
Example: iron (period 4, group 8): [Ar] 4s² then six steps into the d-block → [Ar] 4s² 3d⁶
This works for nearly every element, with a few well-known exceptions like chromium and copper — see electron configuration exceptions. The electron configuration generator will check your answer.
Why the f-block floats underneath
If you put the f-block where it belongs — between groups 2 and 3 in periods 6 and 7 — the table becomes 32 columns wide. That’s accurate but awkward to print or fit on a screen. So the 14-column f-block rows are cut out and placed below, with a marker showing where they slot in.
It’s purely a convenience. Lanthanum through ytterbium (or cerium through lutetium, depending on the table) really are part of period 6.
The group 3 debate
There’s a long-running disagreement about which elements belong in group 3 under scandium and yttrium: lanthanum and actinium, or lutetium and lawrencium. Electron configurations and chemical properties have been used to argue both ways. A 2021 IUPAC project report recommended lutetium and lawrencium, giving an f-block 14 elements wide in each row, but many tables — including textbooks you may use — still show lanthanum. Neither choice changes any element’s chemistry.
Where does helium go?
Helium is an s-block element (1s²), so by electron configuration it “belongs” above beryllium. But chemically it behaves exactly like the noble gases, so nearly every table places it in group 18. Hydrogen’s position is similarly debated: it’s in group 1 by configuration but is not an alkali metal.
Quick answers
What determines which block an element is in? The type of subshell (s, p, d or f) that receives its last electron in the filling order.
How many elements are in each block? In periods 6 and 7: 2 in the s-block, 6 in the p-block, 10 in the d-block and 14 in the f-block per row.
Why is the table not just 32 columns wide? It can be — “long-form” tables do exactly that. The standard layout is simply more compact. See why the periodic table is shaped the way it is.
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