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The standard periodic table has an odd silhouette: a tall column on the far left, a gap-toothed top row, a sunken middle block, and two loose rows parked underneath. It looks like a compromise — and it is. Every feature of that shape comes from how electrons fill orbitals, plus one practical decision about page width.
Why the rows have different lengths
The length of each row (period) is the number of electrons needed to fill the subshells that belong to it:
| Period | Subshells filled | Elements |
|---|---|---|
| 1 | 1s | 2 |
| 2 | 2s, 2p | 8 |
| 3 | 3s, 3p | 8 |
| 4 | 4s, 3d, 4p | 18 |
| 5 | 5s, 4d, 5p | 18 |
| 6 | 6s, 4f, 5d, 6p | 32 |
| 7 | 7s, 5f, 6d, 7p | 32 |
s subshells hold 2 electrons, p hold 6, d hold 10 and f hold 14. Add them up and you get the row lengths: 2, 8, 8, 18, 18, 32, 32. See the s, p, d and f blocks and quantum numbers.
Why there’s a big gap at the top
Periods 1–3 have no d electrons, so they have nothing to put in the d-block columns (groups 3–12). Rather than squeeze elements into the wrong columns, the table leaves a gap so that each group — each column — contains elements with the same outer electron arrangement. That’s the point of the table: elements in the same column behave alike.
Period 1 is an extreme case, with only hydrogen and helium spanning the whole width.
Why two rows float at the bottom
Periods 6 and 7 each have 32 elements. Putting the 14-element f-block where it actually belongs — between groups 2 and 3 — would make the table 32 columns wide, which is awkward on a page or screen. So the lanthanides and actinides are cut out and placed below, with a marker showing where they fit.
The result is the familiar 18-column table. It’s a layout convention, not a statement about chemistry: those elements are fully part of periods 6 and 7.
The 32-column (long-form) table
Some chemists prefer the long-form table, with the f-block in place. It’s longer and thinner, but it shows the sequence of elements honestly, without any cut-and-paste. It’s increasingly used in textbooks and posters, and it makes the reason for the gap-toothed shape of the standard table obvious.
Hydrogen and helium: the awkward pair
- Hydrogen has one electron like the alkali metals, so it sits above lithium — but it’s a nonmetal gas. Some tables float it at the top on its own, or place it above carbon (half-filled shell) or fluorine (one short of full).
- Helium has a full s² configuration, like the group 2 metals, so by electron configuration it “belongs” above beryllium. But chemically it’s a noble gas, so it’s placed above neon. Chemistry wins over configuration here.
The group 3 question
Which elements sit under scandium and yttrium: lanthanum and actinium, or lutetium and lawrencium? Tables differ, and the choice changes where the f-block is cut out. A 2021 IUPAC project report favoured lutetium and lawrencium, but both versions are still common.
Other shapes people have tried
Mendeleev’s arrangement was never the only option. Over the past 150 years, hundreds of alternative designs have been proposed:
- Spiral tables — elements arranged in a continuous spiral, so there’s no break at the end of each row. Charles Janet, Theodor Benfey and others designed influential versions.
- Left-step table — Charles Janet’s 1928 design orders blocks as f, d, p, s from left to right and moves helium above beryllium. It follows electron filling very strictly and has a loyal following among chemists and physicists.
- 3D and pyramidal tables — showing how blocks nest inside each other.
- Circular and wheel tables — popular for art and posters.
None has displaced the standard layout, which balances accuracy, familiarity and fitting on a page. But each reveals a different aspect of the underlying pattern.
What stays the same in every design
However it’s drawn, every periodic table encodes the same two facts:
- Elements are ordered by atomic number — the number of protons.
- Elements with similar outer electron configurations are grouped together, so their chemistry repeats periodically.
That’s the “periodic” in periodic table. See how to read the periodic table and the history of the periodic table.
Quick answers
Why are there 18 columns? 2 for the s-block, 10 for the d-block and 6 for the p-block. The f-block’s 14 columns are placed below.
Why is the periodic table not a rectangle? Because the early periods have fewer subshells to fill, so they have fewer elements.
Will the table get bigger? If element 119 is made, it will start an eighth row. Theory predicts that row would add a new g-block with 18 more columns. See how new elements are made.
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