periodic table

The periodic table arranges the chemical elements in order of atomic number and stacks them so that elements with similar chemistry land in the same column. It is the most-reproduced chart in science, and it is not a filing system — it is a claim. The claim is that chemical behaviour repeats, and that where an element falls tells you how it will act.

The shape

feature	count	what it means
periods (rows)	7	one row per electron shell being filled
groups (columns)	18	shared outer-electron count, hence shared chemistry
blocks	4 (s, p, d, f)	which type of orbital is filling
f-block columns	14	lanthanides and actinides, usually printed below
elements	118	rows 1–7 complete as of 2016

The rows get longer as you go down because bigger atoms open up more orbital types: 2 elements in period 1, then 8, 8, 18, 18, 32, 32.

Mendeleev's bet

Others got there first or nearly so. John Newlands published his "law of octaves" in 1864; Lothar Meyer had a 28-element table the same year. What made Dmitri Mendeleev's 1869 table win was nerve. He began arranging elements by atomic weight on 17 February 1869 and printed the result in the journal of the Russian Chemical Society that May — and where an element did not fit, he refused to bend the pattern. He declared the measurement wrong, or the element missing.

Then he named the missing ones. In 1871 he predicted the properties of three unknown elements he called eka-boron, eka-aluminium and eka-silicon ("eka" is Sanskrit for "one").

predicted	found	by	year
eka-aluminium	gallium	Paul-Émile Lecoq de Boisbaudran	1875
eka-boron	scandium	Lars Fredrik Nilson	1879
eka-silicon	germanium	Clemens Winkler	1886

Lecoq de Boisbaudran initially suspected Mendeleev of trying to take credit for gallium. He later conceded the prediction was right. A table that only sorted what was already known would have been useful; a table that correctly described things nobody had ever seen was something else.

What fixed it

Mendeleev ordered by atomic weight, which is almost right and occasionally wrong. Tellurium is heavier than iodine but has to come first to make the chemistry work.

• 1913 — Antonius van den Broek proposed that nuclear charge, not weight, sets an element's place.
• 1913 — Henry Moseley proved it with X-ray spectroscopy, measuring nuclear charge from aluminium to gold. Ordering became integer-valued and the tellurium–iodine problem dissolved. Moseley was killed in the First World War two years later.
• 1913 — Frederick Soddy coined "Isotope" for atoms of one element with different weights, explaining why weight had been a leaky proxy all along.
• 1913 — Niels Bohr quantised the Electron's energy levels and gave periodicity a physical cause: the columns repeat because outer-shell configurations repeat.
• 1945 — Glenn Seaborg moved the actinides out of the d-block and into the f-block, producing the table's modern shape.

The current table

IUPAC maintains the reference version. The latest release is dated 4 May 2022 and carries the abridged standard atomic weights from CIAAW's 2021 table. Where an element has no isotope with a characteristic natural abundance, the printed value is the mass number of the longest-lived confirmed nuclide, in square brackets — which is why uranium gets 238.029 and oganesson gets a bracketed integer.

Two things the table still argues about:

• Group 3. IUPAC reports in 1988 and 2021 endorse scandium, yttrium, lutetium and lawrencium. Many textbooks still print lanthanum and actinium there.
• Where hydrogen goes. It sits atop group 1 with one s-electron, but it is not a metal, and no placement is comfortable.

Why it works at all

Chemistry is run by outer electrons. Elements in one column have the same outer configuration, so they bond the same way, and that is the whole periodic law. Everything else on the chart — the block colours, the staircase between metals and non-metals, the row lengths — falls out of how orbitals fill. See Atom and Electron.

Related: chemical element · Isotope · hydrogen · uranium · Atomic nucleus