The History of the Periodic Table: From Döbereiner to Mendeleev and Beyond

By the middle of the 19th century chemists knew about 60 elements, and they had a nagging feeling that there was an order hidden among them. Some elements behaved like families. Their atomic weights seemed to follow patterns. Several people glimpsed the pattern before one of them had the confidence to use it to predict the unknown.

Döbereiner’s triads (1829)

The German chemist Johann Döbereiner noticed that certain elements came in groups of three with similar chemistry — and that the middle element’s atomic weight was roughly the average of the other two.

• Lithium, sodium, potassium: (7 + 39) ÷ 2 = 23, and sodium is 23.
• Chlorine, bromine, iodine: (35.5 + 127) ÷ 2 ≈ 81, and bromine is 80.
• Calcium, strontium, barium

It was a hint that atomic weight and chemical behaviour were connected, but most elements didn’t fit into triads.

Chancourtois’s telluric screw (1862)

The French geologist Alexandre-Émile Béguyer de Chancourtois arranged the elements by atomic weight in a spiral around a cylinder, so that similar elements lined up vertically. It was a genuinely periodic arrangement — but his paper was published without the diagram, and it was largely ignored.

Newlands’ law of octaves (1864–1865)

In England, John Newlands arranged the elements by atomic weight and noticed that every eighth element seemed to resemble the first, like the notes of a musical scale. He called it the law of octaves. The Chemical Society mocked the idea — one member asked if he’d tried arranging the elements alphabetically — partly because the pattern broke down after calcium. Decades later the Royal Society recognized his contribution with the Davy Medal.

Mendeleev’s table (1869)

Dmitri Mendeleev, a professor in St Petersburg, was writing a chemistry textbook and wanted a logical way to organize it. He wrote the properties of each element on cards and arranged them in order of atomic weight, grouping similar elements together. In 1869 he published his periodic table.

What set Mendeleev apart were three bold decisions:

1. He left gaps. Where no known element fitted the pattern, he left a space rather than forcing the next element in.
2. He predicted the missing elements. He described their properties in detail — atomic weight, density, the formulas of their oxides — and called them eka-aluminium, eka-boron and eka-silicon (“eka” is Sanskrit for “one”).
3. He trusted chemistry over atomic weight. When the order of weights put an element in the wrong family, he swapped them. Tellurium (127.6) is heavier than iodine (126.9), but he placed tellurium first because iodine clearly belonged with the halogens.

The predictions come true

• Gallium (1875) matched eka-aluminium.
• Scandium (1879) matched eka-boron.
• Germanium (1886) matched eka-silicon remarkably closely: Mendeleev predicted an atomic weight of about 72 and a density of 5.5 g/cm³; germanium came in at 72.6 and 5.35.

Those successes turned the periodic table from a curiosity into a law of nature. Around the same time the German chemist Lothar Meyer had developed a very similar table independently, but Mendeleev’s predictions are why his name is attached to it. Element 101, mendelevium, is named in his honour.

Noble gases: a whole new column (1890s)

In the 1890s William Ramsay and colleagues discovered argon, helium, neon, krypton and xenon — elements that didn’t fit any existing group. Rather than breaking the table, they slotted in neatly as a new column, group 18. See the noble gases.

Moseley and atomic number (1913)

Mendeleev’s ordering by atomic weight had awkward exceptions (tellurium and iodine; argon and potassium; cobalt and nickel). In 1913 the young English physicist Henry Moseley measured the X-rays given off by different elements and found that their frequencies stepped up regularly with a whole number — the charge on the nucleus. That number, the atomic number, was the true basis of the table. Ordering by atomic number fixed every exception at once and showed exactly which elements were still missing.

Moseley was killed at Gallipoli in 1915, aged 27. Many believe he would have won a Nobel Prize.

Quantum mechanics explains why (1920s)

The periodic table was built on observation. The explanation came with quantum mechanics: the rows and blocks reflect how electrons fill shells and subshells. Elements in the same group have the same arrangement of outer electrons, which is why they behave alike. See electron configuration rules.

Seaborg’s rearrangement (1944)

Glenn Seaborg moved the actinides into a separate row beneath the lanthanides, giving the table the shape we use today. See the actinides.

The table today

The seventh row was completed in 2016 with the naming of nihonium, moscovium, tennessine and oganesson. See the newest elements. 2019 was declared the International Year of the Periodic Table, marking 150 years since Mendeleev’s first version.

Quick answers

Who invented the periodic table? Dmitri Mendeleev is credited with the first periodic table that made successful predictions (1869), though Döbereiner, Newlands, Chancourtois and Meyer all contributed.

How was Mendeleev’s table different from today’s? It was ordered by atomic weight rather than atomic number, had no noble gases, and had gaps for undiscovered elements.

How many elements did Mendeleev know? About 63 or 64, depending on how a few early discoveries are dated. Drag the discovery-year slider on the periodic table to 1869 to see them.