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The periodic table did not appear all at once. One of the first people to look for order in atomic weights was a German chemist who had never been to university. This post covers who Johann Döbereiner was, what his triads were, how well they hold up with modern numbers, and why NCERT still teaches them.
Who was Döbereiner?
His entry in the Neue Deutsche Biographie gives his dates as 13 December 1780 in Hof, Bavaria, to 24 March 1849 in Jena. He grew up poor and began a pharmacy apprenticeship at 14. Several business ventures failed. Then, in 1810, he was appointed professor of chemistry, pharmacy and technology at the University of Jena, a post he held until his death, though he had never been able to study at a university. Goethe took a close interest in his work.
He was also a pioneer of catalysis. The NDB records that in 1823 he made a mixture of hydrogen and oxygen ignite instantly using spongy platinum, which led to a lighter that was made in large numbers.
What were the triads?
The NDB says he began grouping substances "by their analogies" in 1817. He set out the idea in an 1829 paper in Poggendorff's Annalen, translated at Classic Chemistry. He wrote that he had long suspected "the atomic weight of bromine might be the arithmetical mean of the atomic weights of chlorine and iodine."
His examples included lime, strontia and baryta (compounds of calcium, strontium and barium), lithia, soda and potash (lithium, sodium and potassium), and sulphur, selenium and tellurium. He also tried, less successfully, to fit iron, manganese and chromium oxides into the scheme. He noted that the chemical strength of the elements in each group also changed in order, with the middle member in between.
The NCERT Class 11 chapter Classification of Elements and Periodicity in Properties presents three of these triads in Table 3.1 and says the "Law of Triads" seemed to work only for a few elements, so it was dismissed as coincidence.
Worked example: do the triads hold today?
Use the IUPAC abridged atomic weights:
Calcium, strontium, barium. Mean of Ca and Ba = (40.078 + 137.33) / 2 = 88.704. Actual Sr = 87.62. Gap = 1.08, about 1.2 per cent.
Chlorine, bromine, iodine. Mean = (35.45 + 126.90) / 2 = 81.175. Actual Br = 79.904. Gap = 1.27, about 1.6 per cent.
Lithium, sodium, potassium. Mean = (6.94 + 39.098) / 2 = 23.019. Actual Na = 22.990. Gap = 0.029, about 0.1 per cent.
All three come close, but none is exact. A close fit in three cases is not proof of a law, which is the point NCERT makes.
Why did the triads matter?
Döbereiner noticed a real pattern: elements with similar chemistry have atomic weights that are spaced in regular ways. Triads could not show how different groups relate to each other, because each triad stood alone. The next step, arranging all known elements in one list by atomic weight, came in the 1860s. Our post on John Newlands and the law of octaves picks up that thread, and Mendeleev's periodic table finishes it.
Which mistakes does this help you avoid?
Calling triads a law of nature. In an exam, describe them as an early observation that worked for a few groups only.
Mixing up the middle member. In each triad the middle element by atomic weight is the one in between: Na, Sr, Br.
Forgetting why it failed. Most elements could not be placed in triads.
The JEE (Main) 2026 syllabus starts its periodicity unit from the modern periodic law, so triads are background rather than a core topic there, while NCERT Class 11 covers them in section 3.2. Check the current syllabus for your board or exam.
What can a student take from Döbereiner's way of working?
Test a hunch with numbers. He did not just say bromine looked like chlorine and iodine. He computed the mean and compared it.
Say where the idea fails. His paper admits that some groups lack a third member and that his copper triad did not fit.
Keep practical and theoretical work together. The same man built a platinum lighter and searched for order among the elements.
If periodic trends still feel like lists to memorise, the Fermi Concept Library covers every concept in chemistry with its own lesson, so you can see how each trend follows from atomic structure. Then test yourself with our post on periodic trends and their exceptions.
Try it now: check whether sulphur, selenium and tellurium form a triad using the IUPAC values, and work out the percentage gap.
