Before there was a periodic table, there was chaos.

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Excerpt: Mendeleyev’s Dream.

Dmitri Mendeleyev invented the periodic table in 1869.
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▼ In the following excerpt from " Mendeleyev’s Dream: The Quest for the Elements," author Paul Strathern describes the state of chemistry in the years leading up to Dmitri Mendeleyev's invention of the modern periodic table.
In the 18th and 19th centuries, several elements were being discovered almost every decade. This profusion of new elements with an ever-widening range of properties soon began to provoke questions. Precisely how many elements were there? Had most of them already been discovered? Or would there perhaps turn out to be innumerable elements? This soon led to more profound speculations. Somehow, amongst all these elements, there must be some kind of fundamental order. Dalton had discovered that the atoms of each element had different weights — but surely there had to be more to it than this? Berzelius had noticed that elements appeared to have different electrical affinities. Likewise, there appeared to be groups of different kinds of elements with similar properties — metals which resisted corrosion (such as gold, silver and platinum), combustible alkali metals (such as potassium and sodium), colorless, odorless gases (such as hydrogen and oxygen) and so forth. Was it possible that there was some kind of fundamental pattern behind all this?
Chemistry had achieved its scientific status and continuing success largely through experiment, and such theoretical thinking was viewed at best as mere speculation. Why should there be some kind of order amongst the elements? After all, there was no real evidence for such a thing? But the rage for order is a basic human trait, not least amongst scientists. And these speculations eventually began to find support, if only from scraps of evidence.
The first of these came from Johan Dobereiner, the professor of chemistry at the University of Jena. Dobereiner was the son of a coachman, and was largely self-educated. He managed to obtain a post as a pharmacist, and eagerly attended the regular local public lectures on science. In 1829 he noticed that the recently discovered element bromine had properties which seemed to lie midway between chose of chlorine and iodine. Not only that, its atomic weight lay exactly halfway between chose of these two elements.
Dobereiner began studying the list of the known elements, recorded with their properties and atomic weights, and eventually discovered another two groups of elements with the same pattern.
Strontium lay halfway (in atomic weight, color, properties, and reactivity) between calcium and barium; and selenium could be similarly placed between sulphur and tellurium. (▪ ▪ ▪)

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ssangra

It must hv required deep study and awareness of patterns to be able to classify these so well......I also do wonder if there is another pattern within a pattern for some of the rare earths