Periodic table - Wikipedia
https://en.wikipedia.org/wiki/Periodic_table
SummaryHistoryOverviewAtomic structures of the elementsVariationsPeriodic trendsClassification of elementsFuture extension beyond the seventh periodIn 1817, German physicist Johann Wolfgang Döbereiner began to formulate one of the earliest attempts to classify the elements. In 1829, he found that he could form some of the elements into groups of three, with the members of each group having related properties. He termed these groups triads. Chlorine, bromine, and iodine formed a triad; as did calcium, strontium, and barium; lithium, sodium, a…
DA: 37 PA: 37 MOZ Rank: 73