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[britannica]
He is known as the father of chemistry who named oxygen and found out that it is the key element for combustion. -
He discovered triads of elements that have similar properties.
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He arranged the elements based on their weight which is currently called atomic weight. He then realised that every 7 elements have similar properties. Due to noble gases haven't been discovered in that period, he only a periodicity of 7 instead of 8. -
In 1869, Mendeleev created a periodic table for elements based on their periodicity where he puts elements with similar properties together and also leave gaps for unknown elements. -
He determined and arranged the elements according to their atomic number.