Final answer:
Johann Dobereiner first noticed the existence of triads, or groups of three elements with similar properties, in 1829, laying groundwork for the modern periodic table.
Step-by-step explanation:
The German chemist Johann Dobereiner was the first to notice the existence of various groups of three elements, which he termed triads. In 1829, Dobereiner placed elements like lithium, sodium, and potassium into these groups.
These triads were based on both physical and chemical properties and aligning elements by their atomic masses revealed a pattern, where the average atomic mass of the lightest and heaviest elements closely matched that of the middle element. Despite pioneering this approach, Dobereiner's triad system had limitations as not all known elements could be classified within this system, which was later refined by others leading to the development of the modern periodic table.