Final answer:
The definition of the first molecular breakthrough varies, however, one significant early discovery in the understanding of molecules was by Robert Boyle in the 17th century. Another major advance was provided by Perrin's confirmation of Brownian motion in the early 20th century. For organic molecules, the discovery of fullerene in 1985 marked a significant modern breakthrough.
Step-by-step explanation:
Identifying the first molecular breakthrough is a complex task as it depends on how breakthrough is defined. There were numerous pivotal moments in chemistry, but one significant early discovery was by Robert Boyle, who in the 17th century developed ideas about the behavior of gases and suggested that small particles could combine to form molecules. Later, John Dalton used these concepts for his atomic theory. Another major advance was by Jean-Baptiste Perrin in the early 20th century, who confirmed Einstein's theory through his observation of Brownian motion, providing direct evidence of atoms and molecules' existence and determining Avogadro's number.
In the context of organic molecular breakthroughs, the discovery of fullerene (a 60-carbon molecule) in 1985 by Harold Kroto, Richard Smalley, and Robert Curl could be considered a major breakthrough as well. Although computational chemist Elena Galpern predicted such a molecule in 1973, her work was not widely recognized until later.