Which theory belongs in place of the question mark in tran’s timeline?

Assuming that the timeline that you're referring to is in the year of 1803, the theory is probably belong to Dalton
In 1803, John Dalton proposed a theory that would change the field of chemistry for the rest of the time, which is the discovery of atom

hope this helps

Non metal is an element or substance that is not a metal metal is a solid material that is typically hard, shiny, malleable, fusible, and ductile, with good electrical and thermal conductivitymetalloid is an element (e.g., germanium or silicon) whose properties are intermediate between those of metals and solid nonmetals. they are electrical semiconductors.