Dalton's Atomic Theory. 1) All matter is made of atoms. Atoms are indivisible and indestructible. 2) All atoms of a given element are identical in mass and. In chemistry and physics, atomic theory is a scientific theory of the nature of matter, which states that matter is composed of discrete units called atoms. Democritus. The greatest contribution of Democritus in astronomy is the atomic theory-the belief that everything in the world consists of particles called atoms. Democritus, ancient Greek philosopher who is credited with the birth of atomic theory. Credit: phil-fak.uni-duesseldorf.de In chemistry and physics, the atomic theory explains how our understanding of the atom has changed over time. Atoms were once thought to be the smallest pieces of matter. Democritus ( chosen of the people ) was an Ancient Greek philosopher born in Abdera, Thrace, Greece. He was an influential pre-Socratic philosopher who formulated an. Democritus; Born: c. 460 BC Abdera, Thrace: Died: c. 370 BC (aged around 90) Era: Pre-Socratic philosophy: Region: Western philosophy: School: Pre-Socratic philosophy Democritus, known in antiquity as the ‘laughing philosopher’ because of his emphasis on the value of ‘cheerfulness,’ was one of the two founders of ancient. Democritus (460—370 B.C.E.) Democritus was born at Abdera, about 460 BCE, although according to some 490. His father was from a noble family and of great wealth. atomic theory = explaining macroscopic phenomenon through the behavior of microscopic atoms, in this case, kinetic behavior temperature = velocity of atoms.