Atomic theory traces itself back to the pre-Socratics when the philosopher Leucippus (d. 370 BCE) and his student Democritus (371 BCE) proposed that everything is composed of indestructible particles moving through empty space. These thinkers later became known as the Atomists.
The Atomists claimed that atoms, given their movement is at the microscopic level, are invisible to the naked eye. Leucippus, for example, held that the universe consists of two different elements: the “solid” and the “empty.” All solid matter is constituted of these moving, tiny atomic particles in empty space or in a void. He also believed that all observable changes within the cosmos are due to the motion of these atoms within this space/void. Each and every atom is an eternal and unchanging entity that is both indestructible and indivisible. Leucippus believed that each atom is capable of joining with other atoms to form different substances and objects or that they can be mutually repelled.
Democritus, a contemporary of the famous Socrates, posited atoms to be the fundamental substances of all things and suggested that they come in a range of sizes and shapes. Their properties also determined the characteristics of different substances. Democritus famously stated that “Nothing exists except atoms and empty space; everything else is opinion.” Atoms that make up liquids are smooth and can move freely past one another (water atoms are smooth and round), solid substances have more rigid atoms that move less and can connect with other atoms (atoms constituting iron, for instance, possess hooks that interlock to give the metal its solidity), and atoms in the air are light and move freely and independently. When the substances that are formed out of the atoms break down the atoms themselves remain intact. Democritus believed that the psyche (soul) is also made of atoms, which he referred to as “fire atoms.”
The Atomists were one of the competing Pre-Socratic schools of philosophy to conceptualize concerning the metaphysical nature of existence. Importantly within this milieu, there were also disagreeable philosophical views. For example, the views of the Atomists were in contrast to the ideas proposed by the likes of Parmenides (b. 515 BCE) and Zeno (d. 425 BCE), both of whom claimed change to be an illusion. There was also a group of incorporeal monists within the Italian school. These thinkers were monists (they believed that everything consisted of a single substance) but did not believe everything to be physical. Pythagoras, for example, believed that the fundamental reality of all things is immaterial, which he claimed is mathematics, thus positing numbers to be the basic rational principle behind all things.
Stanford Encyclopedia of Philosophy. Leucippus. Available.
Weeks, M., Baiasu, R., Fletcher, R., Szudek, A. and Talbot, M. 2019. How Philosophy Works: The Concepts Visually Explained. London: Dorling Kindersley Ltd.