The concept of the atom originated round 440 BC. Leucippus of Miletus came up with the idea. He and his pupil, Democritus refined and extended the idea in the years after. Almost all of the original writings of Leucippus and Democritus are lost. About the only sources we have for their atomistic ideas are found in quotations of other writers.
Leucippus and Democritus reasoned that all matter is composed of atoms, which are bits of matter too small to be seen. These atoms CANNOT be further split into smaller portions. Leucippus and Democritus theorized that splitting stops when it reaches indivisible particles and does not go on infinitely. This was their theory, and the name "atom" originates from this.
In other words, there is a lower limit to the division of matter beyond which we cannot go. Atoms were impenetrably hard, meaning they could not be divided. In Greek, the prefix "a" means "not" and the word "tomos" means cut. Our word atom therefore comes from atomos, a Greek word meaning uncuttable.
Dan Milx, Physics Ph.D, York University, Toronto
The word is derived from the Greek word "atomos", meaning indivisible. Greek philosophers speculated whether matter was continuous or discrete. For example, did water stay liquid
water regardless of how small an amount you have, or was there some "smallest particle" of water? It was a philosophical, not a scientific, question that gave rise to the concept of a smallest "indivisible" unit of matter.
In 1803, John Dalton formulated the "atomic theory" of matter based on experiments that quantified the weights of elements formed when compounds were broken down. Based on experimental evidence, Dalton proposed that atoms really do exist as fundamental units of all elements.
Twentieth century physics was able to probe atoms and prove that they are not, in fact, indivisible. But the name stuck, and still has meaning in that atoms are the smallest, indivisible part of any element. If you do divide them into smaller pieces of protons, neutrons and electrons, you no longer have the original element.
Paul Walorski, B.A., Part-time Physics/Astronomy Instructor
'The important thing is not to stop questioning. Curiosity has its own reason for existing. Never lose a holy curiosity.'