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.
Answered by: Dan Milx, Physics Ph.D, York University, Toronto
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.
Answered by: Paul Walorski, B.A., Part-time Physics/Astronomy Instructor
'You cannot hope to build a better world without improving the individuals. To that end each of us must work for his own improvement, and at the same time share a general responsibility for all humanity, our particular duty being to aid those to whom we think we can be most useful.'