It has been taken for granted that atoms are the elements of all materials around us, which was originally proposed by Democritus as early as 400 BC. Nevertheless, the definition of atoms used in Physics has been dramatically changed since the discovery of electron, the first sub-atomic particle, in 1897. In Greek, atom means something that cannot be divided further, which was exactly what Democritus and his teacher, Leucippus, in their mind when they argued what atoms might be.
A few years later, the seemly well-understood electromagnetic wave was first described by particles called photons by Planck. In 1924, de Broglie postulated in his Ph.D thesis that all particles, such as electrons, can also be described by matter wave, which was then experimentally confirmed by JJ Thomson's son. What does this imply?
Well, it means that we have to abandon the language and the way of decribing a particle in Newtonian mechanics. In fact, the Heisenberg's uncertainty principle is by no means a discouraging theorem. Instead, it simply tells the difference between Quantum Mechanics and Newtonian Mechanics.
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