The idea that matter is made up of discrete units is a very old idea. atomos, meaning "uncuttable", was coined by the ancient Greek philosophers Leucippus & Democritus (5th century BC) However, these ancient ideas were based on metaphysical reasoning rather than empirical evidence.
atØmic theory: is a scientific theory of the nature of matter, which states that matter is composed of discrete units called ATOMS. It began as a philosophical concept in ancient Greece and entered the scientific mainstream in the early 19th century when discoveries in the field of chemistry showed that matter did indeed behave as if it were made up of atoms. The word atom comes from the ancient greek adjective atomos, meaning "indivisible". 19th century chemists began using the term in connection with the growing number of irreducible chemical elements. Around the turn of the 20th century, through various experiments with electromagnetism and radioactivity, physicists discovered that the so-called "uncuttable atom" was actually a conglomerate of varioussubatomic particles
Since atoms were found to be divisible, physicists later invented the term "elementary particles to describe the "uncuttable"