<P> 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 . While seemingly apropos, 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 various subatomic particles (chiefly, electrons, protons and neutrons) which can exist separately from each other . In fact, in certain extreme environments, such as neutron stars, extreme temperature and pressure prevents atoms from existing at all . </P> <P> Since atoms were found to be divisible, physicists later invented the term "elementary particles" to describe the "uncuttable", though not indestructible, parts of an atom . The field of science which studies subatomic particles is particle physics, and it is in this field that physicists hope to discover the true fundamental nature of matter . </P> <P> The idea that matter is made up of discrete units is a very old one, appearing in many ancient cultures such as Greece and India . The word "atom" was coined by the ancient Greek philosophers Leucippus and his pupil Democritus . However, these ideas were founded in philosophical and theological reasoning rather than evidence and experimentation . Because of this, they could not convince everybody, so atomism was but one of a number of competing theories on the nature of matter . It was not until the 19th century that the idea was embraced and refined by scientists, as the blossoming science of chemistry produced discoveries that could easily be explained using the concept of atoms . </P> <P> Near the end of the 18th century, two laws about chemical reactions emerged without referring to the notion of an atomic theory . The first was the law of conservation of mass, formulated by Antoine Lavoisier in 1789, which states that the total mass in a chemical reaction remains constant (that is, the reactants have the same mass as the products). The second was the law of definite proportions . First proven by the French chemist Joseph Louis Proust in 1799, this law states that if a compound is broken down into its constituent elements, then the masses of the constituents will always have the same proportions, regardless of the quantity or source of the original substance . </P>

Who was the first one to proposed the idea about atom