Around the 5th century BC, Greek philosopher Democritus invented the concept of the atom (from Greek meaning “indivisible”). The atom, eternal, constant, invisible and indivisible, represented the smallest unit and the building block of all matter. Democritus suggested that the varieties of matter and changes in the universe arise from different relations between these most basic constituents. He illustrated the concept of atom by arguing that every piece of matter could be cut to an end until the last constituent is reached. Today the word atom is used to identify the basic component of molecules that create all matter, but it is known that the atom itself is made of particles even more fundamental, some of which are elementary. The first theoretical and experimental models of the structure of matter came as late as the 19th century, which is the time marked as the beginning of modern science. At that time a more empirical approach, mainly in chemistry, opened a new era of scientific investigations.