R.2.1 The idea that matter is composed of atoms is an old concept that was first proposed
by the Greek philosophers Empedocles and Democritus around 500 and 400 BC.
R.2.2 The scientific community accepted the existence of the atom in modern times, initially
proposed by the chemist Dalton in the nineteenth century, and supported
and consolidated by Cannizzaro 50 years later.
R.2.3 An atom is too small to be seen directly or even with the help of modern and sophisticated
devices such as a microscope, because its diameter is estimated to be in the
order of 10−10 m.
R.2.4 Atoms are composed of protons, neutrons, and electrons.
R.2.5 In modern times, J. J. Thomson, around 1897, is credited with the discovery and
study of the electron.