Atoms and Early Atomic Models

Since ancient times, one of the fundamental questions for mankind has concerned the constitution of matter. The first answer was offered by the Greek philosophers, by means of the concept of atom. In the late eighteenth century, chemists began to understand how substances combine to form compounds in various reactions. In the mid-nineteenth century, also physicists began to use the ideas of atoms and molecules to explain the properties of gases. In this chapter we will show how in the late nineteenth century the idea of the atom as an indivisible entity and primary constituent of matter began to falter, together with the atomic model proposed by Lord Kelvin[aut]Thomson, William 1st Baron Kelvin. We will also discuss some aspects related to the field of spectroscopy, and we will introduce two fundamental discoveries which were made in those years: that of cathode rays, thanks to the experimental works of J. Plücker[aut]Plücker, Julius, J. W. Hittorf[aut]Hittorf, Johann Wilhelm, E. Goldstein[aut]Goldstein, Eugen and W. Crookes[aut]Crookes, William, and that of X-rays, made by W. C. Röntgen[aut]Röntgen, Wilhelm Conrad. We will conclude by discussing the invention of the cloud chamber, by C. T. R. Wilson[aut]Wilson, Charles Thomson Rees.