Matter consists of small particles called molecules which again consist of atoms. Until the end of the 19th century, it was believed that atoms were indivisible, but later, it was found that atoms are consisted of smaller subatomic particles-electrons, protons and neutrons.
Electrons are light, negatively charged particles while protons are heavy, positively charged particles and neutrons are heavy, neutral particles. The exact structure of the atom and the forces responsible for holding these particles together were the subject of investigation in those times.
Several models were proposed to explain atomic structure : the Thomson model, the Rutherford model, the Bohr model and so on. In subsequent sections we will study the Rutherford and Bohr models of the atom.
Two students working in Rutherford’s laboratory, Geiger & Marsden, bombarded highly energetic α-particles onto a thin Gold foil.
They observed that
(i) Most of the α-particles passed undeviated.
(ii) A small number of α-particles underwent small deviation.
(iii) An extremely small number of very few α-particles deviated through large angles, and about one in a million α-particles retraced their path.
The Thomson model had predicted a small deviation of a few degrees as the positive and negative charges are distributed throughout the whole atom in this model. The experimental results were very different.
Rutherford later wrote : “It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you had fired a 15 – inch shell at a piece of tissue paper and it came back and hit you.”