CHEMISTRY

TOPIC:    ATOMIC STRUCTURE – (Canal Rays & Protons)

By Kingsley Idiagbor, B.Sc. (Hon.), PGDCs, NCE, MNSM

                    Outline

a.                  What are canal rays?

b.                 Properties of canal rays.

c.                  Rutherford’s alpha scattering experiment

What are canal rays?

In 1919, Goldstein repeated Thompson’s experiment with the cathode rays (See – Atomic Structure – (Cathode rays)) but used a perforated cathode. When electric discharge was passed through the tube through the perforated cathode, he observed that besides the cathode rays there was some other kind of rays which passed through the holes in the cathode. These other type of rays traveled at opposite direction to that of the cathode rays. For want of better term, Goldstein named the rays – Canal rays.

Properties of Canal rays

Investigations were further conducted on the canal rays.  They were subjected to magnetic and electric fields.  The rays were deflected and moved towards the negative electric plate. Therefore, canal rays must consist of positively charged particles.

The properties and nature of the canal rays varied with the atomic mass[i] of the gaseous element used in filling the discharge (or cathode ray) tube.

Hydrogen is obviously the lightest of canal rays.  Their canal rays were called pro’ton (literally means the first particle), from where the term proton was derived.

The mass of these particles that we call canal rays were found to correspond with that of protons which is 1.0 x 10^-27kg.

Rutherford’s Alpha-Scattering Experiment

Ernest Rutherford (1920) bombarded a thin gold foil with alpha (ά) particles (See Figure) and observed that a good number of the alpha (ά) particles passed through unhindered. However, some other ά- particles were deflected at wide angles greater than 180^o. Rutherford summarized his observations in the postulations of the Nuclear Theory.

a. Those ά- particles that passed through undisturbed suggested that a great volume of space or region exist in the atom;

b. There must be a mass in the atom which is positively charged as to repel some of the ά- particles. This mass, Rutherford called the nucleus.

c. The nucleus is at the core of the atom, to which the mass of the entire atom is concentrated on.

This experiment is perhaps the greatest proof of the existence of the proton –a positively charged particle at the core of the atom (the nucleus).