Discovery of proton

Production of anode rays

Since the atom is electrically neutral there must be positively charged particles present in the atom to neutralize the negative charges of the electrons. Goldstein experimentally proved the existence of protons in the atom.

Experiment

Production of anode rays

In this a discharge tube having a perforated cathode is used. When a high voltage of about 10,000 volts is applied to a discharge tube having a perforated cathode and containing air at very low pressure of about .001 mm of mercury, a faint red glow is observed behind the cathode. These rays are formed at the anode and when these rays strike the walls of the discharge tube they produce a faint red light. Since they are formed at the anode (positive electrode) they are known as anode rays or positive rays.

Properties of anode rays

• Travel in straight lines: They cast a shadow of the objects placed in their way.

• Produce mechanical effect: A paddle wheel placed in their path starts rotating.

• Rays are positively charged: Anode rays are deflected towards the negative plate of an electric field.

• The nature of the anode rays depends upon the gas taken in the discharge tube. Different gases give different types of positive rays, which contain particles having different masses and different charges. Therefore the e/m ratio is not constant for positive ray particles obtained from different gases.

In the case of hydrogen the e/m ratio is the highest as the positive particles obtained from hydrogen are the lightest. The positive particles obtained from hydrogen gas are called 'protons'. It comes from the Greek word 'Proteios' meaning 'of first importance'.

Formation of positive rays

When high electrical voltage is applied to a gas, its atoms break up into negatively charged particles (electrons) and positively charged particles. These positively charged particles formed by the removal of electrons from the gas atoms are called positive rays.

Characteristics of a proton

Mass

A proton is actually a hydrogen atom, which has lost its electron. Since the mass of an electron is small, the mass of a proton is equal to the mass of a hydrogen atom. As the mass of hydrogen atom is 1 a.m.u., the relative mass of a proton is 1 a.m.u. The mass of a proton is 1840 times that of an electron. The absolute mass of a proton is 1.6 x 10^-24 gram.

Charge

The proton is equal and opposite to the charge of an electron. So the absolute charge of a proton is 1.6 x 10^-19 coulomb of positive charge. This being the smallest positive charge carried by any particle, it is taken as 1 unit positive charge. The relative charge of a proton is +1 (plus one).

Effect of low pressure in the discharge tube

When the gas atoms in the discharge tube are at atmospheric pressure they collide with the electrons preventing them from reaching the anode. As no electrons reach the anode no current flows through the discharge tube. When the gas pressure is very low there are few gas atoms in the discharge tube. As such there is no hindrance to the movement of electrons and the gas conducts electricity.