Structure of Atom

Discovery of Subatomic Particles

Discovery of Electron

In 1830, Michael Faraday demonstrated that if electricity is passed through a solution of an electrolyte, it results in chemical reactions at electrodes. The reaction result in liberation and deposition of matter at the electrodes. These results suggested the particulate nature of electricity.

In mid 1850s many scientists began to study electrical discharge in cathode ray discharge tubes. Faraday was the main scientist to conduct such experiments.

Structure of Cathode Ray Tube: This tube is made of glass. It contains two thin pieces of metal which are called electrodes, sealed inside. The electrical discharge through the gases can only be observed at very low pressure and at very high voltage. Evacuation of glass tube helps in manipulating the pressure of different gases in the tube. When enough high voltage is applied across the electrodes, current starts to flow through a stream of particles. These particles move from the negative electrode (cathode) to the positive electrode (anode). These particles are called cathode rays or cathode ray particles.

The flow of current was further checked by making a hole in the anode, and coating the tube (behind anode) with phosphorescent material zinc sulphide. When these rays (after passing through anode) hit the coating, a bright spot is formed on the coating.

Summary of Results of Experiment:

• Cathode rays move from cathode to anode.
• These rays are not visible but their behavior can be observed with the help of fluorescent materials because such materials glow when hit by cathode rays.
• When there is no electrical or magnetic field, these rays travel in straight lines.
• When either electrical or magnetic field is present, cathode rays behave as negatively charged particles. This suggests that cathode rays consist of negatively charged particles, viz. electrons.
• The characteristics of cathode rays do not depend upon the material of electrodes or on the nature of gas present in cathode ray tube.

Thus, it was concluded that electrons are basic constituent of all the atoms.

Charge to Mass Ratio of Electron

In 1897, J. J. Thomson measured the ratio of electrical charge (e) to the mass of electron (m_e) by using cathode ray tube. He applied electrical and magnetic field perpendicular to each other and also to the path of electrons. According to Thomson, the amount of deviation of particles from their path in the presence of electrical or magnetic field depends upon following:

• The magnitude of the negative charge on the particle. When charge on the particle is greater interaction with electric or magnetic field is greater and so there is greater deflection.
• The mass of the particle: lighter the particle, greater the deflection.
• Deflection of electrons from original path increases with increase in voltage. In other words, deflection increases with increase in strength of magnetic field.

Based on his observations, Thomson gave the value of ratio of charge to mass as follows:

`e/(m_e)` = 1.758820 × 10¹¹ C kg^-1

Here, m_e is mass of electron in kg and e is the magnitude of charge on electron in coulomb (C). As electrons are negatively charged, the charge on electron is –e.

Charge on Electron

A Millikan devised the oil drop experiment (1906-14) to determine the charge on electron. According to Millikan, charge on electron is -1.6 × 10^-19 C. The present accepted value of electrical charge is -1.602176 × 10^-19 C

The mass of electron was determined by combining these results with Thomson’s value of e/m_e ratio

m_e `=(1.602176xx10^(-19)C)/(1.758820xx10^(11)C\kg^(-1))`

= 9.1094 × 10^-31 kg

Discovery of Proton and Neutron

Electrical discharge carried out in modified cathode ray tube led to discovery of canal rays. These rays carry positively charged particles. The characteristics of positively charged particles are as follows:

• Mass of positively charged particles depends upon the nature of gas present in cathode ray tube. These are simply the positively charged gaseous ions.
• The charge to mass ratio of particles depends on the gas from which they originate.
• Some of the positively charged particles carry a multiple of the fundamental unit of electrical charge.
• The behavior of positively charged particles in magnetic or electrical field is opposite to that observed for electrons.
• The smallest and lightest positive ion was obtained from hydrogen and was called proton.

In 1932, Chadwik discovered neutron by bombarding a thin sheet of beryllium by α-particles.

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