Discovery of Electrons and Cathode Ray Experiments

The study of the structure of the atom is grounded in a fundamental rule regarding the behavior of charged particles.
Primary Rule of Interaction: "Like charges repel each other and unlike charges attract each other."
Insights into atomic structure were primarily obtained through experiments involving electrical discharge through gases.

Historical Milestone (1830): Michael Faraday demonstrated that passing electricity through an electrolyte solution triggers chemical reactions at the electrodes.
Observations: These reactions resulted in the liberation and deposition of matter at the electrodes.
Theoretical Implications: Faraday formulated laws of electrolysis (further studied in advanced classes) that suggested electricity has a particulate nature.

Developmental Period: In the mid-1850s, many scientists, following Faraday, began studying electrical discharge in partially evacuated tubes.
Cathode Ray Discharge Tube Components: - A tube made of glass. - Two thin pieces of metal known as electrodes sealed inside. - The negative electrode is called the cathode. - The positive electrode is called the anode. - A vacuum pump used to adjust the pressure of different gases by evacuation.
Required Experimental Conditions: - Electrical discharge through gases is only observable at very low pressures. - The application of very high voltages across the electrodes is necessary.
Process of Flow: When high voltage is applied, a current begins to flow through a stream of particles moving from the negative electrode (cathode) toward the positive electrode (anode). These are designated as cathode rays or cathode ray particles.

Detection Method: Because the rays themselves are not visible, their flow is verified using a perforated anode (an anode with a hole).
Fluorescence/Phosphorescence: The tube wall behind the anode is coated with a phosphorescent material, specifically zinc sulphide ( $ZnS$ ).
Observation: When the cathode rays pass through the anode hole and strike the $ZnS$ coating, a bright spot is developed on the coating.
Real-World Application: Television picture tubes are essentially cathode ray tubes. The pictures observed on television screens result from fluorescence on the screen surface, which is coated with specific fluorescent or phosphorescent materials.

Summarized results from the cathode ray experiments include:

Direction of Travel: Cathode rays start from the cathode and move specifically toward the anode.
Visibility: These rays are not visible to the naked eye; their behavior is only observable through their interaction with materials (fluorescent or phosphorescent) that glow upon impact.
Linear Propagation: In the absence of external electrical or magnetic fields, these rays travel in straight lines.
Behavior in Fields: In the presence of electrical or magnetic fields, the behavior of cathode rays is identical to that expected from negatively charged particles.
Conclusion on Composition: This behavior suggests that cathode rays consist of negatively charged particles, which were subsequently named electrons.
Independence of Material: The characteristics of cathode rays (electrons) do not depend upon the material of the electrodes used or the nature of the gas present in the cathode ray tube.
Conclusion: Electrons are a basic constituent of all atoms.

Historical Milestone (1897): The British physicist J.J. Thomson measured the ratio of electric charge ( $e$ ) to the mass of the electron ( $m_e$ ).
Methodology: He utilized a cathode ray tube and applied both electrical and magnetic fields perpendicular to each other as well as to the path of the electrons.
Deviation Factors: The amount of deviation of the particles from their path in the presence of an electrical or magnetic field depends on specific variables: - Magnitude of Negative Charge: The greater the magnitude of the charge on the particle, the stronger the interaction with the electric or magnetic fields, resulting in greater deflection. - Mass of the Particle: The lighter the particle, the greater the deflection observed from the original path.
Experimental Control: By balancing the electrical and magnetic field strengths, it is possible to bring the electron back to the path it would follow in the absence of such fields (hitting point B on the screen).