Chemistry — Class 11

Discovery of Electron

Structure of Atom · Cathode Ray Experiments

J.J. Thomson Cathode Rays Discharge Tube Electron

Chapter 2 · Structure of Atom

Introduction

Discovery of the Electron

The electron was discovered by J.J. Thomson at the end of the 19th century during the studies of the passage of electricity through gases at extremely low pressures. These experiments were known as discharge tube experiments.

What is a Cathode Ray Tube?

A cathode ray tube is made of glass containing two thin pieces of metal, called electrodes, sealed in it. The electrical discharge through the gases could be observed only at very low pressures and at very high voltages.

When sufficiently high voltage is applied across the electrodes, current starts flowing through a stream of particles moving in the tube from the negative electrode (cathode) to the positive electrode (anode). These were called cathode rays or cathode ray particles.

Why does this matter?

Before Thomson's discovery, atoms were thought to be indivisible. The existence of cathode rays — particles smaller than atoms — was the first evidence that atoms have internal structure. This led to the discovery of the electron, the first subatomic particle known to science.

Experiment Setup

The Discharge Tube Experiment

A sealed glass tube is fitted with two metal electrodes — a cathode (negative) and an anode (positive) — connected to a high-voltage DC source. The tube is evacuated to very low pressure using a vacuum pump. Under these conditions, a stream of particles — the cathode rays — flows from the cathode to the anode.

Discharge tube experiment diagram showing cathode, anode, and cathode rays

Fig 1. Gas discharge tube with cathode and anode connected to a high-voltage source

Key Points

Two conditions are essential for observing the discharge: (i) very low pressure inside the tube, and (ii) very high voltage across the electrodes. Only when both conditions are met does the stream of cathode rays become visible.

Confirmation

Confirming the Direction of Flow

The flow of current from cathode to anode was further checked by making a hole in the anode and coating the tube behind the anode with a phosphorescent material — zinc sulphide (ZnS).

When these rays, after passing through the anode, strike the zinc sulphide coating, a bright spot is developed on the coating. This confirmed that cathode rays originate at the cathode and travel toward the anode.

Cathode ray experiment showing fluorescent screen and properties

Fig 2. Cathode ray experiments demonstrating fluorescent effects and straight-line travel

How ZnS Works

Zinc sulphide is a phosphorescent material — it glows when struck by fast-moving particles. By making a hole in the anode, the rays pass through and hit the ZnS coating behind it, producing a visible bright spot. This is the same principle used in old CRT television screens.

Property i
i

Cathode Rays Travel in Straight Lines

Cathode rays produce a sharp shadow of a solid object placed in their path. When an opaque object (such as a Maltese cross) is placed between the cathode and the fluorescent screen, a well-defined shadow appears on the screen.

Cathode rays casting shadow of Maltese cross on fluorescent screen

Fig 3. Shadow of a Maltese cross on the fluorescent screen proves cathode rays travel in straight lines

What This Proves

The sharpness of the shadow tells us that cathode rays do not bend around objects the way light can (diffraction). Instead, they travel in straight lines from the cathode, similar to how particles in a beam would behave. If cathode rays were waves, we would expect some bending around the edges of the object.

Property ii
ii

Cathode Rays Are Material Particles

If a light paddle wheel (e.g., made of mica) is placed in the path of cathode rays, the wheel begins to rotate. This shows that cathode rays are not just a form of energy or radiation — they are made up of material particles that possess momentum and can exert force on physical objects.

Paddle wheel rotating when struck by cathode rays

Fig 4. Light mica paddle wheel rotates when struck by cathode rays, proving they carry momentum

What This Proves

For the wheel to rotate, the cathode rays must have mass and velocity (i.e., momentum). A wave or pure energy (like light) cannot spin a physical wheel. This was crucial evidence that cathode rays are not electromagnetic waves but actual particles — what we now know as electrons.

Experiment

Watch the Experiment

See the cathode ray discharge tube experiment in action. Observe how cathode rays travel from the cathode to the anode, produce fluorescence on the ZnS screen, cast sharp shadows, and cause the paddle wheel to rotate.

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