Discovery of Cathode Rays and Radioactivity

In this article, we’ll learn about Cathode rays and radioactivity. Let’s begin

Postulates of Dalton’s Atomic Theory

1. Matter is composed of very tiny, indivisible particles called atoms.
2. All atoms of a particular element are identical, but the atoms of a different element are different from the atoms of all other elements.
3. Atoms of one element cannot be turned into the atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.
4. Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.

Moreover, Dalton’s atomic theory successfully explained the law of conservation of mass, the law of constant composition, and the law of multiple proportions.

Dalton’s Early Atomic Model:

Billiard Ball” model

He envisioned atoms as solid, hard spheres, like billiard(pool) balls, so he used wooden balls to model them.

Introduction

Democritus (460–370 Bc) and other early Greek philosophers described the material world as made up of tiny indivisible particles called atoms, meaning “indivisible” or “uncuttable.”

The atomic theory was first given by John Dalton, a British school teacher, in 1808, who considered the atom the basic particle of matter.

Dalton was able to explain the laws of constant composition and multiple proportions. However, neither Dalton nor other scientists had any direct evidence for the existence of atoms.

In this chapter, we will begin with the experimental observations made by scientists towards the end of the nineteenth and starting of the twentieth century. These proved that atoms are made of sub-atomic particles.

Before we summarize the current atomic model, we briefly consider some milestone discoveries that led to this model. First, we will understand that the atom is composed of electrically charged particles, some with positive and others with a negative charge.

In this session, let us learn more about how the electron was discovered.

Explanation

As we discuss the progress of our current atom model, please note that the same charge particles repel one another, whereas particles with opposite charges attract each other.

Cathode Rays and Electrons

An insight into the atom’s structure was found from experiments on electrical discharge through gases. In the mid-1850s, many researchers, mainly Faraday, began to study electrical discharge in partially evacuated tubes, known as cathode ray discharge tubes.

Later, J.J Thomson concluded that cathode rays are streams of negatively charged particles that we now call electrons.

Thomson was the first to show that the atom was made of even smaller things. He used cathode rays to discover the electron.

Discovery of Electron

The first cathode-ray tube (CRT) was invented by Michael Faraday (1791-1867). He began to study electrical discharge through a glass tube pumped almost empty of air, known as a cathode ray discharge tube.

a. A cathode ray tube is prepared by a glass containing two thin slices of metal, called electrodes, coated in it.

b. The electrical discharge through the gases could be seen only at extremely low pressures and very high voltages.

c. Removing the glass tubes could balance the pressure of different gases.

d. When adequately high voltage is applied across these electrodes, current starts flowing through particles in the tube from the negative electrode (cathode) to the positive electrode (anode).

e. These particles are known as cathode rays or cathode ray particles.

f. The current stream from the cathode to the anode was additionally observed by making a hole in the anode and coating the tube behind the anode with phosphorescent zinc sulfide.

g. After passing through the anode, these rays strike the zinc sulfide coating, and a bright spot is developed on the coating.

Results:

The cathode rays go from the cathode and move to the anode. These rays are not visible; however, their behavior can be seen using particular materials (fluorescent or phosphorescent), which glow when hit by them.

1. Without any field, cathode rays travel in a straight-line path.
2. When the electrical or magnetic field is applied, the behavior of cathode rays is related to that expected from negatively charged particles, signifying that the cathode rays consist of negatively charged particles called electrons.
3. The characteristics of cathode rays (electrons) do not change by changing the material of electrodes and also the nature of the gas inside the cathode ray tube.

Therefore, we can say that electrons are the basic constituent of all atoms.

1. Charge to Mass Ratio of Electron: Thomson’s Experiment

In 1897, British physicist J.J. Thomson determined the ratio of electrical charge (e) to the mass of the electron (m_e) by employing a cathode ray tube and electrical and magnetic field perpendicular to each other and the path of electrons.

i. Since the electron is a negatively charged particle, the electric field deflected the rays in one direction (at point A), and the magnetic field deflected them in the opposite direction (at point C).

ii. Thomson changed the strengths of the fields to balance the effects by allowing the electrons to travel in a straight line to the screen (at point B).

The extent of variation of the particles from their path in the electrical or magnetic field depends on the following:

1. Small drops of oil are permitted to fall over in between the electrically charged plates.
2. Millikan measured how varying the voltage between the plates impacted the fall rate.
3. From this information, he determined the negative charge on the drops.
4. Because the charge on any drop was always some integral multiple of 1.602 X 10^-19 C, Millikan deduced this value to be the charge of a single electron.

The mass of the electron (m_e) was calculated by combining these results with Thomson’s value of the e/m_e ratio. Mass of electron= m_e =

Radioactivity

Different types of rays were discovered in the later half of the nineteenth century.

Wilhelm Röentgen, in 1895 discovered x-rays that are not deflected by the electric and magnetic fields and have a very superior penetrating power through the matter.

In 1896, the French scientist, Henri Becquerel, found that a uranium compound spontaneously produces high-energy radiation. This spontaneous emission of radiation from this type of element is known as radioactivity, and the elements are called radioactive elements.

This field was established by Marie Curie, Piere Curie, Rutherford, and Fredrick Soddy.

i. Furthermore, the study of radioactivity, principally by the British scientist Ernest Rutherford, revealed three types of radiation: alpha (α), beta (β), and gamma (γ).

ii. Rutherford showed that the paths of α and β radiation are bent by an electric field, although in opposite directions, while γ radiation is unaffected by the field.

From His finding, he concluded that:

• α and β rays consist of fast-moving electrically charged particles.
• α-rays are high-energy particles carrying two positive charge units and four atomic mass units.
• β-rays are negatively charged (-1) particles similar to cathode rays, i.e., electrons.
• The γ-rays are high-energy radiation similar to X-rays, neutral in nature, do not consist of particles, and carry no charge.
• Each alpha particle has a mass of about 7400 times that of an electron.

1. The magnitude of the negative charge on the particle. The larger the magnitude of the charge on the particle, the more contact with the electric or magnetic field will be, therefore, the greater the deflection.
2. The mass of the particles: If the particles are lighter, the deflection will be greater.
3. The strength of the electrical or magnetic field: With the rise in the voltage or the strength of the magnetic field, the deflection of electrons from their original path will also increase.

iii. Knowing the strengths, Thomson was able to calculate a value of e/m as:

e/m_e = 1.76 X 10⁸ coulombs per gram

Where m_e = the mass of the electron in g, and e = the magnitude of the charge on the electron in coulomb (C).

As the electrons are negatively charged, the charge on an electron is –e.

2. Charge on the electron: Millikan Experiment

R.A. Millikan (1868-1953) invented a method known as the oil drop experiment (1906-14) to determine the charge of the electrons.

Procedure:

1. Small drops of oil are permitted to fall over in between the electrically charged plates.
2. Millikan measured how varying the voltage between the plates impacted the fall rate.
3. From this information, he determined the negative charge on the drops.
4. Because the charge on any drop was always some integral multiple of 1.602 X 10^-19 C, Millikan deduced this value to be the charge of a single electron.

The mass of the electron (m_e) was calculated by combining these results with Thomson’s value of the e/m_e ratio. Mass of electron= m_e =

Radioactivity

Different types of rays were discovered in the later half of the nineteenth century.

Wilhelm Röentgen, in 1895 discovered x-rays that are not deflected by the electric and magnetic fields and have a very superior penetrating power through the matter.

In 1896, the French scientist, Henri Becquerel, found that a uranium compound spontaneously produces high-energy radiation. This spontaneous emission of radiation from this type of element is known as radioactivity, and the elements are called radioactive elements.

This field was established by Marie Curie, Piere Curie, Rutherford, and Fredrick Soddy.

i. Furthermore, the study of radioactivity, principally by the British scientist Ernest Rutherford, revealed three types of radiation: alpha (α), beta (β), and gamma (γ).

ii. Rutherford showed that the paths of α and β radiation are bent by an electric field, although in opposite directions, while γ radiation is unaffected by the field.

From His finding, he concluded that:

• α and β rays consist of fast-moving electrically charged particles.
• α-rays are high-energy particles carrying two positive charge units and four atomic mass units.
• β-rays are negatively charged (-1) particles similar to cathode rays, i.e., electrons.
• The γ-rays are high-energy radiation similar to X-rays, neutral in nature, do not consist of particles, and carry no charge.
• Each alpha particle has a mass of about 7400 times that of an electron.