Thomson’s Atomic Model also called as ‘Plum Pudding Model’ was the most accepted Atomic Model during the year 1904-1910, which emphasized on the inner structure of the Atom. This post will discuss what is Thomson’s Atomic Model, postulates of J.J. Thomson’s Model, How does Plum Pudding Model Work, applications and limitations.

What is Thomson’s Atomic Model

Thomson’s Atomic Model (also called as ‘Plum Pudding Model‘) explains the inner structure of the atom theoretically. ‘J.J. Thomson’ was a great physicist who has the credit of discovering ‘Electrons’ and was awarded Nobel Prize for his discovery. He conducted his experiment using Cathode Ray Tube (CRT). His theory states that an Atom is made up of thousands of Electrons and an Electron is 2000 times lighter than Proton. He called the ‘Electrons’ as “Corpuscles”.

Introduction to Thomson’s Atomic Model

Fig. 1 – Introduction to Thomson’s Atomic Model

Thomson’s Atomic Model states that an Atom is electrically neutral and is made up of ‘Corpuscles’ which act as Negative ions. He also assumed that these Negative ions or Electrons moved in an orbit in a sphere which had equal positive charges. This scientific model was compared to Plum Pudding, where the distribution of Electrons was considered as ‘Plums’ (Raisins) in a positively charged Space which was referred to as Pudding. Hence the name Plum Pudding Model was attributed to Thomson’s Atomic Model.

Schematic Representation of Thomson’s Atomic Model, Atomic Model compared to Plum Pudding

Fig. 2 – (a) Schematic Representation of Thomson’s Atomic Model (b) Atomic Model Compared to Plum Pudding

Postulates of Thomson’s Atomic Model

According to Thomson,

  • An Atom consists of a positively charged sphere with electrons encased in it.
  • Since the positive and negative charges are equal in magnitude, an Atom is electrically neutral.

How does Thomson’s Atomic Model Work

J.J. Thomson conducted series of experiments to examine the nature of Cathode Rays. It was an attempt to conclude that Cathode Rays carried negative charge. The apparatus built by him was called the Cathode Ray Tube which is popularly known today as ‘Electron Gun’.

Fig. 3 shows the Schematic representation of the same. Electrons from Cathode on the left side of the Tube travels through the slit (Anode), and comes in contact with the Phosphorescent screen at the right end of the Tube. He found that the beam could be deflected by electric field while turning off the Magnetic field. He also identified that by changing and adjusting the strengths of Electric and Magnetic field, the Cathode Rays did not deflect.

Cathode Ray Tube Experiment

Fig. 3 – Cathode Ray Tube Experiment for Thomson’s Atomic Model

At low pressure of about 1 millimetre of mercury, Cathode Rays appeared slightly divergent and dark near the cathode. This dark space extends through the Tube at further low pressure appearing Fuzzy. When it reaches glass or when it comes in contact with Phosphorescent Screen, it glows as the result of the energy absorbed by the Cathode Rays. He measured the deflection, length of the apparatus, strength of electric and Magnetic field to calculate Charge to Mass Ratio. He observed that this ratio was independent of the metal (charged Plates) and the gas in the Tube. These observations made him conclude that these negatively charged particles were constituents of Atoms.

He observed that when a uniform magnetic field of strength B is applied perpendicular to the direction of motion of charged particles, the particles deflect in a circular path. The radius R of this circular path is obtained by setting the magnetic force ‘qvB’ equal to mass ‘m’ times the centripetal acceleration v2/R. In his further experiments, Thomson adjusted the electrical and Magnetic fields so that the Cathode Ray beam was straight without deflection. By equating the magnitudes of Electric and Magnetic forces, he could determine the speed v.

Applications of Plum Pudding Model

  • J. Thomson’s Cathode Ray experiment paved the way for the development of modern Electronics. It formed the basis of the discovery behind Transistors and Diodes.
  • From the experiments, researchers found that Cathode Rays traveled in a straight line which helped them to study the properties of Waves.
  • Cathode Ray Tubes were developed (which is popularly known as Electron Gun) and is widely used in many Electronic devices.
  • His experiments led to the revolution in Quantum Physics.

Limitations of Plum Pudding Model

Scientists in the 19th century believed that J.J. Thomson’s Atomic Model was conceived mathematically for studying the stability of the Atom. He considered Electrons to be Static which caused instability of the Atom. Plum-Pudding Model concentrated more on the structure of the Atom and not it’s constitution. Thomson never was able to visualize spectral emission of Hydrogen when Electron transition occurs. Niels Bohr further proved that the Radius of Thomson’s Atomic Model was actually the Radius of the Positive Sphere i.e. the linear extension of the Atom. This model eventually had to be abandoned.

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