Thomson’s Model of an Atom
Cathode Rays Experiment:
cathode ray experiment : discovery of electrons
Sir Joseph John Thomson was trying to relate the conductivity of gasses applying electric field.
For the purpose he used a glass tube with minimum pressure and applied a high electrical voltage (nearly 100000 Volt and more potential).
To his surprise, he observed rays of particles coming out from negatively charged electrode (called cathode) and moving to positively charged electrode (Anode). Because the rays moving from Cathode to Anode , so it was called Cathode rays.
This observation helped Thomson to declare that atom is divisible.
Properties of Cathode Rays :
- They consists of negatively charged particles.
- They move in straight line.
- They are deflected in presence of electric and magnetic field.
- They have particle character.
- They penetrate into air much larger length than that of Hydrogen atoms.
- The properties of Cathode Rays are independent of Gas taken in Glass tube.
Thomson’s Atomic Model: J.J. Thomson was the first to put forward a model to explain the structure of inside of an atom.
Thomson’s atomic model is also called Watermelon model or Christmas pudding model.
He compared the electrons with the raisins in the Spherical Christmas pudding and to seeds in a watermelon. Therefore when energy is provided to the atom, negative charged particles come out of the atom while positive charge remain inside. With this approach, he was able to explain discovery and properties of cathode rays.
Postulates of Thomson’s Model:
- An atom consists of a positively charged sphere , with electrons set within the sphere.
- An atom is electrically neutral as the positive and negative charges within it are equal.
Draw backs of Thomson’s Model:
- It could not explain the stability of an atom, i.e. how a positive charge in the atom holds the negatively charged electrons.
- It could not explain the position or arrangement of electrons inside an atom.
- It could not give any idea about size of the atom.