EARLY THEORIES OF THE ATOMIC STRUCTURE
The first clear atomic hypothesis for the existence of atoms was presented in 1803 by John Dalton
(1766-1844). He suggested that:
1. All matter is composed of extremely small particles called atoms.
2. Atoms cannot be created nor destroyed-in a chemical reaction, atoms of different
elements are separated, joined or rearranged. They are never changed into atoms
of another element.
3. All atoms of the same element are alike and different from those of any other
element.
4. A compound is a specific combination of atoms of more than one element.
Towards the end of the 19th century, it was realized that the atom is not indivisible but consists of
a number of sub-atomic particles. In 1897, J.J. Thomson (1856–1940) (Prof of Physics at
Cavendish Lab at Cambridge University) discovered the electron by experimenting with the
cathode ray tube. A cathode ray tube is made of glass containing two thin pieces of metal, called
electrodes, sealed in it as shown in the Figure below. 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). Because they are emitted from
the cathode, they were called cathode rays or cathode ray particles. The flow of current from
cathode to anode was further checked by making a hole in the anode and coating the tube behind
anode with phosphorescent material zinc sulphide. When these rays, after passing through anode,
strike the zinc sulphide coating, a bright spot on the coating is developed.
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The first clear atomic hypothesis for the existence of atoms was presented in 1803 by John Dalton
(1766-1844). He suggested that:
1. All matter is composed of extremely small particles called atoms.
2. Atoms cannot be created nor destroyed-in a chemical reaction, atoms of different
elements are separated, joined or rearranged. They are never changed into atoms
of another element.
3. All atoms of the same element are alike and different from those of any other
element.
4. A compound is a specific combination of atoms of more than one element.
Towards the end of the 19th century, it was realized that the atom is not indivisible but consists of
a number of sub-atomic particles. In 1897, J.J. Thomson (1856–1940) (Prof of Physics at
Cavendish Lab at Cambridge University) discovered the electron by experimenting with the
cathode ray tube. A cathode ray tube is made of glass containing two thin pieces of metal, called
electrodes, sealed in it as shown in the Figure below. 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). Because they are emitted from
the cathode, they were called cathode rays or cathode ray particles. The flow of current from
cathode to anode was further checked by making a hole in the anode and coating the tube behind
anode with phosphorescent material zinc sulphide. When these rays, after passing through anode,
strike the zinc sulphide coating, a bright spot on the coating is developed.
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