The Atomic theory of atom was first proposed by the John Dalton. His theory called as Dalton’s Atomic Theory, regarded the atom as the ultimate particle of the matter.
Many experimental observations are made by the scientists at the nineteenth and the twentieth century. they established that the atoms can be further divided into the sub-atomic particles, i.e, electrons, protons, neutrons. This concept is very different from that of the Dalton’s. He was able to explain the law of conservation of mass, law of constant composition and law of multiple proportion very successfully. However, he failed to explain the results of many experimental observations like when the glass or ebonite when rubbed with silk or fur generate electricity. how??
Discovery of Electron
In 1850s, the scientist known as Micheal Faraday began to study electrical discharge in particularly evacuated tubes, known as the cathode ray discharge tubes. A cathode ray tube contain electrodes, made up of glass which is a two thin pieces of metal, sealed in it. when high voltage is applied to the electrodes, current starts flowing from the negative (cathode) to the positive (anode). These were called cathode ray particles or cathode rays. The flow of the current from the cathode electrode to anode electrode was checked by making a hole in the anode with phosphorescent material Zinc Sulphide.When these rays, after passing through anode it strikes the Zinc sulphide coating, a bright spot on the coating in devloped.
The results of this experiment are:
- The cathode rays start from the cathode electrode and move towards the anode electrode.
- These rays themselves are not visible but their behavior can be observed with the help of the Zinc Sulphide coating on the anode.
- In the absence of the electrical or magnetic field, these rays travel in the straight lines.
- In the presence of electrical or magnetic field, the behavior of cathode rays are similar to that of the electrons, negatively charged particles.
- The cathode ray characterstics does not depends upon the material of electrodes and the nature of the gas present in the cathode ray tube.
Thus, we can say that the electrons are the basic constituent of the atom.
Charge to mass Ratio
J.J Thomson discovered the first subatomic particle electron. He measured the ratio of electrical charge (e) to the mass of electron (me) by using the cathode ray tube and applying electrical and magnetic field perpendicular to each other and also to path of the electrons.
The deviation from the path of the particles in the presence of electrical and magnetic field depends upon:
- Greater the magnitude of the charge on the particle, greater is the interaction with the electric and magnetic field and thus greater is the direction.
- Lighter the particle greater the deflection.
- The deflection of electrons from its original path increases in the voltage across the electrodes, or the strength of the magnetic field.
When only electric field is applied, the electrons deviate from its path and hit the cathode ray tube at point A. Similarly when only magnetic field is applied, electron strikes the cathode ray tube at the point C. By calculating accurate measurements on the amount of deflections which is observed by the electrons on the electric field strength or magnetic field strength, Thomson was able to determine the value of the e/me as: e/me = 1.758820 10^11 C/kg.
Here, me is the mass of the electron in kg and e is the magnitude of the charge on the electron in coulomb (C).
Charge of the electron
Oil drop experiment helps to determine the charge on the electrons. R.A. Millikan found that the charge of the electron to be -1.6 x 10^-6. The mass of the electron was founded by combining the results with the Thomson’s value of e/me ratio
me = e/me= 1.6022 x 10^-6 C/ 1.758820 x 10^11 Kg^-1
Discovery of Protons and Neutrons
The discovery of particles carrying positive charge is determined by the electrical discharge carried out in the modified cathode ray tube, also known as canal rays. The characteristics of these positively charged particles:
- The positively charged particles depend upon the nature of gas present in the cathode rays tube but the cathode rays do not. These are simply the positively charged gaseous ions.
- The charge to the mass ratio of the particles is found to depend on the gas from which these are originated.
- Some of the positively charged particles has a multiple of the fundamental unit of electrical charges.
- The behavior of these positively charged particles field is opposite to that observed for electron or cathode rays.
Proton is the smallest and lightest positive ion, obtained from hydrogen. These particles were discovered by Chadwick by bombarding a thin sheet of beryllium by alpha-particles. When electrically neutral particles which carry a mass slightly greater than that of the protons was emitted. He named this particles as neutrons.
Distribution of these charged particles was explained by different atomic models of the atom. Although among for all, some of these models were not able to explain the stability of atoms, two of these models, proposed by J.J Thomson and Ernest Rutherford.
Thomson Model of Atom-
J.J Thomson in 1898, proposed that a spherical shape is possessed by the atom in which the positive charge is uniformly distributed. The electrons are embedded into it in such a manner, to give the most stable electrostatic arrangement. Plum pudding, raisin pudding or watermelon were the different names given to this modal. An important feature of Thomson model is that the mass of the atom is assumed to be uniformly distributed over the atom. Although this model was able to explain the overall neutrality of the atom, but was not consistent with the results later experiments.
Rutherford Model of Atom-
Rutherford’s famous alpha-particle scattering experiment. A stream of high energy alpha-particles from a radioactive source was directed at a thin foil of gold metal. This gold foil had a circular fluorescent zinc sulphide screen around it. Whenever these alpha-particles struck the screen, a tiny flash of light was produced at that point.