Answer:
Atom is the smallest particle of matter that can take part in a chemical reaction.
Atom is made of electron, proton and neutrons.
Atom is not capable of independent existence.
Two or more atoms combine together to form molecules.
Electron:
Negatively charged particle discovered on the basis of ‘cathode ray discharge tube’ experiments.
Conclusion from ‘cathode ray discharge tube’ experiment:
Cathode rays start from cathode and move toward anode.
These rays are not visible but their behaviour can be observed with fluorescent or phosphorus sent material.
In the absence of electric or magnetic field these travels in strait lines.
In the presence of electric or magnetic field the behaviour of cathode rays is similar to negatively charged particles which suggest that these rays contain negatively charge particles called electron.
Proton:
Positively charged particle discovered on the basis of anode ray experiment.
Some of the characteristics of anode rays, also called canal rays, are:
These travel in straight line and posses mass many times the mass of an electron.
These are not originated from anode.
These are deflected by electric and magnetic field.
Unlike cathode rays, the positively charged particles depend upon the nature of the gas from which these originate.
Neutron:
Neutral particles discovered by bombarding a thin sheet of beryllium by α- particles.
Conclusion from α- particles scattering experiment:
Most of the α-particles passed through foil undeflected, indicating most of the space in atom is empty.
Some of the α-particles are deflected to certain angles, which means that there is positively mass present in atom.
Only some of the α-particles suffered large deflections, which means that the positively charged mass must be occupying very small space.
Strong deflections or even bouncing back of α-particles from metal foil indicate the direct collision with positively charged mass in atom.
Comparison between the subatomic particles of an atom
subatomic particles comparison
Thomson model of atom:
atom is considered asa uniform positively charged sphere with radius about 10-10 m, in which electrons are is uniformly distributed.
Electrons are embedded in such a manner to give most stable electrostatic arrangement.
Mass of atom is assumed to be uniformly distributed in atom.
Also known as plum pudding raisin pudding orwatermelon model.
Rutherford’s Nuclear Model of Atom:
Based upon α-particles scattering experiment.
Most part of the atom is empty.
Atom possesses a highly dense, positively charged centre, called nucleus of the order 10-13 cm.
Entire mass of the atom is concentrated inside the nucleus.
Electrons revolve around the nucleus in circular orbits.
Electrons and the nucleus are held together by electrostatic forces of attraction.
Drawbacks of Rutherford’s Model:
It doesn’t explain the stability of atom.
It doesn’t say anything about the electronic distribution of electrons around nucleus.
