Introduction
Atom is the smallest particle of the element which cannot reside in the free state but that can take part in chemical reaction.
(Noble Gasses like Helium, Neon atoms can exist in free state.)
Atom is defined as the smallest particle of the element that cannot exist independently and retain all its chemical properties.
In early attempts of understanding matter, in years 1803 – 1805, John Dalton defined atom as the indivisible particle of the element. This theory was well in belief until J.J.Thomson Cathode rays experiment and Rutherford alpha particles experiment was not observed.
Experiments of J. J. Thomson (in year 1897) ; it was concluded that atom consists more elementary particles namely electrons.
Sub Atomic Particles:
According Modern Atomic Theory, atoms are composed of particles. The three main sub-atomic particles are proton, neutron and electron.
Properties of electron, proton and neutron:
| Parameters | Electron | Proton | Neutron |
| Position | Present outside the nucleus and revolves in the orbits | Present inside the nucleus | Present inside the nucleus |
| Mass | 9.108 × 10-28 g | 1.67 ×10-24 g | 1.67 × 10-24 g |
| Charge | -1.602 × 10-19 coulombs | 1.602 × 10-19 coulombs | Zero |
| Representation | e– | p+ | n |
Various theories were given to explain the structure of atom. These theories can be arranged in order of the discoveries of sub atomic particles.
1. Discovery of electrons (Cathode rays): In 1878, William Crooks carried out discharge tube experiments and discovered new radiations and called them cathode rays.Since these rays travel from the cathode towards anode.
Later J.J Thomson, in year 1897, studied on characteristics of cathode rays and concluded that cathode rays are negatively charged particles, now called electrons. The name electron was given by Johnson Stony.
2. Discovery of Nucleus: Rutherford famous α – particle scattering experiment (in year 1908-1913) resulted into discovery of Nucleus.
3. Discovery of Protons (Canal rays Or Anode rays): In 1886, E. Goldstein carried out discharge tube experiments and discovered new radiations and called them canal rays. These rays were made up of positively charged particles and led to the discovery of proton.
Later in year 1920, Rutherford termed the nucleus of Hydrogen as Proton.
4. Discovery of Neutrons: By the year 1920, all Scientists started believing that nucleus contains proton and entire mass of the atom. But In year 1932, James chadwick announced that nucleus also contain charge-less particle which he termed as Neutron.
