Chemistry - Structure of Atom

Subatomic particles : 

Dalton’s atomictheory was able to explain the laws ofchemical combination successfully. However, it failed to explain some properties of matter. For example, it could not explain why substances like glass or ebonite when rubbed with silk or fur, generate electricity. Discovery of subatomic particles in late nineteenth and early twentieth century set a blow to Dalton’s atomic model of hard sphere. Three important subatomic particles which are of concern to were found. Chemistry are proton, electron and neutron. Proton and neutron are present in the atomic nucleus and together are called nucleons. Electrons are present in the extranuclear part of an atom.The properties of electron, proton and neutron are summarised in Table.

Properties of subatomic particles

Discovery of electron : 

In the year 1897, J. J. Thomson investigated the cathode rays and found that the cathode rays are a stream of very small, negatively charged particles which are 1837 times lighter than a hydrogen atom and are present in all atoms. Later these particles were named as electrons.

Cathod ray tube experiment

Discovery of proton : 

In the year 1911, Ernest Rutherford found in the experiment of scattering of α-particles by thin gold foil (see Figure) that a few α-particles bounce back. From this he inferred the presence of massive and positively charged nucleus inside the atom. Following the discovery of nucleus in an atom, Rutherford found (1919) that fast moving α-particles transmuted nitrogen into oxygen with simultaneous liberation of hydrogen.

He further showed that other elements could also be transmuted, but hydrogen was always emitted. 

On this basis Rutherford proposed that the hydrogen nucleus must be contained inside  nuclei of all the elements. Hence, the hydrogen 

nucleus was renamed as proton.

Rutherford's scattering experiment

Discovery of neutron : 

Existence of an electrically neutral and massive particle in the nucleus was predicted by Ernest Rutherford in 1920 to account for the disparity in atomic number and atomic mass of an element. In the year 1932, James Chadwick measured velocity of protons knocked out from paraffin by an unidentified radiation from beryllium.

Atomic number and atomic mass number

From that he determined the mass of the particles of the unidentified neutral radiation which came out to be almost the same as that of a proton. He named this particle as ‘neutron’ which was predicted by Rutherford earlier.