Structure, shape, and size of Nucleus: The Rutherford’s α-scattering experiment established that the mass of an atom is concentrated within a small positively charged region at the center which is called nucleus of an atom.
Experimentally, the nuclear radius is given by :
where A = mass number of the particular nucleus.
Ro = 1.3 Fermi = 1.3 × 10^-15 m. i.e. the nuclear radius is of order of 10^-15.
Constituents of the Nucleus :
i) Proton -Electron Hypothesis:-
Before the discovery of the neutron, the nucleus of an atom was thought to be composed of protons & electrons, which gives mass & charge to the nucleus.
e.g. The Helium nucleus whose mass is four times that of the proton but its charge is only +2e was thought to be composed of four protons and two electrons.
The proton-electron hypothesis of nucleus easily explain β -emission from the radioactive nuclei but it failed to explain the following arguments.
a) Nuclear Size:
Nuclei are only of the order of 10^-15 m. According to the Uncertainty principle
“the minimum energy required to confine an electron in such a small region must be of the order of 100MeV but in β-emission, it is found only 2 or 3 MeV”
Hence electrons cannot be present within the nucleus.
b) Nuclear Magnetic Moment :
An electron has a magnetic moment of the order of Bohr Magneton.
c) Nuclear Spin :
A proton and an electron each have a spin 1/2.
Thus nuclei with an even number of the proton plus even number of electrons should have integral spin( 1, 2, 3,….) while those with an odd number should have half-integral spin( 3/2, 5/2,……)
This prediction is however not satisfied. As the deuteron has a mass about two times that of the proton, i.e. an odd number of the particle.
Therefore it’s spin should be half-integral,
However, the observed spin of the deuteron is one. This shows that the electron as a separate particle does not exist in the nucleus of an atom.
ii) Proton -Neutron Hypothesis :
In 1932, neutron was discovered. It is an uncharged particle of mass slightly more than that of a proton and a spin of 1/2.
All these properties fit perfectly with the observed properties of the nuclei when it is assumed that protons & neutrons are tightly packed together.
The proton provides the positive charge to the nucleus and proton & neutron together provide the mass of the nucleus.
e.g. H atom consists of only one proton, & He atom consists of two protons.
An atom zXA contains Z protons & A – Z neutrons. The nuclear particles proton & neutron are commonly named as nucleons.
The hypothesis that nuclei are composed of protons & neutron explains the existence of isotopes.
The nuclei of all isotopes of an element contain the same number of protons but have a different number of neutrons.
e.g. 17Cl(35) and 17Cl(37) are two isotopes of Chlorine having 18 and 20 neutrons.