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The Rutherford’s α-scattering experiment established that the mass of an atom is concentrated within a small positively charged region at the centre which is called nucleus of an atom. 

Experimentally, the nuclear radius is given by :
    where,  A = mass number of 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 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 it’s 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 nuclie but it failed to explain following arguments. 
a) Nuclear Size : Nuclie are only of the order of 10^-15 m. According to 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 has a spin 1/2.
Thus nuclie with an evwn number of 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 deutron has a mass about two times that of proton, i.e. an odd number of particle. 
Therefore it’s spin should be half integral,
However, the observed spin of deutron is one.This shows that the electron as a separate particle does not exist in 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 nuclie, when it is assumed that protons & neutrons are tightly packed together. 
The proton provide 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. 
Verification : The hypothesis that nuclie are composed of protons & neutron explains the existence of isotopes. 
The nuclie of all isotopes of an element contains the same number of protons but have different number of neutrons. 

e.g. 17Cl(35) and 17Cl(37) are two isotopes of Chlorine having 18 and 20 neutrons.