In 1897 J.J. Thomson discovered the electron in the rays emitted from the cathode of discharge tube filled with gas at
Again 1910 Thomson suggested a model for describing atom , known as 'Thomson's atomic model' which suggests that
atom consists of positively charged sphere of radius 10-8cm in which electrons were supposed to be
Thomson atomic model failed as it could not give convincing explanation for several phenomenon such as, spectrum
of atoms, alpha particle scattering and many more.
In 1909 Gieger and Marsden employed α-particles (Helium ion) as projectile to bombard thin metallic foil.
According to Thomson atomic model since all positive charge of atom was neutralized by the negatively charged electrons, there would be rare event for an α-particle to suffer a very large deflection , as expected force of repulsion would not be very strong.
Surprisingly experiments of Gieger and Marsden showed large deflections of alpha particles that were many orders of magnitude and more common then expected.
This result of Gieger and Marsden α-particle scattering experiment was explained by Sir Rutherford in 1911.
Rutherford proposed a new atomic model in which electrons were located at much greater distance from the positive charge.
Rutherford proposed that all the positive charge , and nearly all the mass of the atom, was concentrated in an extremely small nucleus.
The electrons were supposed to be distributed around the nucleus in a sphere of atomic radius nearly equal to 10-8cm.
In explaining this experiment Rutherford made simple assumptions that both the nucleus and α-particles (Helium ion) were point electrical charges and the repulsive force between them is given by Coulombís inverse square law at all distances of separation.
These assumptions made by Rutherford were not valid if α-particle approaches the nucleus to a distance comparable with the diameter of the nucleus.
From this experiment there emerged a picture of internal structure of atoms and it also confirmed the existence of the atomic nucleus.
Approximate values for size and electrical charge of nucleus were calculated using data of various scattering experiments.
2. Nuclear Composition
Atomic nuclei are build up of protons and neutrons.
Nucleus of hydrogen atom contains only single proton.
Charge on a proton is +1.6x10-19 C and its mass is 1836 times greater then that of electron.
Neutrons are uncharged particles and mass of a neutron is slightly greater then that of a proton.
Neutrons and protons are jointly called nucleons.
Number of protons in nuclei of an element is equal to the number of electrons in neutral atom of that element.
All nuclei of a given element does not have equal number of neutrons for example99.9 percent of hydrogen nuclei contains only one proton , some contain one proton and one neutron and a very little fraction contains one proton and two neutrons.
Elements that have same number of protons but differ in number of neutrons in their nucleus are called ISOTOPES.
Hydrogen isotope deuterium is stable but tritium is radioactive and it decays to changes into an isotope of helium.
In heavy water instead of ordinary hydrogen deuterium combines with oxygen.
Symbol for nuclear species follows the pattern AXZ where
X= Chemical symbol of element
Z= Atomic number of element or number of protons in the nucleus of that element.
A= Mass number of nuclide or number of nucleons in the nucleus. A=Z+N where N is the number of neutrons in the nucleus.
In symbolic form
(1) hydrogen = 1H1 and Deuterium = 2H1
(2) Chlorine isotopes are 35Cl17 and 37Cl17
3. Atomic mass
Atomic masses refer to the masses of neutral atoms , not of bare nuclei i.e., an atomic mass always includes the masses of all its electrons.
Atomic masses are expressed in mass units (u).
One atomic mass unit is defined as one twelfth part of the mass of 12C6 atom.
So the mass of 12C6, the most abundant isotope of carbon is 12u.
Value of a mass unit is
We now calculate the energy equivalent of mass unit. We know that Einsteinís Mass-Energy relation is
Δm = 1.60x10-27 Kg and
c = 3x108 m/s
ΔE = (1.60x10-27) x (3x108)2
but 1eV = 1.6 x 10-19 J
ΔE = .931 x 109 eV
ΔE = 931 MeV
Thus 1 amu = 931 MeV
Mass of proton is 1.00727663 u which is equal to 1.6725 x 10-27kg or 938.26 MeV.
Mass of neutron is 1.0086654 u which is equal to 1.6748 x 10-27kg or 939.55 MeV.
4. Isobars and Isotones
Nuclei with same A but different Z are known as Isobars for example 40K19 and 40Ca20 share same mass number 40 but differs in one unit of Z.
Although isobaric atoms share same mass number but they differ slightly in their masses.
This very slight difference in masses of isobaric atoms is related to difference between energies of two atoms since small mass difference corresponds to considerable amount of difference in energies.
Nuclei with same number of neutrons but different number of protons are called Isotones for example 198Hg80 and 198Au79
5. Size of nucleus
First estimate of size of nucleus was provided by Rutherford scattering experiment.
In Rutherfordís scattering experiment incident alpha particles gets deflected by the target nucleus as long as the distance approached by the alpha particles does not exceeds 10-14m and Coulombís law remains consistent.
Apart from Rutherfordís scattering experiment various other experiments like fast electrons and neutron scattering experiments were performed to determine the nuclear dimensions.
Since electrons interact with nucleus only through electric forces so electron scattering experiments gives information on distribution of charge in the nucleus.
A neutron interacts with nucleus through nuclear forces so neutron scattering provides information on distribution of nuclear matter.
It was found that the volume of a nucleus is directly proportional to the number of nucleons it contains which is its mass number A.
If R is the nuclear radius then relationship between R and A is given as
Where value of R0 ≅ 1.2 x 10-15 ≅ 1.2 fm and is known as nuclear radius parameter.
Since R3 is proportional to A this implies that density of nucleas (ρ = m/V) is a constant independent of A for all nuclei.
The density of nuclear matter is approximately of the order of !17 Kg/m3 and is very large compared to the density of ordinary matter.
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