Atoms and Nuclei Questions and Answer PDF
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CLASS-12 PHYSICS UNIT-VIII
ATOMS AND NUCLEI
1. Line Spectrum of Hydrogen atom________________
(i) LYMAN SERIES Belongs to the ultraviolet region.
(ii) BALMER SERIES Belongs to visible regions.
(iii) PASCHEN SERIES Belongs to the infrared region.
(iv) BRACKETT SERIES Belongs to infrared region
(v) P-fund SERIES Belongs to .rared region.
2. lonisation Energy:
It is defined as the minimum energy required to ionise an atom. The ionisation energy of hydrogen atom is 13.6 e.V
3. Ionisation Potential:
It is the potential difference through which particles accelerate to ionise an atom.
So, the ionisation potential of hydrogen is 13.6V.
Ionisation potential = 13.6zV/n²
where z = atomic number, n = 1, 2, 3..
4. Excitation Energy :
It is the minimum energy required to excite an atom in the ground state to one of the higher stationary states.
5. Limitations of Bohr’s Atomic model
(i) It could not explain the fine structure of spectral lines in the hydrogen atom.
(ii) It could not explain the difference in the intensities of emitted radiations.
(iii) It could not explain the wave properties of electron.
(iv) It does not explain why the orbits are circular while elliptical orbits are also possible
The phenomenon of emission of radiation is termed as radioactivity and the substances which emit these radiations are called radioactive substances. It is of two types:
(A) Natural Radioactivity:
The nuclei of several elements of high atomic weight (Uranium, Radium) are unstable and undergo spontaneous disintegration into more stable ones. This phenomenon is called natural radioactivity. This activity takes place only by the emission of a or B
particles or by y-radiation.
(B) Artificial Radioactivity:
Artificial radioactivity is a phenomenon of producing 12 radioactive isotopes of stable elements by 16 bombarding them with certain suitable particles.
(i) In dating of archeological and geographical specimens.
(ii) In an energy reactor.
(iii) In biological areas etc.
7. Statistical Law of Radioactive Decay:
(a) Law of Chance:
Radioactive atoms are unstable and disintegrate at random.
(b) Law of Probability:
A large number of radioactive atoms are present in a given sample and the disintegration occurs as per the statistical law of probability.
(c) Law of Disintegration :
The rate of disintegration of the radioactive substance at any instant is directly proportional to the number of atoms present at that instant. and if This law was given by Ruthorford and Soddy.
8. Radioactive Disintegration Constant:
Radioactive disintegration constant of a radioactive substance is equal to the reciprocal
of the time during which the number of atoms of that substance falls to the th part of its original
9. Half Life Period:
Half life period of a radioactive substance is the time in which the 2v number of atoms of the given substance is reduced to half of its original value.
sis ba T 0.693/, where = Decay constant.
10. Average or Mean Life Period:
Average life period of a radioactive substance is defined as the total life time of all the atoms present in a given quantity of that material divided by the total number of atoms present in that much quantity of the material.i g
Mathematically T= 1/ Π