1. What are the cathode rays?
The radiation consisting of negatively charged particles (electron) which originate from the
surface of cathode when electricity is passed through gases at low pressure are called cathode rays.
2. What is the effect of electric and magnetic fields on cathode rays?
Effect of magnetic field
In 1895, J.Perrin showed that cathode rays are deflected by magnetic field in a curved path,
perpendicular to the line joining north & south poles.
Effect of electric field
Cathode rays are deflected towards positive plate(anode) of applied electric field.It shows that Cathode
rays consist of negatively charged particles.
3. e/m ratio of cathode rays doesn’t depend upon the nature of gas inside the gas
Cathode rays consists of streams of electrons. Since electrons are present in all kinds of matter, therefore
ionization of all the gases present inside the gas discharge tube results in the production of same type of
cathode rays.Hence e/m ratio of cathode rays doesn’t depend upon the nature of gas.
4. Why it is necessary to decrease the pressure in the discharge tube to get cathode
The gases conduct electricity due to ionization of their atoms. However, gases are not
conventional conductors, but under extreme conditions of low pressure and high voltage, they can
conduct electricity. At low pressure the electrons which emit from the surface of cathode, effectively
collide with fewer gases atoms and ionize them. The electrons thus ejected move towards anode and
hence constitute the cathode rays.
5. How the mass of electron can be calculated from e/m ratio and the charge on
From the value of charge and charge per mass ratio, the mass of electron can be calculated as :
As e C
1.7588 10 1.6022 10
19 1.6022 10−
It means that 1 kg of electrons have 1.7588x1011
6. What are the positive rays? Why they are also known as canal rays?
The radiations consisting of positively charged ions produced by the ionization of the gaseous atoms
in the gas discharge tube, are known as positive rays. Since these radiations were studied after their
passage through holes or canals, therefore, these radiations are also known as canal rays.
31 9.1095 10−
coulombs of charge.
7. How positive rays are produced ?
When electric current is passed through a gas at low pressure,cathode rays are emittedfrom the surface
of cathode.When these high energy cathode rays collide with gaseous atoms in the gas discharge tube,
they knock off electrons from the gaseous atoms and form the positively charged ions. These ions
collectively move towards cathode and are collectively positive rays.
M + e→M + 2e
8. How proton was discovered from positive rays?
The e/m ratio of positive rays is smaller than that of electrons and depends upon nature of gas used in
the gas discharge tube. When hydrogen gas was used in the gas discharge tube, e/m ratio was found to
be maximum in comparison to any other gas because the mass of positive particle obtained from
hydrogen gas was lightest among all the positive particles.Its charge was found equal but opposite tothat
of electron. This particle was named as proton by Rutherford. The mass of proton is 1836 times more
than that of an electron.
9. How much is the H-atom heavier as compared to an electron?
9.A Justify that the e/m ratio of positive rays depends upon the nature of gas in the gas
The positive rays are formed by ionization of gaseous atoms in the gas discharge tube. The mass of the
positive ions depends upon the mass of gaseous atoms in the gas discharge tube, so the e/m ratio is
different gases is different. It is greater for lighter gases & less for heavier gases.
10. How neutron was discovered by James Chadwick?
James Chadwick bombarded a stream of ∝-particles from Polonium on a very thin beryllium foil. He
observed that highly penetrating radiations were produced. Thee radiations were called neutrons as they
were electrically neutral. The nuclear reaction is :
He Be C n
2 + − − − − − −> +
11. What species are formed by the decay of neutrons ?
Free neutrons decay into a proton with the emission of electron and a neutrino.
n P e n
0 − −− > + + − +
12. What happens when fast neutrons are bombarded on nitrogen atoms?
A fast neutron ejects an ά-particle from nucleus of nitrogen atom and boron is produced.
N n B He
7 + − − − −− > +
13. What happens when slow neutrons are bombarded on Cu- atoms?
When slow neutrons hit the Cu-metal, γ-rays are produced. Radio active copper emits β-rays and is
converted into Zn.
Cu n Cu hv rays
29 ( )
+ − − − −− > + γ −
Cu Zn e
− − − − − − > +−
14. How nucleus was discovered by Rutherford ?
Rutherford in 1911performed some experiments on scattering of high speed α-particles which were
emitted from a radioactive metal.
The source of α -rays was a radioactive metal radium or polonium, placed in a hole made in a lead block.
The beam of α -rays was directed to a very thin gold foil of thickness 0.00004cm. A circular plate coated
with ZnS was placed around the foil in order to detect the scattered α -particles. Whenever an α -particle
struck the screen, a flash of light was produced at that point.
Rutherford observed that most of α -particles passed through the foil without any deflection from
their path and struck the screen at its centre, causing illuminations. A few of them were deflected at ma
Rutherford proposed that rebounding α -particles must have collided with central heavy portion of the
atom which he called NUCLEUS.
15. What are the defects of Rutherford’s atomic model?
According to the classical electromagnetic theory, if a charged particle (e.g electron) is accelerating
around an oppositely charged particle (nucleus), it would continuously lose energy and its speed will
decrease and will gradually come closer and closer to the nucleus. The ultimate result will be that the
electron will fall into the nucleus thereby making the atom unstable i.e according to Rutherford’s the
atom would collapse. But actually it does not happen. Therefore Rutherford’s atomic model cannot
explain the stability of the atom.
16. Why the distance gap between the adjacent orbits goes on increasing from lower to
higher orbits ?
According to Bohr’s atomic model, the radius of an orbit is hydrogen atom is given by the relation.
r = Z
From this relation, we conclude that
r ∝ n
radius is directly proportional to sq. of number of orbit .Therefore,there is a large increase in the
radius of an orbit in going from lower to higher orbits.Hence the distance gap between the
adjacent orbits goes on increasing from lower to the higher orbits. i.e.
r2 – r1 < r3 – r2 < r4 – r3 ..............
17. Hydrogen and He+ are monoelectronic systems but the size of He+ is much smaller as
compared to H-atom.Why?
r = n
r = n
r = n
r = n
2/Z x a0
2/Z × 0.5290A
2/Z x a0
2/Z × 0.5290A
As the nuclear charge in He+ is greater due to the presence of two protons in the nucleus, therefore its
orbit is pulled more closer to the nucleus and its radius or size becomes smaller as compared to that of
18. How will you show that the radius of 3rd orbit is 9 times greater as compared to the
Radius of first orbit for H-atom i.e n = 1 , Z=1
/2 x 0.529
r1 = n2
x 0.529 0A
/1 x 0.529 0A
= 0.529 0A ----------- (1)
Radius of third orbit for H-atom i.e n =3 , Z=1
r3 = 3
2 × 0.529 oA
= 9 x 0.529 0A ---------(2)
Dividing eq(2) by eq(1)
r3 = 9 r1
19. Calculate the value of ao in the equation for the radius of H-atom
21. Why the energy of an electron is written with negative sign ?
En = –2.178 × 10-18 × [
The negative value of energy shows the decrease in the energy of electron ,that is the electron
is under the force of attraction of the nucleus.
22. Which quantum number is known as sub-shell quantum number ?
Azimuthal quantum number is known as sub-shell quantum number.It is denoted by the letter l and
describes the shape of the sub-shell.Its values ranges from 0,1,2 upto (n-1)
If n=1 it is S-sub shell which is spherical in shape.
If n=2 it is p-sub shell which is dum-bell in shape
If n=3 it is d-sub shell which is sausage in shape
If n=4 it is f-sub shell which is complicatedl in shape
23. What is the difference between orbit and orbital?
The circular or elliptical path in which an electron
moves around the nucleus is known as orbit.
It is denoted by K ,L ,M etc It is denoted by s ,px ,py etc.
The number of electrons in an orbit are calculated
24. What is the relationship between
i- energy and wavelength ?
The region around the nucleus where the
probability of finding the electron is maximum is
known as orbital.
The number of electrons in an orbital are
calculated by 2(2l+1)
E=hc/λ or E ᾰ 1/ λ
ii- frequency and wavelength
c=νλ or ν =c/λ or ν ᾰ 1/ λ
25. How the wavelength of moving particles is related to the momentum of electron?
λ = h/mv
λ ᾰ 1/mv
26. State Heisenberg’s uncertainty principle.
It states that it is impossible to determine the exact position and momentum of a small particle like
∆x. ∆p ≥ h/4π
∆x. =uncertainty in position
∆p = uncertainty in momentum
27. Why 4s orbital is lower in energy than 3d orbital ?
“ n + l value for 4S orbital is 4+0=4 whereas for 3d orbital it is 3+2=5.Therefore,according to n+l rule,
4S orbital is lower in energy than 3d orbital.
28. Write down the electronic configuration of Mn+2 ,Zn+2 , Cd+2 ,Al+3
Electronic configuration of Mn = 1S2
29. What is n+l rule ?
It states that the added electrons always enter into the sub-shells in the increasing order of their n+l
values.If two sub-shells have same n+l value ,then the sub-shell with lower value of n will have low
30. What is Zeeman and Stark effects ?
The splitting of spectral lines in the spectrum of H-atom in the presence of magnetic field is called
Zeeman effect whereas splitting of spectral lines in the spectrum of H-atom in the presence of electric
field is called Stark effect.
31. How the defects in Bohr’s atomic model are removed by the dual nature of electron
and Heisenberg’s uncertainty principle?
32. Calculate the wave number of 1st
spectral line in Lymen series.
33. Calculate the wave number of limiting line in the Balmer series.
34. What are the X-rays ?
X-rays are the electromagnetic radiations of very short wave length which are produced due to the inner
shell ionization of gaseous atoms in the gas discharge tube brought about by the cathode rays.
35. How X-rays are produced ?
When high energy cathode rays are bombarded on a heavy metal used as target anode inside a
gas discharge tube,the electromagnetic radiations of very short wave length and high frequency are
emitted.These radiation are known as x-rays. The frequency of these radiations depends upon the
nature of metal used as target.X-rays of high frequency are emitted from a metal of high atomic number
whereas x-rays of low frequencies are emitted from a metal of low atomic number.
36. How atomic number was discovered from x-rays ?
In 1913-14, Henry Moseley used 38 different metals (from Al to Au) as target anode in the
gas discharge tube and measured the frequencies of emitted x-ays.
37. State Moseley’s law
It states that the frequency of X-rays is directly proportional to the square of atomic number of the
element emitting it.
38. What is the Schrodinger Wave equation?
39. Write down the main points of Quantum mechanical theory.
40. What are the Quantum numbers?
Quantum numbers are the set of four numerical values which are obtained by solving Schrodinger wave
equation for H-atom.These quantum numbers completely describe an electron around the nucleus.These
i-principal quatum number
ii-azimuthal quantum number
iii-magnetic quantum number
iv-spin quantum number