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Structure of Atom
The term Permanu (meaning Atom) was first used by Maharishi Kanad, an Indian saint and philosopher
to describe the ultimate particle of matter.
Atom is a Greak word which means indivisible. In 1804 John Dalton,” matter was composed of extremely
small particles called atoms which were structure less possessed fixed mass and were indivisible”.
Study of Cathode Rays
In 1879 William Crookes studied the conductance of electricity through gases at low pressure. For this
purpose he took a discharge tube which is a long glass tube fitted with a stop-cock. This tube is
connected to a vacuum pump and the pressure inside the discharge tube is reduced to as low as 0.1 mm
Now when a high voltage (nearly10000 Volts) is applied between the electrodes it is found that the
cathode. The presence of these rays is detected from the fact that glass wall of the discharge tube
opposite to the cathode, begins to glow with a faint greenish light. Obviously this must be due to the
bombardment of the walls by some rays emitted from the cathode. These rays were cathode rays.
Discharge tube experiment production of cathode rays
Properties of Cathode Rays
1.
2.
3.
4.
5.
Cathode rays travel in a straight line.
Cathode rays carry negative charge.
Cathode rays are deflected by magnetic field.
Cathode rays are made up of material particle.
Cathode rays are produce heating effect.
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6. Cathode rays produce X-rays.
7. Cathode rays posses penetration effects.
8. Cathode rays affected the photographic plate.
All these observation led to the two important conclusions:
i.
ii.
Cathode rays are made up of material particles.
Cathode rays carry negative charge.
The negatively charged material practices constituting the cathode rays are called electrons.
1. Determination of charge mass ratio (e/m) for electrons : Sir J.J Thomson, studied the deflection of
cathode rays under the simultaneous application of electric and magnetic fields, applied
perpendicular to each other and also to the path followed by the electron. According to Thomson,
the extent of deflection under the influence of applied electric and magnetic field is directly
proportional to the
i.
Magnitude of negative charge and
ii.
Strength of the applied electric and magnetic field
iii.
And it is inversely proportional to the mass of the particle.
Now, it is possible to adjust the two fields, so that the cathode rays strike the florescent screen at the
same position as they do when neither field is applied. From the magnitudes of electric and magnetic
field thus applied it is possible to calculate charge(e) to mass(m) ratio of the electrons.
𝑒
108 𝑐𝑜𝑢𝑙𝑜𝑚𝑏
= 1.76 ×
𝑚
𝑔
e = charge on electron
m = mass of electron
Two important conclusions can be drawn from this observation:
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i.
The e/m ratio for the particles in the cathode rays was found to be same irrespective of the
nature of cathode or the nature of gas taken in the discharge tube.
ii.
The electrons are universal constituent of all matter.
2. Determination of charge on electron: R.A.MILLIKAN (1909), The first accurate determination of the
charge of an electron (e) was made by R.A.Millikan (1909) by his famous oil-drop experiment. The
apparatus used by him is shown in figure. Small oil droplets are produced by an atomizer. The oildrops enter the apparatus through a small and are allowed to fall under gravitational force between
two charged metal plates. The moment of single droplets is observed by a microscope. The space
between the charged plates is irradiated with X-rays. The X-rays ionize the molecules of air. One or
more electrons, thus produced may be absorbed by an oil droplet. The oil droplet as a result
becomes negatively charged. By measuring the velocity of a given oil droplets as it falls freely under
influence of gravity and then in an electric field, it is possible to calculate charge (q) on the droplets.
Millikan found charge on oil all droplets could be expressed as whole number multiple of −1.602 ×
10−19 coulomb. This was therefore, the smallest possible negative charge, i.e., the charge on
individual electron.
3. Determination of mass of electron : By knowing the value of e/m ratio (by J.J.Thomson’s
experiment) and charge on electron (by Millikan oil-drop method),it is possible to calculate the exact
mass of an electron
𝑒
= 1.76 × 108 coulomb/g
𝑚
𝑒 = 1.60 × 10−19 𝑐𝑜𝑢𝑙𝑜𝑚𝑏
∴ Mass of electron, m =
e
1.60 × 10−19 coulomb
=
e/m 1.76 × 108 coulomb/g
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= 9.1 × 10−28 𝑔 = 9.1 × 10−31 𝑘𝑔.
An electron is a fundamental particle of atom carrying one unit negative charge and having mass
𝟏
nearly equal to 𝟏𝟖𝟑𝟕th of mass of an atom of hydrogen
Anode Rays or Canal Rays (Discovery of Proton)
We know that an atom is electrically neutral. The emission of negatively charged electrons from the
atoms of the gases in the discharged tube suggests that some positively charged particles must also be
present in them. The first direct proof for the presence of positively charged particles in an atom was
provided by the discharge tube experiment conducted by Goldstein (1886) using perforated cathode as
shown in figure. On passing the electric discharge at a low pressure, he observed a new type of luminous
rays passing through the holes or perforations of the cathode and moving in a direction opposite to the
cathode rays. Thus, these rays consisted of positively charge particles moving away from the anode and
were named as positive rays or anode rays or canal rays.
Properties of anode rays:
1.
2.
3.
4.
5.
6.
7.
8.
9.
They travel in straight lines and cast a shadow of the object placed in their path.
They consist of material particles. They rotate a light paddle wheel placed in their path.
These causes fluorescence when strike with the zinc sulphide screen (larger than cathode rays).
These rays are deflected in electric and magnetic fields and direction of deflection indicates a
positive charge on these particles.
These rays can be also penetrating through thin metal foils but penetration power is much less
than cathode rays.
These also affects the photographic plate
These can also ionize the gas molecules (ionization power of anode rays is more than cathode
rays).
The value of the positive charge (e) on the particles constituting the anode rays depends upon
of the charge on the electron but with positive sign. Thus , the particles constituting anode rays
may carry one, two, three unit of positive charges depending upon the number of electron
knocked out from the gaseous atom
The values of e/m ratio of these particles are much smaller than that of the electron and it also
depends upon the nature of gas enclosed in the discharge tube. When hydrogen gas is taken in
the discharge tube, the value found to be maximum(e/m = 9.58 × 104 C/g). This is quite
expected because the hydrogen atom is the lightest. The magnitude of positive charge is same
as on the electron(+1.6 × 10−19 ∁). The mass of the positively charged particle may be
calculated as follows :
𝑒
(1.6 × 10−19 ∁)
𝑚=
=
= 1.67 × 10−24 𝑔
𝑒/𝑚 (9.58 × 104 ∁/𝑔)
This mass is nearly same as that of hydrogen atom
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Radioactivity
Rutherford and Villard showed experimentally that by placing the radioactive element in a cavity made
in a block lead and applying electric or magnetic field on the radiation being emitted and then allowing
them to fall on photographic plate. Three types of radiations are emitted:
1. ∝ −𝑟𝑎𝑦𝑠 ∶ These are deflected slightly towards the negative plate. This shows that ∝-rays are
positively charged. The particles present in them are called ∝-particles. Each ∝-particle has same
positive charge (=+2) and four unit mass (= 4u). Hence, it is called helium nucleus and represented as
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2𝐻𝑒.
2. 𝛽 − 𝑟𝑎𝑦𝑠 ∶These are deflected more than ∝-rays towards the positive plate. This shows that 𝛽-rays
are negatively charged. The particles present in them are called 𝛽-particles. Each 𝛽-particle has
same charge and mass a that of electron. Hence ,it is called electron and represented as −10𝑒
3. 𝛾 − 𝑟𝑎𝑦𝑠 ∶These are not deflected on any side and move straight. This shows that 𝛾-rays do not
have any charge. In fact 𝛾-rays are electromagnetic radiations of high energy.
Note: Since, ∝ and 𝜷 rays differ in mass, therefore deflection is more in 𝜷-rays due to the small mass or momentum
(mv). This is the reason that these rays also differ in their in their velocity. Penetrating power kinetic energy etc.
Rutherford’s Scattering Experiment and Nuclear Model of Atom
i.
Since, most of the ∝-particle went undeflected, there must be large empty space within the
atom.
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ii.
iii.
Since, few ∝-particle were deflected, there must be some positively charged body at the centre
of the atom. Since, ∝-particle are quite heavy, the positively charged centre must be quite heavy
and almost whole of the mass of the atom may be concentrated in it. Rutherford named it
nucleus.
Since, a very few ∝-particle suffered large deflections or even retraced their path, this indicated
that the nucleus is rigid and occupies a very small volume of the atom.
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