Structure of Atom 1 Flashcards
what suggested the patriculate nature of matter?
In 1830, Michael Faraday showed that if
electricity is passed through a solution of an
electrolyte, chemical reactions occurred at the
electrodes, which resulted in the liberation
and deposition of matter at the electrodes. He
formulated certain laws which you will study
in class XII. These results suggested the
particulate nature of electricity.
what isa cathode ray tube
A cathode ray tube is made of glass
containing two thin pieces of metal, called
electrodes, sealed in it. The electrical
discharge through the gases could be
observed only at very low pressures and at
very high voltages. The pressure of different
gases could be adjusted by evacuation.
what are cathode rays?
When
sufficiently high voltage is applied across the
electrodes, current starts flowing through a stream of particles moving in the tube from
the negative electrode (cathode) to the positive
electrode (anode). These were called cathode
rays or cathode ray particles. The flow of
current from cathode to anode was further
checked by making a hole in the anode and
coating the tube behind anode with
phosphorescent material zinc sulphide. When
these rays, after passing through anode, strike
the zinc sulphide coating, a bright spot on
the coating is developed
result of cathode ray experiment
(i) The cathode rays start from cathode and
move towards the anode.
(ii) These rays themselves are not visible but
their behaviour can be observed with the
help of certain kind of materials
(fluorescent or phosphorescent) which
glow when hit by them.
(iii) In the absence of electrical or magnetic
field, these rays travel in straight lines
(Fig. 2.2).
(iv) In the presence of electrical or magnetic
field, the behaviour of cathode rays are
similar to that expected from negatively
charged particles, suggesting that the
cathode rays consist of negatively
charged particles, called electrons.
(v) The characteristics of cathode rays
(electrons) do not depend upon the
material of electrodes and the nature of
the gas present in the cathode ray tube.
Thus, we can conclude that electrons are
basic constituent of all the atoms.
how did jj thomson find out charge to mass ratio of electron
In 1897, British physicist J.J. Thomson
measured the ratio of electrical charge (e) to
the mass of electron (me
) by using cathode
ray tube and applying electrical and magnetic
field perpendicular to each other as well as to
the path of electrons
what does deflection of particles depend upon
i)the magnitude of the negative charge on
the particle, greater the magnitude of the
charge on the particle, greater is the
interaction with the electric or magnetic
field and thus greater is the deflection.
(ii) the mass of the particle — lighter the
particle, greater the deflection.
(iii) the strength of the electrical or magnetic
field — the deflection of electrons from
its original path increases with the
increase in the voltage across the
electrodes, or the strength of the
magnetic field.
what were the observation of thomson’s charge to mass ratio experiment
When only electric field is applied, the
electrons deviate from their path and hit the
cathode ray tube at point A. Similarly when
only magnetic field is applied, electron strikes
the cathode ray tube at point C. By carefully
balancing the electrical and magnetic field
strength, it is possible to bring back the
electron to the path followed as in the absence
of electric or magnetic field and they hit the
screen at point B. By carrying out accurate
measurements on the amount of deflections
observed by the electrons on the electric field
strength or magnetic field strength, Thomson
was able to determine the value of e/me
as:
e
e
m
= 1.758820 × 1011 C kg–1 (2.1)
Where me
is the mass of the electron in kg
and e is the magnitude of the charge on the
electron in coulomb (C).
how was charge on electron found out
R.A. Millikan (1868-1953) devised a method
known as oil drop experiment (1906-14), to
determine the charge on the electrons. He
found that the charge on the electron to be
– 1.6 × 10–19 C. The present accepted value of
electrical charge is – 1.6022 × 10–19 C. The
mass of the electron (me
) was determined by
combining these results with Thomson’s value
of e/me
ratio.
what are canal rays
Electrical discharge carried out in the
modified cathode ray tube led to the discovery
of particles carrying positive charge, also
known as canal rays.
what are the characteristics of canal rays
(i) unlike cathode rays, the positively
charged particles depend upon the
nature of gas present in the cathode ray
tube. These are simply the positively
charged gaseous ions.
(ii) The charge to mass ratio of the particles
is found to depend on the gas from which
these originate.
(iii) Some of the positively charged particles
carry a multiple of the fundamental unit
of electrical charge.
(iv) The behaviour of these particles in the
magnetic or electrical field is opposite to
that observed for electron or cathode
rays.
what is the smallest proton
The smallest and lightest positive ion was
obtained from hydrogen and was called
proton. This positively charged particle was
characterised in 1919.
what are neutrons
a need was felt
for the presence of electrically neutral particle
as one of the constituent of atom. These
particles were discovered by Chadwick (1932)
by bombarding a thin sheet of beryllium by
α-particles. When electrically neutral particles
having a mass slightly greater than that of
the protons was emitted. He named these
particles as neutr ons. The important
