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1.HOW atoms is Discover?
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3.Elements and their valency
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4.1 INTRODUCTION
4.1 Introduction
All matter is composed of tiny particles called atom. An atom is the
4.2 Dalton’s Atomic Theory smallest particle of an element that can exist and still have properties
of an element.
4.3 Charged particles in The first theory regarding atom is given by Dalton but later on it
matter discarded by scientists. After that J. J. Thomson, Rutherford,
4.4 Thomson Atomic Model Bohr and many other scientist gave their atomic model.
4.5 Rutherford model of an 4.2 DALTON’S ATOMIC THEORY
atom
The main postulates of this theory are
4.6 Bohr model of atom (1) Each element is composed of extremely small particles called
4.7 Atomic number and atoms.
mass number (2) Atom is indestructile.
(3) Atoms of a particular element are all alike but differ from atoms
4.8 Electronic configuration
of another element.
principles
Merits
4.9 Valence electrons and (1) It explains the law of conservation of mass.
valency
(2) It explains the law of definite proportion.
Demerits
“IIT-JEE FOUNDATION” (1) It fails to explain that why should atoms of an element differ in
their masses.
*4.10 Quantum numbers (2) Discovery of isotopes and isobars, proved that atoms of same
element may have different atomic mass (isotopes) and atoms of
*4.11 Some Important Terms
different kinds may have same atomic masses (isobars).
*4.12 Radioactivity (3) The discovery of various sub atomic particles like electron,
proton, neutron etc. lead to the idea that the atom was no longer the
smallest, in divisble particle of matter.
4.3 CHARGED PARTICLES IN MATTER
4.3.1 Electron
Properties of electron
(a) Electron was disovered by Sir J. J. Thomson
(b) Charge is 1.6 × 10–19 coulomb/gm
(c) The molar mass is 5.48 × 10–4 g/mole
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(e) In 1897, J. J. Thomson determined the e/m value of the electron –1.7588 × 108 coulomb/gram.
(f) Robert A. Millikan, gave the value of charge by oil drop experiment –1.6022 × 1019 Coulomb =
4.8 × 1010 esu. (g) mass of electron 9.1096 × 10–31 Kg = 0.000548 amu.
Cathode Rays
Scientists William Crookes 1879, Julius Pleuckar 1889 studied the electrical conduction through
gases at low pressures. Sir William Crookes designed discharge Tube.
(a) The electron was discovered at extremely low pressure and high voltage of the order of 10,000 volts.
J. J. Thomson’s Experiment and The Discovery of Electron
Properties of the cathode rays shows that these consist of streams of negatively charged particles having
very small mass. Sir J. J. Thomson called these particles corpuscles of negative electricity. Later,
G. J. Stoney called these particles electrons.
In 1897, an English physcist, J. J. Thomson studied the effect of electric field on cathode rays.
The discharge tube used by Sir J. J. Thomson consisted of the following parts:
a cathode (C),
a cylindrical metal disc having a fine hole at its centre, acts as anode (A) – this accelerates the
particles of the cathode rays,
another metal disc (D) having a fine hole at its centre in line with the hole in the anode.
the cathode rays (consisting of electrons) pass through these holes and strike the fluoresecnt screen
at the point E.
Two flatplates P1 and P2, which can be connected to a source of high voltage.
an electromagnet, generating field opposite to the field generated by the plates P1 and P2.
In the absence of any electrical or magnetic field, the cathode rays strike the fluorescent screen at point
E, and can be seen as a bright spot there.
When a high electric field is applied across the plates P1 and P2, the spot on the screen moves towards the
positively charged plate. Here, in figure the plate P2 is positively charged.
The bending of cathode rays towards the positive plate showed that the cathode rays consist of negatively
charged particles. The deflection suffered by the beam of cathode rays (or beam of electrons) in Thomson’s
experiment depends upon the strength of the electric field applied across the electrodes.
Cathode ray have the following properties
(i) Travel in straight line
(ii) Rays are deflected by electric and magnetic field
(iii) Produce X-rays
(iv) Produce heating effect
(v) Produce green glow
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(vi) Penetrate through metal like Al (Aluminium)
(vii) Effect the photographic plates
(viii) The ratio of charge to mass i.e. charge/mass is same for all the cathode rays irrespective of the gas
used in the tube.
4.3.2 Proton
Properties of proton: (a) Discovered by Goldstein (b) Charge of +1.602 × 10–19 coulomb (c) Mass is
1.672 × 10–27 kg or 1.0072 amu
Positive Rays-Discovery of Proton
Perforated
cathode
Positive
Cathode ray rays
+
Cathode
(a) E. Goldstein in 1886 discovered proton by repeating the same discharge tube experiments by using
a perforated cathode.
(b) When a high potential difference was applied, not only cathode rays were produced but also a new
type of rays were produced simultaneously from anode moving towards cathode and passed through the
holes of the cathode. These termed as canal ray or anode ray.
(c) Characteristics of Anode rays are as follows
(i) Travel in straight line and cast shadow
(ii) Deflected by the magnetic and electric fields
(iii) These rays have kinetic energy and produces heating effect also.
(iv) Unlike cathode rays, their e/m value is dependent upon the nature of the gas taken in the tube
(v) These rays can pass through thin metal foils.
(vi) They are capable to produce ionization in gases.
4.3.3 Neutron
Properties of Neutron (n): (a) Chadwick (1932), discovered neutral particles which was called neutron.
Nuclear reaction is as follows 4 Be 9 2 He 4
6 C12 0 n 1
(b) A neutron is a subatomic particle which has a mass 1.675 × 10–24 g, approximately 1 amu.
4.3.4 Meson or Pions ()
(a) Discovered by Yukawa in 1935.
(b) On the basis of charge, the meson is of three types, -meson, µ-meson and neutral meson(°).
(c) -meson are called pions.
(d) It tells about the stability of nucleus.
4.4 THOMSON ATOMIC MODEL(THE RAISIN PUDDING MODEL) (PLUM
PUDDING MODEL): FIRST ATOMIC MODEL
J. J. Thomson suggested atom is a sphere (Pudding) of positive electricity with a electrons (raisins)
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