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, Atom
y Each element is composed of smallest Definition
particles known as ‘ATOM’.
y Atom the name is derived from Greek word Each element is composed of
Atom means - ‘Not to be cut’. smallest particles called ‘ATOM’.
Concept Ladder
Democritus was the person
who first suggested the
existence of ATOM & coined
the name ATOMOS means
Not to be cut or Indivisible.
DALTON’S THEORY OF ATOM
J. Dalton developed his famous theory of
atom in 1803. The main postulates of
Rack your Brain
y Atom was considered as a samllest, dense
and hard indivisible particle.
y Each element consists of a specific type of Why and how do atoms combine
atoms. together to form compound
y The properties of elements differ because of atoms?
difference in kinds of atom contained in them.
y This theory provides a satisfactory basis for
law of chemical combinaiton.
Limitations of Dalton’s Theory
y Dalton’s theory fails to explain why atoms of Concept Ladder
different kinds should differ in valency and
mass etc.
y The discovery of isobars and isotopes showed Atoms is indestructible,
that atoms of same elements have different i.e., it cannot be created or
destroyed.
Atomic Structure
atomic masses (isotopes) and atoms of
different kinds may have same atomic masses
(isobars).
2.
,y The discovery of various sub-aomic particles
like protons, electrons, X-rays etc. during late Concept Ladder
19th century lead to idea that the atom was no
longer an indivisible and smallest particle of
the matter. R.S. Mullikan measured the
charge on an electron by
DISCOVERY OF FUNDAMENTAL PARTICLES oil drop experiment. The
y Atom consist of several sub-atomic particles charge on each electron is
like neutron, proton, electron, neutrino, –1.602 × 10–19 C.
positron etc. Out of these particles, electron,
proton and the neutron are called fundamental
subatomic particles.
Rack your Brain
(1) Electron
y Electron discovered by J.J. Thomson (1897)
and it is negatively charged particle. Why, cathode rays do not depend
Cathode Ray Experiment upon the nature of gas or the
y William Crookes in 1879 studied the electrical cathode meterial used in discharge
discharge in partially evacuated tubes called tube?
as cathode ray discharge tube.
y Discharge tube is made of glass, about 60
cm long containing two thin pieces of metals
known as electrodes, sealed in it. This is
Atomic Structure
called crooke’s tube. Negative electrode is
called as cathode and positive electrode is
called anode.
3.
, y When a gas enclosed at low pressure (~10–4
atm) in discharge tube is subjected to a high Rack your Brain
voltage (~10,000 V), invisible rays originating
from the cathode and producing a greenish Why do cathode rays produce
glow behind the perforated anode on the glass fluorescence?
wall coated with phosphorescent material
ZnS is observed. These rays
Properties
y Cathode rays travel in straight line. Previous Year’s Question
y Cathode rays produce mechanical effect, as
they can rotate the wheel that placed in their Cathode rays have
path. [AIPMT]
y Cathode rays consist negatively charged (1) Mass only
particles called as electron. (2) Charge only
y Cathode rays travel with high speed. (3) No mass and charge
y Cathode rays can cause fluorescence. (4) Mass and charge both
y Cathoderays heat the object on which they
fall due to transfer of kinetic energy to the
object. Concept Ladder
y When cathode rays fall on heavy metals,
X-rays produced.
y Cathode rays possess ionizing power that is Order of specific charge
they ionize the gas through which they pass. e e e
< <
y The cathode rays produce scintillation on the m n m p m e
photographic plates. mass of proton mp
y They can penetrate through thin metallic = = 1837
mass of electron me–
sheets.
Atomic Structure
4.
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