Download STRUCTURE OF AN ATOM

STRUCTURE OF AN ATOM
LESSON OBJECTIVES
1.
Dalton’s theory
2.
Discovery of fundamental particles
3.
Thomson’s model of an atom
4.
Rutherford’s model
5.
Concept of atomic number and mass number
6.
Drawback of Rutherford’s model
DALTONS ATOMIC THEORY - 1800
ATOMS CAN NEITHER BE CREATED NOR DESTROYED
 ATOMS ARE FURTHER DIVIDED INTO SUB –ATOMIC
PARTICLES ELECTRONS PROTONS AND NEUTRONS
• All matter is made of atoms.
• Atoms of an element are identical.
• Each element has different atoms.
• Atoms of different elements combine in constant ratios
to form compounds.
• Atoms are rearranged in reactions.
• His ideas account for the law of conservation of mass
(atoms are neither created nor destroyed) and the law
of constant composition (elements combine in fixed
ratios).

DICOVERY OF CATHODE RAYS ( ELECTRONS )
J.J THOMSON
Gas at
According to Michael faraday when
low pressure
Cathode
chemical substances in aqueous or
T
rays
p
molten form in the presence of
Cathode
electricity deposition or accumulation –
of substances occurs which is
electrolysis
High voltage
–
+
gases are poor conductors of
electricity at normal pressures but
Discharge tube experiment-Production of
by decreasing pressure they can
behave as electrolytes

A cathode ray tube is made of glass containing
two pieces of metal called electrodes

When sufficient high voltage and pressure is
applied across the electrodes , current start
flowing through a stream of particles moving in
the tube

These are cathode rays which are –ively charged

These rays are no visible but their behavior can
be observed with the help of certain kind of
materials (television tube )
PROPERTIES OF CATHODE RAYS
In the absence of electrical or magnetic field , these
rays travel in straight lines
They are material particles as they produce mechanical
motion in a small paddle wheel
The characteristics of cathode rays don’t depend upon the
Material of electrodes and nature of the gas present in
cathode ray tube
They are deflected from their path by electric and magnetic
fields.
e/m ratio of electron
By using cathode tube and applying electrical
and magnetic field perpendicular to each
other as well as to the path of electrons
* magnitude of –ive charge is greater ,
greater is interaction and greater is
deflection
*lighter the particle greater deflection
*deflection increases with increase in voltage
and strength of magnetic field
* refer figure 2.2 from text book page.no 28
draw figure and write the values of e/m and
DISCOVERY OF ANODE RAYS ( PROTONS )
E.GOLDSTEIN
Here in this experiment perforated
cathode is used where some rays
are observed behind the
+
perforated cathode
*charge / mass ratio depend on
the gas from which they
originate
Perfo
cath
–
H2 gas inside
at low pressur
PROPERTIES OF ANODE RAYS

They travel in straight line

They are deflected by electric and magnetic field

The nature of anode rays depends upon the nature of gas

e/m ratio of anode rays is not constant and depends on the gas present in the di
Neutron was discovered by chadwick by bombarding a thin sheet of beryllium by al
when electrically neutral particles having a mass slightly greater than that of prot
9
4
Be +
4
12
He →
2
1
C
6
n
0
Charge in
Symbol
Unit charge
Unit mass
particles
Proton
Neutron
Electron
Mass
Coulomb
Negligible
J.J Thomson plum pudding model

Atom possess spherical shape in which positive charge is
uniformly distributed

Electrons are embedded into it in such a manner as to
give the electrostatic arrangement

It is not consistent with the results of new experiments
Positive sphere
Electron
Rutherfords nuclear model of an atom
Rutherford’s Experiment - Results

A beam of  particles aimed at thin gold foil.

Most of the particles passed through – most of the space in
atom is empty

A few came back – presence of concentrated mass at
centre

Others deflected at various angles – repulsions between
two positively
charged particles
RUTHERFORD ATOMIC MODEL :
Atom consist of two parts:
(a)Nucleus: Almost the whole mass of the atom and
positive charge is concentrated in this small region
(b)Extra nuclear part:this is the space around the
nucleus
in which electrons are revolving at high speeds in
fixed path
© Electrons and nucleus are held together by
electrostatic forces of attraction
Rutherfords drawbacks :
Rutherfords drawbacks

According to electromagnetic theory of Maxwell ,charged particles when
accelerated should emit radiations

So an electron in an orbit emit radiation ,the energy carried by radiation
comes from electronic motion

So orbit will thus continue to shrink ,but this does not happen

Rutherford model cannot explain the stability of an atom

How electrons are distributed around the nucleus

Atomic mass n atomic number