CHEMISTRY BY HIMANSHU SIR
SPECIAL BATCHES FOR XI AND XII
Atomic Structure
Matter :- Anything which occupies space have mass and have fixed volume. Matter is made up
of small particles called atoms.
Q:- what is an atom, why does it occur?
Cathode ray experiment:- Cathode rays are so named because they are emitted by the negative
electrode, or cathode, in a vacuum tube.was done by using a high electrical potential between the
anode and the cathode to ionize the residual gas in the tube; the ions were accelerated by the
electric field and released electrons when they collided with the cathode.Cathode rays are
invisible, but their presence was first detected in early vacuum tubes when they struck the glass
wall of the tube, exciting the atoms of the glass and causing them to emit light, a glow called
fluorescence.
Properties of Cathode Rays
They travelled in straight lines
Produced a shadow when obstructed by objects could pass through
thin metal foils without disturbing them.
SOME FACTS
Thomson model :-
Size of atom -> 10-8 size of nucleus -> 10-15
Isotopes:-atoms of same element having same atomic number but different atomic mass number
example :-1H1 (protium), 1H2(deuterium), 1H3(tritium).Isobar:-atoms of
ATOMIC NUMBERx
the different element
ATOMIC MASS having
different atomic number but same atomic mass.example :- 18Ar , 19K , 20Ca .Isotone:-the species
40 40 40
having same number of neutrons are called isotones.
No. of neutrons = atomic mass – atomic number
example :- 6C14 (no. of neutrons = 14 – 6 = 8), 7N15 (no. of neutrons = 15 – 7=8) , 8O16(no. of neutrons
= 16 – 8 = 8)
Isoelectronic species:-The isoelectronic species are those elements, which have the same no. of
electrons inside their atoms. Like an example is neon, and sodium ion, both of which consists of 10
electrons each.
“The elements, which are isoelectronic with each other, differ in their chemical and physical
properties, like as the positive charge on a species increases, it's reactivity increases due to the
increased electropositive nature of that species”.
ATOMIC MODELS We know the fundamental particles of the atom. Now let us see, how
these particles are arranged in an atom to suggest a model of the atom.
1|Page
, CHEMISTRY BY HIMANSHU SIR
SPECIAL BATCHES FOR XI AND XII
Thomson’s Model:
J.J. Thomson, in 1904, proposed that there was an equal and opposite positive charge enveloping
the electrons in a matrix. This model is called the plum – pudding model after a type of Victorian
dessert in which bits of plums were surrounded by matrix of pudding.
This model could not satisfactorily explain the results of scattering experiment carried out by
Rutherford who worked with Thomson.
Rutherford’s Model:
α– particles emitted by radioactive substance were shown to be dipositive Helium ions (He ++)
having a mass of 4 units and 2 units of positive charge.
Rutherford allowed a narrow beam of α–particles to fall on a very thin gold foil of thickness of
the order of 0.0004 cm and determined the subsequent path of these particles with the help of a
zinc sulphide fluorescent screen. The zinc sulphide screen gives off a visible flash of light when
struck by an a particle, as ZnS has the remarkable property of converting kinetic energy of α
particle into visible light. [For this experiment, Rutherford specifically used α particles because
they are relatively heavy resulting in high momentum].
Majority of the a–particles pass straight through the gold strip with little or no deflection.
Some α–particles are deflected from their path and diverge.
Very few α–particles are deflected backwards through angles greater than 90°.
Some were even scattered in the opposite direction at an angle of 180°
Conclusions:
The fact that most of the α - particles passed straight through the metal foil indicates the
most part of the atom is empty.
The fact that few α - particles are deflected at large angles indicates the presence of a
heavy positively charge body i.e., for such large deflections to occur α - particles must
have come closer to or collided with a massive positively charged body.
The fact that one in 20,000 have deflected at 180° backwards indicates that volume occupied by
this heavy positively charged body is very small in comparison to total volume of the
atom. Conclusions of a-Scattering Experiment:
2|Page
SPECIAL BATCHES FOR XI AND XII
Atomic Structure
Matter :- Anything which occupies space have mass and have fixed volume. Matter is made up
of small particles called atoms.
Q:- what is an atom, why does it occur?
Cathode ray experiment:- Cathode rays are so named because they are emitted by the negative
electrode, or cathode, in a vacuum tube.was done by using a high electrical potential between the
anode and the cathode to ionize the residual gas in the tube; the ions were accelerated by the
electric field and released electrons when they collided with the cathode.Cathode rays are
invisible, but their presence was first detected in early vacuum tubes when they struck the glass
wall of the tube, exciting the atoms of the glass and causing them to emit light, a glow called
fluorescence.
Properties of Cathode Rays
They travelled in straight lines
Produced a shadow when obstructed by objects could pass through
thin metal foils without disturbing them.
SOME FACTS
Thomson model :-
Size of atom -> 10-8 size of nucleus -> 10-15
Isotopes:-atoms of same element having same atomic number but different atomic mass number
example :-1H1 (protium), 1H2(deuterium), 1H3(tritium).Isobar:-atoms of
ATOMIC NUMBERx
the different element
ATOMIC MASS having
different atomic number but same atomic mass.example :- 18Ar , 19K , 20Ca .Isotone:-the species
40 40 40
having same number of neutrons are called isotones.
No. of neutrons = atomic mass – atomic number
example :- 6C14 (no. of neutrons = 14 – 6 = 8), 7N15 (no. of neutrons = 15 – 7=8) , 8O16(no. of neutrons
= 16 – 8 = 8)
Isoelectronic species:-The isoelectronic species are those elements, which have the same no. of
electrons inside their atoms. Like an example is neon, and sodium ion, both of which consists of 10
electrons each.
“The elements, which are isoelectronic with each other, differ in their chemical and physical
properties, like as the positive charge on a species increases, it's reactivity increases due to the
increased electropositive nature of that species”.
ATOMIC MODELS We know the fundamental particles of the atom. Now let us see, how
these particles are arranged in an atom to suggest a model of the atom.
1|Page
, CHEMISTRY BY HIMANSHU SIR
SPECIAL BATCHES FOR XI AND XII
Thomson’s Model:
J.J. Thomson, in 1904, proposed that there was an equal and opposite positive charge enveloping
the electrons in a matrix. This model is called the plum – pudding model after a type of Victorian
dessert in which bits of plums were surrounded by matrix of pudding.
This model could not satisfactorily explain the results of scattering experiment carried out by
Rutherford who worked with Thomson.
Rutherford’s Model:
α– particles emitted by radioactive substance were shown to be dipositive Helium ions (He ++)
having a mass of 4 units and 2 units of positive charge.
Rutherford allowed a narrow beam of α–particles to fall on a very thin gold foil of thickness of
the order of 0.0004 cm and determined the subsequent path of these particles with the help of a
zinc sulphide fluorescent screen. The zinc sulphide screen gives off a visible flash of light when
struck by an a particle, as ZnS has the remarkable property of converting kinetic energy of α
particle into visible light. [For this experiment, Rutherford specifically used α particles because
they are relatively heavy resulting in high momentum].
Majority of the a–particles pass straight through the gold strip with little or no deflection.
Some α–particles are deflected from their path and diverge.
Very few α–particles are deflected backwards through angles greater than 90°.
Some were even scattered in the opposite direction at an angle of 180°
Conclusions:
The fact that most of the α - particles passed straight through the metal foil indicates the
most part of the atom is empty.
The fact that few α - particles are deflected at large angles indicates the presence of a
heavy positively charge body i.e., for such large deflections to occur α - particles must
have come closer to or collided with a massive positively charged body.
The fact that one in 20,000 have deflected at 180° backwards indicates that volume occupied by
this heavy positively charged body is very small in comparison to total volume of the
atom. Conclusions of a-Scattering Experiment:
2|Page
