Download rutherford gold foil experiment for the structure of atom

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DALTON ATOMIC THEORY.
In 1808, a British school teacher John Dalton gave his atomic
theory called Dalton atomic theory. The main points of Dalton
atomic theory are given below:
1. Matter is composed of extremely small indivisible particles
called atoms
2. Atoms can neither be created nor destroyed
3. Atoms of the same elements are identical in all respects, i.e
size, shape, and mass, but differ from those of other elements.
4. Atoms combine in a simple whole number ratio to form a
compound.
RUTHERFORD GOLD FOIL EXPERIMENT FOR THE
STRUCTURE OF ATOM
Introduction. After discovery of electrons and protons, the
question was raised about the arrangement of these particles
inside the atom. In order to answer this question, Rutherford
performed experiment in 1911 to find out location of
fundamental particles inside atom.
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Apparatus and materials.

Alpha particles source like radioactive elements

Gold foil

Zinc sulphide (ZnS) Screen

Movable microscope

Lead shield
Experiment. Rutherford obtained α particles from radioactive
elements and bombarded it on gold foil. In order to observe the
extent or scattering of alpha particles, he placed a zinc sulphide
screen behind the gold foil.
Observation: He observed that:

Most of the alpha particles passed undeflected through the
gold foil

Some of the alpha particles were deflected at larger angles
from their original path

A few particles deflected backward on their original path or
completely bounced back
Conclusions or Rutherford atomic model. Based on his
experiment, Rutherford presented the following model for the
structure of atom.

An atom is a neutral particle

Major portion of the atom is empty because most of the alpha
particles passed undeflected

The positive charge particles (proton) are present in the centre
of atom called nucleus. Nucleus has positive charge because
alpha particles are positive so only repel from positive things.
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
The size of the nucleus is very small as compared to the size of
atom because most of the alpha particles passed undeflected.
The negatively charged electrons revolve around the nucleus
such as the planets revolve around the sun in the solar system.
Drawbacks or shortcoming or defects in Rutherford atomic
model.
The following defects were observed in Rutherford atomic
model:

In Rutherford atomic model, electron is a charged
particle revolves around the nucleus in a circular path
but according to classical electromagnetic theory the
charged particles lose energy continuously. Therefore
electrons should lose energy continuously and ultimately
fall in the nucleus but it does not happen.

If
the
revolving
continuously,
electrons
then
the
radiate
atomic
or
emit
spectrum
energy
must
be
continuous instead of line spectrum.
Neil Bohr’s atomic theory.
In order to remove the defects of Rutherford atomic model
that why electrons do not fall in the nucleus, Neil Bohr’s in
1913 gave his atomic theory. The main postulates of Neil
Bohr atomic theory are given below:
1. Electrons revolve around the nucleus in a fixed circular
path called orbit or energy level. Each orbit has a fixed size,
shape and energy.
2. Electron does not absorb or emit energy when revolves
around the nucleus in a fixed orbit. This postulate removed
the defect of Rutherford atomic model that why electron not
fall in the nucleus
3. Electron only absorbs or emits energy when jumps from one
orbit to another and the energy absorbed or released is not
continuous but discontinuous in the form of quantum
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(photon). This postulate removed the second defect that why
atom gives discontinuous or line spectrum
4. Electron releases energy when jumps from higher orbit to
lower orbit but absorbs energy when jumps from lower orbit
to higher orbit.
5. The energy absorbed or released is always equal to the
difference of the energy of the two orbits. ∆E = E2-E1 = hⱱ
Where ‘h’ is planks constant and ‘ⱱ’ is frequency of radiation.
6. Electron only revolves around the nucleus on those orbits
for which the angular momentum (mvr) is equal to ‘nh/2π’.
mvr = nh/2π
where ‘m’ is mass of electron, ‘v, is velocity, ‘r’ is radius of orbit, ‘n’
is any natural number (1,2,3-----) and ‘h’ is a plank constant
(6.6262x 10-34J)
Spectrum. A range of electromagnetic radiations obtained by
passing light through a prism is called a spectrum
Continuous spectrum: Those spectrums in which there is no
clear boundaries line between two adjacent colours are called
continuous spectrum.
Example. When white light is passed through a prism, it disperses
into its component colours and a spectrum is obtained known as
Continuous Spectrum. A continuous spectrum is one in which
colours are diffused in one another without any line of
demarcation.
Line spectrum or discontinuous or atomic spectrum:
Those spectrums in which there is clear boundaries line between
two adjacent colours and colours are separated called
continuous spectrum.
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Example. Electron radiates energy when jumps from higher orbit
to lower orbit. This radiated energy gives a line on the spectrum
and called line spectrum.
Energy levels. A circular path on which electrons revolve around
the nucleus is called energy level or shell. It shows how close the
electrons are to the nucleus
Example. The first energy level is K which is the nearest to the
nucleus.
Energy sub levels or orbital. The three dimensional region around
the nucleus where the probability of finding electrons is
maximum is called orbital. The sub shells are denoted by s, p, d, f
which stand for sharp, principal, diffused and fundamental
respectively. The s-orbital is spherical, p-orbital is dumbbell and d
and f orbital are more complex. All main energy levels consist of
sub energy levels.
Examples. The first shell K (n=1) consists of s-orbital(sub shell), the
second shell L (n=2) consists of s and p, the third shell consists of s,
p and d.
Electronic configuration. The arrangement of electrons in orbits
or shells around the nucleus of an atom is called electronic
configuration.
Explanation: Electrons revolve around the nucleus in a circular
path called shell or orbit. The distribution of electrons in shell or
orbit is called electronic configuration. The maximum number of
electrons in shell or orbit are according to 2n2 formula where ‘n’
is number of shell. Electronic configuration is done with the help
of energy level diagram (Figure-1) where lower energy orbitals
are filled first.
Examples. When ‘n’ is equal to 1 it is called K-shell and maximum
number of electrons K-shell can accommodate are 2 i.e 2n2 = 2(1)2
=2
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Similarly when ‘n’ is equal to 2 it is called L-shell and maximum
number of electrons L-shell can accommodate is 8 i.e 2n2 = 2(2)2 =
8
Fig. 1
n Energy level or
shell
Energy sublevels
Number of
electrons
1
2
3
4
s
s, p
s,p,d
s,p,d,f
2
2, 6
2,6,10
2,6,10,14
K
L
M
N
Total
number of
electron
2
8
18
32
Isotopes
Atoms of the same elements having same atomic number but
different mass number are called isotopes of that element (OR)
Atoms of the same element, which have same number of protons
but different number of neutrons in nucleus are called isotopes. Characteristics:
(I)
same number of electrons.
(2 same number of Protons.
(3) Different number of neutrons.
(4) Different mass number.
(5) Same chemical properties but different physical
properties.
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Representation of isotopes. Isotopes can be represented by
writing atomic number below and mass number above on the
left hand side of element symbol.
Examples. 1. Isotopes of Hydrogen: Hydrogen has three isotopes
and represented by
From above figure, it is clear that all three forms (protium,
deuterium and tritium) have same number of proton and
electron but different number of neutrons. Ordinary hydrogen is
called protium because contains only one proton but no neutron.
II. Isotopes of carbon: Carbon has three isotopes and represented
by
From above figure, it is clear that all three forms (C-12, C-13
and C-14) have same number of protons and electrons but
different number of neutrons. The C-12 contains six neutrons, the
C-13 contains 7 neutrons and C-14 contains 8 neutrons.
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III. Isotopes of chlorine: chlorine is a member of halogen family
and its atomic number is 17. Chlorine has two isotopes and
represented by
Isotopes
Cl-35
Cl-37
No. of electron
17
17
No. of proton
17
17
No. of neutron
18
20
From above figures, it is clear that both forms (Cl-35 and Cl-37)
have same number of protons and electrons but different number
of neutrons. The Cl-35 contains 18 neutrons, the Cl-37 contains 20
neutrons.
IV. Isotopes of Uranium: Uranium has atomic number 92 and has
three isotopes which are given below
From above figure, it is clear that all forms (U-234, U-235 and U238) have same number of protons and electrons but different
number of neutrons. The U-234 contains 142 neutrons, the U-235
contains 143 neutrons and U-238 contains 146 neutrons.
Uses of Isotopes: Isotopes arc mainly used in chemical,
agricultural and medical research and for diagnosing and
treatment of diseases. For instance:
1. Iodine- 131 is used in the treatment of thyroid cancer.
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2. Radium irradiation and cobalt-60 are used in the treatment
of cancer and for diagnosis of tumours.
3. Deuterium, heavy carbon {C-13), heavy nitrogen (N-I 5) and
heavy oxygen (O-18) are being, used as tracer elements in
biochemical and physio-chemical research.
4. Sodium-24 is used for the identification of blood circulation
problems in patients.
Exercise
1.Choose the correct answer
i. (b) proton ii. a) 2 iii. b) 18 iv. c) an electron v. c) radiate
energy vi. a) alpha particles vii. d) double positively charged
viii. a) reside in an orbit
ix. c) different number of neutrons x. d) isotopes
2. Answer theses questions
(i) How many sub energy levels are there in L-energy level? Give
there names and the number of electrons they can
accommodate.
Ans. As L-energy level is the second shell of atom therefore n=2.
There are two sub energy levels in L-shell. They are named as ‘s’
and ‘p’ which stand for sharp and principal. The s-orbital can
only accommodate 2 electrons and p-orbital (px, py and pz) 6
electrons. Therefore, the total number of electrons L-energy
level can accommodate will be 8.
Diagrammatically:
vii. Give electronic configuration of Ne(Z= 10)
Ans.10 Ne : 1s2, 2s2, sp6
K=2 L=8
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7. Give reasons
i. Only a few high energy α-particles rebound after striking the
metal foil in Rutherford’s experiment.
Ans. As major portion of the atom is empty and nuclei only
occupies small portion of atom due to which only a few high
energy α-particles rebound. The size of positive centre (nucleus)
is very small as compared to the whole atom so deflection of
only a few alpha particles (double positive) is possible.
ii. An atom is a neutral particle.
Ans. As the number of electrons (negative) in atom is always
equal to the number of protons (positive), therefore atom is a
neutral particle because cancel the effects of each other.
iii. Electrons continuously revolve around the nucleus but do
not fall into the nucleus.
Ans. Electrons continuously revolve around the nucleus but do
not fall into the nucleus because electron does not absorb or
emit energy when revolves around the nucleus in a fixed orbit.
When electrons revolve in a fixed orbit the centrifugal force of
an electron is balanced by the force of attraction between the
nucleus and the electron
iv. K shell can only accommodate 2 electrons while other shell
can accommodate more than 2.
Ans. The maximum number of electrons can be accommodated by
the help of 2n2 formula. For K shell n= 1, therefore maximum
number of electrons K-shell can accommodate are 2 i.e 2n2 = 2(1)2
= 2. However, for other shell the n value is greater than 1 due to
which accommodate more than 2 electrons. For example when
n=2 then it is called L-shell and maximum number of electrons Lshell can accommodate is 8 i.e 2n2 = 2(2)2 = 8
v. The mass of an atom is present in its nucleus.
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Ans. The mass of an atom is present in its nucleus because both
proton and neutron are heavy particles of atom which are present
inside the nucleus. This was the reason that alpha particles
bounced back when collided with nucleus because mass of an
atom is present in its nucleus.
vi. Physical properties of the isotopes are different but their
chemical properties are the same.
Ans. Chemical properties depend upon number of electrons and
its configuration around the nucleus. Therefore, chemical
properties of isotopes are same because isotopes have same number
of electron and same configuration around the nucleus. However,
physical properties such as mass, density etc will be different from
each other because isotopes have different number of neutrons.
For example, all isotopes of carbon when react with oxygen it
form carbon dioxide (same chemical properties) but masses are
different such as C-12, C-13 and C-14.
vii. Rutherford concluded that most of the space in atom is empty
Ans. Most of the space in atom is empty because most of the alpha
particles passed undeflected from gold foil.
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