Anurag Kumar
Structure Of Atom A project to elaborate the discovery and advancement in the field of existence of atom, structure of atom and its various fundamental particles. Anurag Kumar 12/2/2010 Class: 12 Science Roll No.:2670081 Session : 2010-2011 School : K V No 1 Bathinda Cantt
Anurag Kumar
1. Discovery of Atom
2. Discharge tube Experiment
3. Cathode Rays And Their Properties
4. Discovery of Electrons
5. Discovery of Protons or Anode Rays
6. JJ Thomason’s Model Of An Atom
7. Rutherford’s Alpha Scattering
Experiment
8. Rutherford’s Model Of An Atom
9. Planck’s Quantum Theory
10. Photoelectric Effect
11. Bohr’s Model Of An Atom
12. De Broglie Concept
13. Heisenberg’s Uncertainty Principle
14. Quantum Mechanical Model Of An Atom
15. What are quarks?
16. Short Information about some of
scientists involved in study of atom.
Anurag Kumar
Discovery of Atom: 1. The existence of atom has been proposed since the time of early Indian and
Greek philosophers (400 B.C.) who were of the view that atoms are the
fundamental building blocks of matter.
2. The word atom has been derived from the Greek word “a-tomio” which means
uncut able or non-divisible. These ideas remain dormant for a very long time and
were revived again by scientists in nineteenth century.
3. Near the end of the 18th century, two laws about chemical reactions emerged
without referring to the notion of an atomic theory. The first was the law of
conservation of mass, formulated by Antoine Lavoisier in 1789, which states
that the total mass in a chemical reaction remains constant (that is, the reactants
have the same mass as the products). The second was the law of definite
proportions. First proven by the French chemist Joseph Louis Proust in
1799, this law states that if a compound is broken down into its constituent
elements, then the masses of the constituents will always have the same
proportions, regardless of the quantity or source of the original substance.
4. John Dalton studied and expanded upon this previous work and developed
the law of multiple proportions: if two elements can together form more than
one compound, then the ratios of the masses of the second element which
combine with a fixed mass of the first element will be ratios of small integers. For
instance, Proust had studied tin oxides and found that their masses were either
88.1% tin and 11.9% oxygen or 78.7% tin and 21.3% oxygen (these were tin (II)
oxide and tin dioxide respectively). Dalton noted from these percentages that
100g of tin will combine either with 13.5g or 27g of oxygen; 13.5 and 27 forms a
ratio of 1:2. Dalton found an atomic theory of matter could elegantly explain this
common pattern in chemistry - in the case of Proust's tin oxides, one tin atom will
combine with either one or two oxygen atoms
A page of the book “A New System of
Chemical Philosophy (1808).” By John
Dalton
Anurag Kumar
Discharge Tube Experiment:
Observations : 1. No current flows at 1 atm pressure even at high voltage (about 104 V ).
2. When pressure is reduced to 10-2 atm, gas is found to emit light which
depends upon the nature of gas.
3. Further decrease in pressure stops emission of light but walls opposite to
cathode starts glowing and this phenomenon is called fluorescence.
Result : 1. Fluorescence is due to rays emitted from cathode and hence these rays
are called cathode rays.
Origin of Cathode Rays :
Initially cathode rays originate from the metal forming cathode and then
from the molecules of gas due to the bombardment of molecules of gas by
high speed electrons originate from cathode.
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Properties of Cathode Rays :
J.J.Thomson studied the important properties of cathode rays as follows:
1. Cathode rays travel in a straight line.
2. Cathode rays are made up of material particles because when some paddle
wheel made up of mica is placed in the path of cathode rays it starts rotating.
(references CD).
3. Cathode rays consist of negatively charged particles because when electric field
is applied cathode rays deflects towards positive plate.
4. Cathode rays produce heating effect.
5. They produces x-rays when strikes the surface of hard metal for e.g. tungsten,
etc.
6. They ionize the gas through which they pass.
7. They affect photographic plate.
8. They penetrate the path through which they pass.
9. JJ Thomson performed different efforts to calculate the charge/mass ratio for
electron using different gases.
And value remains constant and comes out to be 1.76 X 108 Coulombs/gram.
Millikan with the help of oil drop experiment found that charge on each electron is
equal to 1.6 x 10-19 coulomb.
So by using charge by mass ratio ,
The mass of electron comes out as 9.1 x 10-28 grams.
So, each electron constituting cathode rays has mass 9.1 x 10-28 grams.
Cathode rays under electric field.
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Discovery of electrons:
1. Atoms were thought to be the smallest possible division of matter until 1897
when Thomson discovered the electron through his work on cathode rays.
A Crookes tube is a sealed glass container in which two electrodes are
separated by a vacuum. When a voltage is applied across the electrodes,
cathode rays are generated, creating a glowing patch where they strike the glass
at the opposite end of the tube. Through experimentation, Thomson discovered
that the rays could be deflected by an electric field (in addition to magnetic fields,
which was already known). He concluded that these rays, rather than being a
form of light, were composed of very light negatively charged particles he called
"corpuscles" (they would later be renamed electrons by other scientists).
2. As cathode rays are made up of material particles which are attracted towards
the positively charged plate. So, cathode rays consist of negatively charged
particles and they were named as Electrons.
3. As J.J.Thomson studied the properties of cathode rays which led to the discovery
of electrons so, it’s J.J. Thomson who discovered electrons.
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Discovery of Protons( Anode Rays) :
As atom as a whole is neutral, since atoms contain negatively charged electrons, it was
thought that some positively charged particles must also be present to make it neutral.
This thought makes the Goldstein to perform Discharge tube experiment again but with
some modifications.
Goldstein used perforated cathodic electrode and cathodic end of the glass tube is
coated with ZnS.
Observations :
1. Luminous rays passing through perforations of cathode and moving in
directions opposite to cathode rays.
2. These rays consists of positively charged particles, these rays are called
Anode rays.
Result :
As these rays passes the cathode ray through the holes and produces green
fluorescence so these are +vely charged and called Anode Rays.
Origin of Anode Rays: Anode rays do not emit from anode but they are actually
produced from the gas present in between cathode and anode. They are
produced as a result of the knockout of electron from these molecules.
Properties Of Anode Rays :
1. They travel in a straight line.
2. They are made up of material particles.
3. These rays are positively charged because these are deflected towards
the negative plate under the influence of electric field.
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Discovery of protons:
The Discovery of anode rays led to the discovery of protons , as it is found that
anode rays are made up of some positively charged particles and these particles
are called Protons.
Charge on a proton: Equal to that on an electron i.e.
1.6 X 10-19 coulombs.
Mass of proton: 1.67 X 10-24 grams.
Anurag Kumar
J.J. Thomson model of Atom:
In 1904, Thomson believed that the corpuscles emerged from the molecules of
gas around the cathode. He thus concluded that atoms were divisible, and that
the corpuscles were their building blocks. To explain the overall neutral charge of
the atom, he proposed that the corpuscles were distributed in a uniform sea of
positive charge; this was the plum pudding model as the electrons were
embedded in the positive charge like plums in a plum pudding (although in
Thomson's model they were not stationary).
Or
Atom is a sphere of positively charged particles in which negatively charged
electrons are embedded. Stability of atom was explained on the basis of
attraction between positively and negatively charged protons and electrons
respectively.
Drawbacks: One major drawback is that it could not explained the Rutherford’s
Alpha Scattering Experiment, hence rejected.
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Rutherford’s α-Scattering Experiment:
Observation:
1. Most of the α-particles passed through the gold foil.
2. A small fraction of α-particles was deflected by small angles.
3. A very few α-particles (~1 in 20,000) bounced back, that is, were deflected by
nearly 1800.
Alpha Scattering Experiment
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Rutherford’s Model of an atom and its
Drawbacks :
Drawbacks :
1. His model cannot explain the hydrogen atomic spectra and of other
elements.
2. When an electron revolves in a orbit the it undergoes acceleration and due
to which it loses energy and hence ultimately fall into the nucleus. But
actually this does not happen so his model is not appropriate.
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Planck Quantum Theory:
After the failure of Electromagnetic wave theory, Max.Planck came into
picture and gives few postulates as:
1. Radiant energy is emitted or absorbed not continuously but discontinuously
in the form of packets called Quantum. & these quantum are called
Photons in case of light.
2. Energy of each quantum is directly proportional to the frequency of
radiation. i.e.
E ∞ V or E = hv
Where h is planks constant such that
h = 6.626 X 10-34 J/s
3. Total amount of energy emitted is whole number multiple of quantum ,
i.e.
E = nhv where n in any natural number
Anurag Kumar
Photoelectric effect:
DEFINITION : The ejection of electrons from the surface of a metal under the
influence of striking photons .
EXPLANATION : Actually electrons are held by the nuclei by some force called
Binding Energy. But photons which fall on the carries energy and hence excite
them to the surface from where electrons start moving in a definite direction with
definite amount of kinetic energy.
CONDITIONS :
Energy of incident photons = Work Function + Kinetic
Energy of electron
hυ = hυ0 + ½ mv2
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Bohr’s Model Of An Atom :
Postulates :
1. Whole mass of atom is present in the central core called nucleus.
2. Electrons revolve around the nucleus in stationary orbits also called energy
levels.
3. Angular momentum of an electron in a given stationary orbit can be given by
4. No. of electrons in a given energy level is given by
Bohr’s Model Of An Atom
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Failures Of Bohr’s Model:
1. The theory could not explain the atomic spectra of atoms containing
more than one electrons.
2. Theory failed to explain the fine structure of spectral lines.
3. Splitting of lines in the magnetic field is known as Zeeman Effect and
splitting of lines in the electric field is known as Stark Effect. Bohr’s theory
could not offer any satisfactory explanation of these effects.
4. Theory failed to explain the shapes of the molecules formed by the
combination of atoms.
5. Bohr’s Theory could not explain the de Broglie’s Relationship and
Heisenberg’s Uncertainty Principle.
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De Broglie Concept:
He stated that moving particles are associated with dual nature i.e. wave and
particle nature.
He co-related the two characters in the form of an equation known as de-Broglie
Equation.
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Heisenberg’s Uncertainty Principle:
He stated that, It is not possible to measure simultaneously the position and the
momentum of a microscopic particle with absolute accuracy or certainty.
Mathematically,
This principle is a prove that electron can never exists inside the nucleus.
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Quantum Mechanical Model Of An Atom:
Erwin Schrödinger Gave the quantum mechanical model of an atom considering
the wave nature of electron. He describes the motion of electron as three
dimensional around the positively charged nucleus and gave the following eqn.
Called as Schrodinger Wave Equation:
Here,
X,Y and Z are Cartesian coordinates
E = total energy of the electron
U = potential energy,
M = mass of electron,
Ψ = Amplitude of wave (or called wave function), and
Ә2Ψ/ӘX2 = second order derivative of Ψ wrt x axis .
It is found that instead of revolving in an orbit electrons actually revolve in orbital
and these orbital aggregate to form orbits.
Ә2Ψ/ӘX2 + Ә2Ψ/ӘY2 + Ә2Ψ/ӘZ2 + 8π2M/h2 (E-U)Ψ = 0
Anurag Kumar
What are Quarks :
A quark (pronounced /ˈkwɔrk/ or /ˈkwɑrk/) is an elementary particle and a
fundamental constituent of matter. Quarks combine to form composite
particles called hadrons, the most stable of which are protons and neutrons, the
components of atomic nuclei. Due to a phenomenon known as color
confinement, quarks are never found in isolation; they can only be found within
hadrons. For this reason, much of what is known about quarks has been drawn
from observations of the hadrons themselves.
There are six types of quarks,
known as flavors: up, down, charm, strange, top, and bottom. Up and down
quarks have the lowest masses of all quarks. The heavier quarks rapidly
change into up and down quarks through a process of particle decay: the
transformation from a higher mass state to a lower mass state. Because of this,
up and down quarks are generally stable and the most common in the universe,
whereas charm, strange, top, and bottom quarks can only be produced in high
energy collisions (such as those involving cosmic rays and in particle
accelerators).
A proton, composed of two up quarks and one down quark. (The
color assignment of individual quarks is not important, only that all
three colors are present.)
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Quarks have various intrinsic properties:
Electric charge, color charge, spin, and mass. Quarks are the only
elementary particles in the Standard Model of particle physics to experience all
four fundamental interactions, also known as fundamental
forces (electromagnetism, gravitation, strong interaction, and weak
interaction), as well as the only known particles whose electric charges are
not integer multiples of the elementary charge. For every quark flavor there
is a corresponding type of antiparticle, known as antiquark, that differs from
the quark only in that some of its properties have equal magnitude but opposite
sign.
The quark model was independently proposed by physicists Murray Gell-
Mann and George Zweig in 1964. Quarks were introduced as parts of an
ordering scheme for hadrons, and there was little evidence for their physical
existence until deep inelastic scattering experiments at SLAC in 1968. All six
flavors of quark have since been observed in accelerator experiments; the top
quark, first observed at Fermi lab in 1995, was the last to be discovered.
Photograph of the event that led to
the discovery of the Σ++c baryon,
at the Brookhaven National
Laboratory in 1974
The strengths of the weak
interactions between the six quarks.
The "intensities" of the lines are
determined by the elements of
the CKM matrix.
Anurag Kumar
Strong interaction and color charge :
Quarks possess a property called color charge. There are three types of color charge,
arbitrarily labeled blue, green, and red. Each of them is complemented by an
anticolor—antiblue, antigreen, and antired. Every quark carries a color, while every
antiquark carries an anticolor.
All types of hadrons have zero total color charge.
Anurag Kumar
5. Werner Heisenberg:
Werner Heisenberg
Born Werner Karl Heisenberg 5 December 1901 Wurzburg, Germany
Died 1 February 1976 (aged 74) Munich, Germany
Nationality German
Fields Physics
Institutions University of Gottingen University of Copenhagen University of Leipzig University of Berlin University of Munich
Alma mater University of Munich
Doctoral advisor Arnold Sommerfeld
Anurag Kumar
Doctoral students Felix Bloch Edward Teller Rudolph E. Peierls Reinhard Oehme Friedwardt Winterberg Peter Mittelstaedt Şerban Ţiţeica Ivan Supek Erich Bagge Hermann Arthur Jahn
Other notable students William Vermillion Houston Guido Beck Ugo Fano
Known for Uncertainty Principle Heisenberg's microscope Matrix mechanics Kramers-Heisenberg formula Heisenberg group Isospin
Notable awards Nobel Prize in Physics (1932) Max Planck Medal (1933)