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.

Anurag Kumar

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.

Anurag Kumar

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.

Anurag Kumar

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.

Anurag Kumar

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.

Anurag Kumar

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

Anurag Kumar

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.

Anurag Kumar

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

Anurag Kumar

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

Anurag Kumar

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.

Anurag Kumar

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.

Anurag Kumar

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.

Anurag Kumar

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

Few Postulates Of Quantum Theory :

Shapes of some orbitals :

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.)

Anurag Kumar

Anurag Kumar

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

Anurag Kumar

1. J.J.THOMSON :

Anurag Kumar

2. ERNEST RUTHERFORD

Anurag Kumar

3. MAX PLANCK :

Anurag Kumar

Anurag Kumar

4. LOUIS De BROGLIE:

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)

Anurag Kumar