STRUCTURE OF ATOM

NAME:- PINAKI BANDYOPADHYAYCLASS:-11-’C’ROLL NO.:-26

CONTENTS

• various model of atom J.J. Thomson model of atom Rutherford alpha-scattering experiment

• Electromagnetic wave theory• Planck’s Quantum theory• Photoelectric effect• Dual behavior of matter• Heisenberg’s uncertainty principle

Various Models of Atom

J.J. Thomson Model of AtomThis model of atom is given by J.J. Thomson in 1904. In this model,

There Is a positive sphere in which positive charge is uniformly

distributed like pulp in melon and electrons is embedded in this

positive sphere like seed in melon.

Rutherford alpha- scattering Experiment

ExperimentRutherford and his student bombarded very thin gold foil alpha-particle. A stream of alpha-particle from radioactive source was directed towards thin gold foil( thickness~ 100nm) of gold metal.

The thin gold foil had a circular fluorescent zinc sulphide screen

around it. Whenever alpha-particle struck the screen , a tiny flash of light was produced at that point.

Experiment

Observation

most of the alpha-particle passed through the gold foil undeflected.

a small fraction of the alpha-particle a deflected by small angles.

a very few alpha-particle(~ 1 in 20,000) bounded back, that is, were deflected by 180 degree.

Conclusion

There is most of the space empty in atom. The positive charge has to be concentrated in a

very small region that repelled and deflected the positively charged alpha-particle.

size of nucleus is very smaller than size of atom i.e., nucleus= 10 to the power -15 & atom= 10 to the power -10.

Schematic molecular view of the gold foil

Why Gold foil Is used?

• Gold is highly malleable; gold leaf can be made in thicknesses around 100 nm or 4 micro-inch.

• Most of the element after gold is radioactive. It is used because gold was known to be a very inert element.

• To increase the probability of an alpha particle hitting a nucleus, the more you have in a given volume, the better you chances. Gold has a density of 19.3 grams per cubic centimeter.

Nucleus & Orbits

Nucleus:- The positive charge and most of the mass of the atom was densely concentrated in extremely small region. This part is called nucleus. It contain neutron & proton.

Orbits:- The electron move in circular path around nucleus called orbit.

Limitation of Rutherford model

• The Rutherford model cannot explain the stability of an atom.

• According to EMW(Electro Magnetic Wave) theory, if a charged particle accelerate then it radiate energy and the path of orbit shrink and electron will spiral into nucleus.

• It say nothing about the electronic structure of the atom.

Electromagnetic wave theory

James Clerk Maxwell (1831-1879) – Scottish

mathematician and physicist developed this.

• Unified existing laws of electricity and magnetism

• Oscillating electric field produces a magnetic field

(and vice versa) – propagates an EM wave

According to EMW, Electron Spiral into Nucleus

Electromagnetic waves

Spectrum of Electromagnetic radiation

Absorption spectrum & Emission spectrum

• The electromagnetic spectrum, broken by a specific pattern of dark lines or bands, observed when radiation traverses a particular absorbing medium. The absorption pattern is unique and can be used to identify the material. This is called absorption spectrum.

• The spectrum of bright lines, bands, or continuous radiation characteristic of and determined by a specific emitting substance subjected to a specific kind of excitation. spectrum of electromagnetic radiation emitted by a self-luminous source. This is called emission spectrum.

Spectrum

Planck’s Quantum theory

Max Planck showed that the energy of light is proportional

to its frequency, also showing that light exists in

Discrete quanta of energy.

Wavelength of a wave:-

The distance used to determine the wavelength of

a wave is shown. Light has many properties

associated with its wave nature, and the

wavelength in part determines these properties.

Frequency of Wave

It Is defined as number of waves that pass a given

point in one second.

Wave number:- It is defined as the number of Wavelengths per unit length.

Planck’s constant:- i.e., h=6.626X10 to the power

-34 Js.

Therefore, E=hv(frequency(nu))

Photoelectric EffectThe photons of a light beam have a characteristic

energy proportional to the frequency of the light.

In the photoemission process, if an electron within

some material absorbs the energy of one photon and acquires more energy than the work function (the electron binding energy) of the material, it is ejected. If the photon energy is too low, the electron is unable to escape the material. Increasing the intensity of the light beam increases the number of photons in the light beam, and thus increases the number of electrons excited, but does not increase the energy that each electron possesses. The energy of the emitted electrons does not depend on the intensity of the incoming light, but only on the energy or frequency of the individual photons. It is an interaction between the incident photon and the outermost electrons.

Electrons can absorb energy from photons when irradiated,

but they usually follow an "all or nothing" principle. All of the

energy from one photon must be absorbed and used to

liberate one electron from atomic binding, or else the

energy is re-emitted. If the photon energy is absorbed,

some of the energy liberates the electron from the atom,

and the rest contributes to the electron's kinetic energy as a

free particle.

Experiment

Technology developed

• "electric eye", light meter, movie film audio track.• photoconductivity: an increase in the electrical conductivity of a

nonmetallic solid when exposed the electromagnetic radiation. The increase in conductivity is due to the addition of free electrons liberated by collision with photons. The rate at which free electrons are generated and the time they over which the remain free determines the amount of the increase.

• photovoltaic's: the ejected electron travels through the emitting material to enter a solid electrode in contact with the photo emitter (instead of traveling through a vacuum to an anode) leading to the direct conversion of radiant energy to electrical energy.

• photo static copying .

Dual behavior of Matter

Wave–particle duality postulates that all particles exhibit

both wave and particle properties.

The photon has momentum as well as

Wavelength, De Broglie, from the analogy,

Gave the following relation:-

Lambda= h/mv=h/p.

h= Planck’ constant,

m= mass,

V(nu)= frequency

p= momentum.

Einstein is most famous for saying "mass is related

to energy". Of course, this is usually written out as

an equation, rather than as words:

E=m X c X c ---------------------(1)

wave's frequency by the equation:

E=h X v(nu) ---------------------(2)

Combining the 1st & 2nd eq.,

Lambda=h/mv (since, c=v)

Dual Behavior of Electron

Heisenberg’s Uncertainty Principle

It states that it is impossible to determine Simultaneously,

the exact position and exact momentum(or velocity) of an

electron.

Mathematically,

x(delta) X p(delta)=h/4X3.14(pie).