Structure of Atom

Structure of Atom

A project to elaborate the discovery and advancement in the field of existence of

atom and its various fundamental particles

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After this animation you will be able toKnow more about the discovery of electron ,

proton and neutron.Thomson, Rutherford and Bohr atomic

models.Rutherford’s Alpha scattering experiment.Quantum mechanical model of atom.Photoelectric effect.


Discovery of atomThe 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.

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.

Scientists work hard to discover the sub-atomic particles and those experiments are as….


Discharge Tube Experiment

William Crooks performed this exp. In cylindrical glass tube as shown in figure.

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 is called fluorescence.

Result : Fluorescence is due to rays emitted from cathode and hence these rays are called cathode rays.

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Properties of cathode rayJ.J.Thomson studied the important properties of

cathode rays as follows:Click The Following To Study :

Cathode rays travel in a straight line .Cathode rays are made up of material particl

es.Cathode rays consists of negatively charged p

articles.


Discovery of electrons

As cathode rays are made up of material particles which are attracted towards the positively charged plate. So , cathode rays consists of negatively charged particles and they were named as Electrons.

As J.J.Thomson studied the properties of cathode rays which led to the discovery of electrons so, its

J.J. Thomson who discovered electrons


Discovery of Protons( Anode Rays)

In 1886, Goldstein performed discharge tube experiment with certain modifications as shown in figure.

He observed new type of luminous rays passing through perforations of cathode and moving in directions opposite to cathode rays. So, these rays consists of positively charged particles called Protons and these rays are called Anode rays.

Anode rays are not emitted from anode but from a space between anode and cathode.


J.J. Thomson model of Atom

In 1904, Thomson proposed that 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.


Rutherford’s α-Scattering ExperimentClick Here To See Experiment And Its Results

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.

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Drawbacks of Rutherford’s Model Of An AtomHis model cannot explain the hydrogen

atomic spectra and of other elements.When an electron revolves in a orbit the it

undergoes acceleration and due to which it looses energy and hence ultimately fall into the nucleus. But actually this does not happened so his model is not appropriate.


Planck Quantum TheoryAfter 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 a re 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 thath = 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


Photoelectric effect

Click Here To see 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 electronhυ = hυ

0 + ½ mv2


Bohr’s Model Of An Atom

Whole mass of atom is present in the central core called nucleus.

Electrons revolve around the nucleus in stationary orbits also called energy levels.

Angular momentum of an electron in a given stationary orbit can be given by

No. of electrons in a given energy level is given by

mvr = nh/2π

No of electrons = 2n2

Click Here to view Bohr’s Model of an Atom


Failures Of Bohr’s ModelThe theory could not explain the atomic spectra of

atoms containing more than one electrons.Theory failed to explain the fine structure of spectral

lines. 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.

Theory failed to explain the shapes of the molecules formed by the combination of atoms.

Bohr’s Theory could not explain the de Broglie’s Relationship and Heisenberg’s Uncertainty Principle.


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.

λ = h/mv

Click here to see the prove


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.

∆x.∆p = h/4π

Quantum Mechanical Model Of An Atom

It is found that instead of revolving in an orbit electrons actually revolve in orbital and these orbital aggregate to form orbits.

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Structure of Atom