Question

hey ♥♡♥♡


explain Rutherford, Bohr model with limitations..

also discovery of protons by Goldstein.

exam tension, be quick ​

Answer 1

Answer :-

So the question asked about the

• Atomic Models of Rutherford and Bohr

• Discovery of a proton by Goldstein.

Anode Ray discharge

Now to be in sequence we may start from the discovery of the proton by Goldstein.

This Discovery of cathode Ray in the discharge tube lead to the discovery of Anode Ray ( or proton).

In the experiment, Goldstein took a positive ray discharge tube having perforated cathode in it and filled with air.

The tube was attached with a vacuum pump ( Pressure of about 0.001 mm of Mercury).

It was provided with a high voltage, thus due to high voltage the air conducted Electricity → Atom of air dissociated into an electron and positive ions ( H⁺ ions, protons)

When these rays of positive excited ions strike the wall of tube it started glowing with faint red color.

These rays are also known as canal rays.

Properties :-

• These rays travel in straight line

• These rays are consist of material particles.

• These rays travel towards negatively charged plate , hence positive in nature.

• When a magnetic field is applied in a direction perpendicular to the path of Anode Ray , it gets deflated in the direction of positive current.

Some facts :-

• Proton is positive in nature and present in every atom.

• Mass of Proton is $1.672 \times 10^{-24} \: g$

• Charge of Proton is $+ 1.6 \times 10^{-19} \: C$

Rutherford's Model of an Atom

In this model Rutherford performed a α-particle scattering experiment.

In this experiment he bombarded α-particles on a thin sheet of gold foil.

He fixed a screen coated with ZnS around the foil to detect the detect the deflection suffered by α-particle.

His observation :-

(i) Most of the α-particles passed undeflected.

(ii) A small amount of α-particles were deflected by a small angle.

(iii) Very few ( nearly 1 in 20,000 ) α-particles bounced back making an angle of 180° .

Theory :-

According to his observations :-

(i) The positively charged particles and most of the mass of the atom is concentrate in a small region i.e the nucleus.

(ii) Nucleus is surrounded by the orbits ( circular paths) in which electron revolve around it in a high speed.

(iii) Electrons are Heald together with Nucleus due to the presence of Electrostatics forces.

Drawback :-

He was unable to explain the stability of the electron .

Bohr's Model :-

He suggested that there are infinite number of orbits possible around Nucleus but electron will move only in energy level only for which angular momentum of electron is provided by

$mvr = n\dfrac{h}{2\pi}$

Where n = orbit no.

According to him , the exchange of energy is possible only when electron from one energy level to another.

$\Delta E = E_2 - E_1 = hv = \dfrac{hc}{\lambda}\: joule$

Where h = planck's constant

$h = 6.63 \times 10^{-34}$

Drawback :-

(i) It was applicable for only monoatomic systems.

(ii) It fails to explain why spectrum of Hydrogen atom is discontinuous.

(iii) It failed to explain Zeeman effect ( effect of magnetic field) and Stark effect ( effect of electric field)

Answer 2

$\Huge{\boxed{\sf{\underline{\underline{Answer}}}}}$

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Rutherford Model : To demonstrate the scattering of alpha particles by gold foil.

$\Large\boxed{RUTHERFORD'S\:MODEL\:OF\:AN\:ATOM}$

⇒ Atom is spherical and consists of 2 parts, nucleus and extranuclear.

⇒ Nucleus is very small in size. It is positively charged highly dense central core in which entire mass of an atom is concentrated.

⇒ Electrons revolves around the nucleus is a circular orbit with very high speed.

• α-Particles ( alpha particles ) : ( +2 charge and 4 mass ) when fast moving alpha particles are bombarded on very thin gold foil, following observations were made :

i) Most of the alpha particles passed straight through the gold foil.

ii) Some of the alpha particles were deflected by the foil by small angles.

iii) One out of 1200 particles appeared to rebound.

$\Large\boxed{OBSERVATION\:MADE\:BY\:RUTHERFORD}$

• Most of the Alpha particles passed through gold foil undeflected.

• A small fraction of the alpha particles was deflected by small angles.

• Very few alpha particles ( in 20000 ) bounced back by nearly 180°.

$\Large\boxed{CONCLUSION\:FROM\:OBSERVATIONS}$

• Most oth space inside the foil is empty.

• Positive charge of atom occupies very less space.

• Mass of the atom is concentrated in the centre with all +ve charge concentrated in small volume within the atom.

$\large\boxed{LIMITATIONS\:OF\:RUTHERFORD'S\:MODEL\:OF\:AN\:ATOM}$

• The electrons are revolving in a circular orbit around the nucleus.

• Any charged particle that revolves would undergo acceleration and radiate energy.

• The revolving electron would lose its energy and finally falls into the nucleus, thus the atom would be highly unstable.

$\Huge\boxed{BOHR'S\:MODEL\:OF\:AN\:ATOM}$

• Positive charge in centre called nucleus.

• Certain spherical orbits known as discrete orbits of electrons.

• Electrons revolve in discrete orbits and do not radiate energy.

• The orbits were termed as energy shells labelled as K,L,M,N or n= 1,2,3,4 ( numbered ).

$\Large\boxed{LIMITATIONS\:OF\:BOHR'S\:MODEL\:OF\:AN\:ATOM}$

• It could not explain line spectra and energy of atoms containing more than 1 electron.

• Failed to explain Zeeman effect ( splitting of spectrum in magnetic field ) and Failed to explain Stark effect ( splitting of spectrum in electric field )

• Could not explain why atoms should combine to form chemical bonds and why molecules are more stable than atom.

DISCOVERY OF PROTONS BY GOLDSTEIN :-

• In 1866 E. Goldstein discovered the presence of new radiations in a gas discharge tube and called them canal rays.

• These rays were positively charged radiations which led to the discovery of subatomic particles called protons.

⇒ The anode rays travel in straight line.

⇒ The anode rays are reflected by the magnetic and electric field. The direction of reflection represents that they were positively charged.

⇒ Like cathode rays rays these rays produce heating effect.

⇒ Unlike cathode rays +vely charged particles depends upon nature of gas present in tube.These are simply the positively charged ions.

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