Structure of Atom

# Discovery of Fundamental Particles

# Nucleus

# Atomic Model

# Bohr Model

# Various Theories

# Wave Model

# Electronic Configuration

Discovery of fundamental particles


Discharge tube experiment

  • In discharge tube experiment physical observation of charges in discharge tube is observed by the flow of fluorescent material.
  • Fluorescent material is the substance which glow as radiation falls on it.
  • In discharge tube only one type of gas should be taken.
  • On applying extremely low pressure and very high voltage there is glow of fluorescent material. This means certain kind of radiation has been produced inside the discharge tube known as cathode rays.
  • Extremely low pressure is required to avoid the molecular collision.
  • High voltage is required to break the molecule.

Discovery of electrons

  • Cathode rays are negatively charged material particles which travel in a straight line which get deflected by magnetic and electric field.
  • Thomson has given the name negatron to these particles which later on replaced by the term electron(by stoney).


  • Symbol e
  • Mass 9.1*10-31 kg
  • Charge -1.6*10-19 C
  • Specific charge = charge/mass = 1.76*108 C/g
  • In case of electrons charge to mass ratio remains constant irrespective of nature of gas.


Discovery of Canal Rays

  • Goldstein repeated the experiment of J.J. Thomson with certain modifications in the discharge tube.
  • He kept the fluorescent material behind the cathode.
  • He used the perforated cathode so that rays can cross the cathode material.
  • On applying the extremely high voltage and low pressure there was glow of fluorescent material, this means certain kind of rays has been produced which travel from anode to cathode. Hence called anode rays or canal rays.
  • Goldstein also observed the changes done by J.J. Thomson in case of anode rays.
  • Anode rays are positively charged material particles which travel in straight path and get deflected by electric and magnetic field.


  • Symbol p+
  • Mass 1.67*10-27 kg
  • Charge +1.6*10-19 C
  • Charge/mass => is not constant
  • Charge by mass ratio is not constant in case of proton, it depends upon the nature of gas.

Discovery of neutrons

  • Discovery of neutrons was done by the nuclear radiation.
  • On striking the alpha particles on the Beryllium atom certain kind of rays were produced which travelled in straight line and had definite mass. These rays don not get deflected in magnetic or electric field and called as neural rays and material particle is called neutron.


  • Symbol n
  • Mass 1.67*10-27 kg
  • Charge : no charge
  • Charge/mass ratio is not defined in case of neutrons
  • Massneutron > Massproton > Masselectron


Discovery of Nucleus

  • When there is bombardment of the alpha particles on a thin gold foil most of the alpha particles passed unaffected. From this it was concluded that most of the space in the atom is empty.
  • Few of the alpha particles got deflected from their path. From this it was concluded that the alpha particles are facing repulsion.
  • Out of 20,000 alpha particles at least one of the particles bounced back i.e. returned by 1800. From this it was concluded that entire mass as well as the positive charge is concentrated on a very small space called nucleus.


  • Radius 10-8 cm
  • Nucleus 10-12 cm to 10-13 cm
  • R = R0 A1/3 R = radius of nucleus     R0 = 1.44*10-13 cm        A = mass number


  • Units : Coulomb and Faraday 1F = 96500 C
  • F = kq1q2/r2 r = distance between the two charges
  • Energy = kq1q2/r2
  • Energy = charge*voltage = Q*V
  • One electron volt = 1ev = 1.6*10-19 J
  • One electron volt is the charge accelerated by the potential difference of 1 volt.

Closest Approach in Rutherford Model

  • R = 4kze2/malhav2alpha
  • R = closest approach
  • Malpha = mass the alpha particles
  • Valpha = velocity of the alpha particles
  • Z = atomic number ( number of protons)


Wave length

  • Distance between two successive crest or trough is known as wavelength.
  • 1 A0 = 10-10 m ( A0 = Angstrom )


  • Number of waves passing through a point in one second is called frequency.
  • Units : Hertz(Hz) or sec-1
  • F=C/lamda

Wave number

  • Number of waves in one meter.
  • Units : m-1
  • wave number= 1/wavelength
  • Energy = hv                                    h = planks constant = 6.626*10-34sec.

Planks Quantum Theory

  • As per planks quantum theory energy radiations are discrete i.e. they are mad with the small packets of energy known as quanta. When number of quanta is present then it is quantum.
  • In case of light radiation quanta is known as photons.
  • E = nhv                                 n=> Number of photons

Electromagnetic Radiations (EMR)

  • When a moving charged body is kept under a magnetic field or moving magnet is kept under electric field then certain type of radiation is developed known as electromagnetic radiation.
  • EMR travels with the velocity of light.
  • EMR does not require any medium to propagate. It can travel in the vacuum also.
  • EMR have two components electric and magnetic. Both are perpendicular to each other and travels in the direction of propagation of wave.
  • Examples of EMR – Radio, Micro, Infrared, Visible, Ultra violet, X-rays, alpha and cosmic rays.


  • When a radiation is passed through a prism and impact produced on photographic film or detector is known as spectrum.


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