Atomic structure|| class 11th

 prepared by AKHILESH TIWARI 

UNIT 2 STRUCTURE OF ATOM

According to Dalton theory of atom(1808) - 

•         All matter is composed of extremely small particles called atoms.
• ·        The atom consists of smaller particles such as electron, proton and neutron.
• ·        These particles are regarded as fundamental particles.
• ·        Dalton theory explained the laws of chemical combination

·        Dalton couldn’t explain the charge produced by rubbing ebonite

 with silk or fur.

DISCOVERY OF ELECTRONS

The electrons were discovered by conducting an experiment by using a simple apparatus known as Cathode rays Discharge tube  tube or Crookes tube.

• ·        It  consists of a sealed glass tube of about 50 cm lengths.
• ·        Two metal electrodes are fused to the ends.
• ·        A side tube is connected to a vacuum pump.
• ·        The discharge tube is filled with a gas under study.
• ·         The two electrodes are connected to a source of high voltage (1000 V) And very low pressure .
• ·        Croockes said that at very low pressure, air being conductor of electricity.

 

OBSERVATIONS  and properties :

• ·        When the discharge tube containing the gas is at 1 atm pressure and at high voltage, the gas remains non conducting.
• ·        When the pressure of the gas inside the tube is less than 1 mm of mercury, a dark space appears near the cathode.  This is known as Crookes dark space.
• ·        When the pressure is reduced to 0.01 mm Hg, it fills the entire tube.
• ·        When the pressure is further reduced to 10^-4 mm Hg, the electric discharge passes between the electrodes  wall of  the tube  coated with ZnS (fluorescent screen) begins to glow.
• ·       

	This is due to the striking of some invisible rays from the cathode.

• ·        These rays which start from the cathode and move towards the perforated anode, so called cathode rays.
•    • They move in straight lines.
• • These rays are called cathode rays or cathode ray particles.
• • These rays deflect towards anode. These rays consist of negatively charged material particles called electrons.
• • Properties of cathode rays don’t depend upon cathode material or gass filled in discharge tube.
• • Cathode rays produce mechanical effects.
• • It indicates that cathode rays themselves are negatively charged.
• • When a magnetic field is applied perpendicular to the path of cathode rays, they  get deflected in the direction expected for negative particles. This direction of deflection shows that cathode rays are –vely charged.
• When cathode rays are allowed to strike a thin metal foil, it gets heated up.
•  Cathode rays produce, x-rays when they strike heavy metals like tungsten, copper etc.
•  Cathode rays produce fluorescence on glass walls, ZnS etc.
•  Cathode rays penetrate through thin metallic foils.
•  Cathode rays affect photographic plates.

DETERMINATION OF CHARGE TO MASS RATIO (e/m) OF ELECTRONS

•  In 1897 J.J. Thomson determined the e/m of the electron.
•  He measured the deflection of cathode rays under the simultaneous influence of electric and magnetic fields.
•  A narrow beam of cathode rays is generated by electric discharge in a gas at low pressure.
•  If an electric field is applied at right angles to the beam, the beam having negative charge is attracted to the positive plate of the field.
•  the beam thus travels a parabolic path and strikes at another point on the screen.
•  A magnetic field is now applied to the beam in a direction at right angles to that of the electric field.
•  The beam undergoes deflection in the opposite direction.
•  Deflection ∝ strenght of magnetic feild and electric field. 

                Deflection ∝ magnitude of negative charge. 

                Deflection ∝ 1/ mass of particles.  

•   The strength of the two fields are so adjusted that the beam strikes the screen at the original position.
• 
  
  
•  Lighters the particle greater the deflection in the field.
•  As voltage of electric field and strength of magnetic field increase deviation from orginal pathway increases .

           e – charge of electron

           Me – mass of electron

Charge on the electron

 

The charge of an electron was determined by Robert Millikan in 1909 by oil drop

experiment.

• ·        In this method, a spray of oil droplets is produced by an atomizer.
• ·        The oil droplets enter the apparatus through a small hole.
• ·        It allowed to fall in between two charged plates.
•         The motion of the droplets is observed with a telescope.
• ·        The space between the charged  plates is irradiated with x-rays.
• ·        The x-rays ionize the molecules of the air.
• ·        One or more electrons produced may be absorbed by an oil droplet.
• ·        The oil droplet as a result becomes negatively charged.
• ·    By measuring the velocity of a given oil droplet as it falls freely under the influence of gravity and then in an electric field, it is possible to calculate the charge on the droplet. Q= n e
• ·    This charge is considered to be electronic charge.
• ·     The charge on the electron is found to be - 1.602 x 10⁻¹⁹ coulombs.