2020, സെപ്റ്റംബർ 9, ബുധനാഴ്‌ച

CHEMISTRY Class 9 Atomic structure part 1

 CHEMISTRY 

ATOMIC STRUCTURE  

1 DALTON ATOMIC THEORY

  • Matter is made up of minute particles called atom
  • Atom cannot be divided during chemical reactions.Atoms can neither be created nor destroyed.
  • Atoms of the same elements are identical in properties,size and mass.
  • Atoms of different elements differ in their properties and mass. 
  • Atom is the smallest particle that can take part in chemical reactions.
  • Compounds are formed when atoms of two or more elements combine in a small ratio.
2 Discovery of electron -Discharge tube experiment -Conducted by J J Thomson

The  experiment conducted by using a simple apparatus known as discharge tube. It consists of a sealed glass tube of about 50 cm legth with two metal electrodes fused to the ends and a side tube connected to a vacuum pump.The discharge tube is filled with the gas under study and two electrodes are connected to a high voltage DC source.

At normal temperature and the pressure, electric current does not pass through the gases. Due to this,  pressure in the discharge tube decreases . Connect a battery to the induction coil for getting DC voltage. The electrode which is connected to the positive terminal of the electrode has positive change and the electrode connected to the negative terminal has negative charge. 

JJ Thomson noticed that, when electricity was passed through the discharge tube at low pressure, the gas walls of the tube at side of the positive plate (anode)were glowing.He found out that some particles formed inside the tube caused this glow.

In the discharge tube,some invisible rays flow from negative electrode to positive electrode (Cathode to Anode). These invisible rays make the fluorescence which is the reason for  glowing.
If a light paddle wheel of a papers is placed in the discharge tube,the paddle wheel rotates. This indicates that cathode rays consist of material particles and produced mechanical effect.

Whenever an object is placed in the cathode rays.It causes a shadow on the walls of the opposite to the negative electrode.This experiment showed that cathode rays travels in straight.


 When the  beam of cathode rays are allowed to pass between electrically charged plate,they are deviated towards the positively charged plate.This shows that particle in the cathode rays are negatively charged. 

3 Inference arrived by J J Thomson

  • The cause of the glow is the invisible rays emitted from the negative metal plate of the discharge tube
  • It is a flow of particles with negative charge 
  • There are particles carrying negative charge in all substances

4 ELECTRON

Electron is the first identified particles in an atom.The existence of electrons in an atom was shown by J J Thomson by passing electricity at high voltage through a gas at low pressure.This great discovery proved Dalton Atomic Theory was wrong.

Charge of an electron          -  Negative 



     charge           -                   1.6 x 10^-19 C


Mass                 -                       1/1837th                                      mass of Hydrogen atom
                                               9.1x10^-31 Kg                                     (9.1 x 10^-28 g )
Thomson model of atom(plum pudding model)
                                
                                
• The negatively charged electrons are embedded in over a sphere of positive charge. 
• The total number of units of positive charge of the sphere are equal to the total number of negatively charged electrons. Hence an atom is electrically neutral. 

Proton

The presence of positively charged particles in an atom was shown by Goldstein by discharge tube experiment.Earnest Rutherford performed experiment by passing alpha particles through thin gold sheet.Through this experiment ,he proved that the entire positive charge of an atom concentrated at a particular point and this point is called nucleus.

Observation and inference of Rutherford gold foil experiment


describe rutherford s scattering experiment - Chemistry -  TopperLearning.com | jmosgwll


Observation                                              

Most of the alpha particles passed through the gold foil without any deviation.                          . Some alpha particles deviated in small angles  
.Only small number of alpha particles bounced back.   
Inference 
Most of the space in an atom is empty
. Since positively charged alpha particle are          deflected,we can infer that there are +vely            charged parts in an atom.These positively            charged parts are concentrated in a small
  space in atoms.      
 There is a small center in an atom which     
     consists of the entire mass and positive charge.The positive charged alpha particles moving towards this center reflect.

Proton

Charge        : positive charge (1.6 x 10^-19 C)

mass            : Same as Hydrogen atom (1.67 x 10^-27 kg)

Rutherford atom model

The structure of the atom presented by Rutherford is similar to that of the solar system. This model is known as the planetary model of atom.


Limitations of Rutherford atom model 

According to electro magnetic theory put forward by James Clerk Maxwell, a charged body in circular motion continuously emits energy as radiations. Electrons which revolve around the nucleus are negatively charged. Hence, while electrons revove around the nucleus, they should emit electromagnetic radiations, lose energy and come closer tothe nucleus. Finally the negatively charged electrons should collapse in to the nucleus. But this does not happen in an atom.Rutherford model failed to explain this.

Neutron

James Chadwick confirmed the presence of neutral particle within the nucleus of an atom. These particles are called neutron. The mass of neutrons determined is slightly higher than that of protons. 




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2021, മേയ് 22 6:12 AM ല്‍, Blogger Unknown പറഞ്ഞു...

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