Saturday, February 25, 2012

                         General Chemistry....
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  1. Atomic structure                 
  2. Periodic table          
  3. Bonding
  1. Chemical Bonds
  2. Inter- molecular forces
                                               Atomic structure
Cathode Rays
                          Cathode ray is a beam of electrons that travel from negatively charged end to positively charged end of vacuumed tube,across a voltage difference between the electrodes placed at each end.The electrode at the negative end is called a cathode; the electrode at the   positive end is called an anode
Properties  of the cathode rays
  1. Cathode rays come out at right angles to the surface of the cathode.                      VIDEO LINK
  2. Cathode rays move in straight lines.
  3. Cathode rays contain material particles having both mass and velocity. Crookes railway tube or paddle wheel circa 1950 VIDEO LINK
  4. Cathode rays contain the smallest unit of negatively charged.       VIDEO LINK
  5. Cathode rays produce X-ray when they strike a metal.

Mechanical properties of cathode rays video link

effects of electric field on cathode rays
6   Cathode rays  ionize a gas through which they pass                                                             



Atomic Models

Thomson 's model:-As a result of Thomson ' experiments,Thomson suggested that atoms consisted rings of negative electrons embedded in sphere of positive charge.The negative and positive charges balance and result in the being neutral.Thomson ' model for the atom was compared to a Christmas pudding,Plum pudding.

     Plum Pudding Model                       Rutherford's atomic model
                             Rutherford's model of the atom

                                       Atomic spectra 

Studies on atomic spectra provided the evidence for the arrangement of electrons and their energies in atom. There are two types of atomic spectra which are atomic emission spectrum and atomic absorption spectrum.

   How Bhor model explain Hydrogen Spectrum  
hydrogen spectrum series   hydrogen line spectrum emission and absorptionatomic spectra Balmer limit and continuum

                                                   shapes of atomic  orbitals
                     s-orbitals are spherical shape and p-orbitals are dumbbell shape.Each atomic orbital can hold  either one or a maximum of two electrons.If the orbital contains two 'paired-up' electrons they will spinning in opposite directions.
              s-orbital                           p-orbitals(dumbbell)   

The filling of atomic orbitals by electrons are governed by the three major guidelines.These are aufbau principle, Pauli's exclusion principle and Hund's rule.

  1. Aufbau principle:-Electrons are filled in various orbitals in the increasing order of their energies.
  2. Pauli'exclusion principle:-No two electron in an atom can have all the four quantum numbers identical.According to this principle ,each orbital can accommodate at the most two electrons with opposite spins.
  3. Hund's rule:-This rule operates when the filling of orbitals are carried out.According to this rule,electrons enter one by one till all of them are singly occupied with parallel spins.


                                                      Relative energies of different orbitals


In 1919,Aston invented the mass spectrometer.This gave chemists a reliable and accurate method of comparing the relative atomic masses of atoms.  There are five stages in mass spectrometer.                                                                                                        
  1. Vaporisation:-the sample of element is vaporised.
  2. Ionisation:- positive ions are obtained from the vapour.
  3. Acceleration:- the positive ions are accelerated by an electric field.
  4. Deflection:-the positive ions are deflected by a magnetic field.
  5. Detection:- the ions are detected and record is made.


                                           Periodic table 


                     Ionization Energies of Elements
The ionization energy of an atom is strongly influenced by three atomic parameters.

  1. The distance of the outermost electron from the nucleus.                                              As this distance increases,the attraction of the positive nucleus for the negative electron will decrease.
  2. The size of the positive nuclear charge.As the nuclear charge gets more and more positive,its attraction for the outermost electron increases and consequently the ionization  energy increases.
  3. The screening(shielding) effect of inner electrons.