CHIMISTRY-STRUCTURE OF ATOM

Cathode ray discharge tube experiments: - 

1.     Cathode rays start from cathode and move toward anode.

2.     These rays are not visible but there behaviour can be observed with fluorescent or phosphorus sent material.

3.     In the absence of magnetic or electric field these travels in strait lines

4.     In the presence or magnetic or electric field the behaviour of cathode.

Cathode rays (electrons) do not depend on  the martial of the electrode and  nature of the gas tin the tube so electro us are basic constituent of all atoms.

 

Mass of electron

 From charge on e^- & e/m_e

 We get,                  

             M_e = 9.1094 ´ 10^-31 kG

DISCOVERY OF NEUTRON:

Discovered by Chadwick (1932).

  By bombarding a thin sheet of beryllium by alpha particles.

  Electrically neutral particles were emitted known as neutrons.

Rutherford’s nuclear model of atom

When beam of high energy α- particles was directed at gold foil then tiny flash of light observed at photographic plate.

Rutherford observed that-

1)    Most of the α-  practical  passed  through gold foil undeflected :

2)    A small fraction of α- particles was deflected by small angles.

3)    A very few α- particles (about 1 in 20000) bounced back means deflected by nearly 180⁰

^{CONCLUSION :}

1)    Most of space in atom is empty because most of α- particles passed 

undeflected.

2)    Few +ve charged α- particles were deflected.

Because + ve charge of the atom present in center in very small volume that repelled & deflected the +ve charged α- particles.

3)    Volume of nucleus is negligible as compared to total volume of atom

On the basis of observation &  conclusion Rutherford proposed model:

1)    +ve charge & most of mass present in the center of atom known as nucleus.

2)    Electrons moves around nucleus with very high speed in circular paths known as orbits.

3)    Electrons and nucleus (protons) are held together by electrostatic force of attraction 

Atomic number (Z) = no of protons in the nucleus of an atom

                                       = no of electrons in a neutral atom

Mass number (A) = number of protons (z) + number of electron (n)

Isobars :- 

              These are atoms with same mass number but different atomic no.

Isotopes: - 

               These are atoms with same atomic number but  different atomic mass no.

Planck’s Quantum theory:

Energy (E) of  quantum of  radiation is directly proportional to its frequency(n)

 i.e.            E=hn

Where,      h = planks constant = 6.626× 10^-34 js

Photo electric effect:-When a beam of light strike a metal surface then electrons were ejected. This phenomena is known as photo electric effect.

Number of electron ejected is directly proportional to intensity (or brightness) of light

     There is characteristic minimum frequency (n₀threshold frequency) below which photoelectric effect is not observed.

    If n > n₀ then electrons comes out with kinetic energy which increases with increase in frequency of light.

Kinetic energy of ejected electrons is given by-

h n = h n₀+ ½(m_eV²)