Rutherford's Model Of An Atom (1911):-

Experiment:-
Bombardment of alpha particles or Helium nuclei on the thin gold foil.

Discovery:-
Nucleus of atom and arrangement of particles.

Explanation:-
Rutherford bombarded large number of alpha particles on a thin gold foil and found their effect on a photographic film. Rutherford observed that almost all the rays pass through the gold foil un deflected, only few were deflected at an angle greater than 90 degree. On the basis of these observations Rutherford gave some points about structure of atom which are given below:

Main Points Of Rutherford's Atomic Model:-
• An atom consist on two parts, one central part called nucleus and other part.
• ‎protons are present in the nucleus of an atom, it has charge +Ze
• ‎Electrons revolve around the nucleus with very high speed. No: of electrons is represented by Z.
• ‎Most of the atomic volume is empty through which large number of alpha particles pass un deflected.
• ‎Nucleus has one millionth part of atomic volume.
• ‎All the mass of an atom is present in its nucleus.
• ‎Attractive force of nucleus on the electrons is counter balance by the centrifugal force due to motion of electrons, so the electrons remain stable at their position.
Weakness of Rutherford's Atomic model :
Or

Objections on the Rutherfors's Atomic Model:-

• According to the Maxwell's theory if a charged particle moves then it emit energy. Thus if an electron moves continously then it should emit energy continously and energy of its orbit go on decreasing and the electron come close to the nucleus untill it will fall in the nucleus and atom destroys, but it never happens.
• ‎If an electron emits energy continously during the motion then elements should give continous spectrum but all the elements give line spectrum. Rutherford failed to explain these objections.

X-Rays and Atomic Number :
(X-Rays discover by W. Roentgen in 1895)

Definition:-
Rays which emit through anode in Crooke's discharge tube when cathode rays strike with it are called X-Rays.

Explanation:-
William Roentgen observed that in crooke's dischrage tube when cathode rays strike with anode then some invisible rays emit through the anode. Roentgen called these rays as X-Rays.
X for unknown. These rays are also called Roentgen Rays.
X- Rays are only the waves of energy with very short wave lenght. These rays can also pass human easily flash without making holes and can affect the photographic film, so these are used in medical field to recieves the picture of inner parts of the body.

Study of X- Rays by Hennery Mosley in 1911:-
In 1911 Hennery Mosley found that by increasing the atomic mass of anode frequency of X- rays emiting through it increases. He found that it is due to inacrease in number of protons in the nuclei of atoms of anode. Mosley called the number of protons in the nucleus of an atom of an element is called its atomic number.

Atomic Number (Z):
Number of protons in the nucleus of an atom of an element is called its atomic number OR total number of electrons in an atom is called its atomic number. In an atom always number of protons and electrons are equal. Atomic number is represented by Z.

Bohr's Theory (Given by Neil's Bohr In 1913):-

Introduction:-
Bohr was a worker in Rutherford's laboratory. He gave a theory in 1913 to remove the points of this theory are given below:
• Electron revolves around the nucleus of an atom in circular orbit called stationary states of electrons.
• ‎In each stationary state or orbit electron has a definite energy level, so the stationary state or orbits are also called energy levels.
• ‎As a long as an electron remians in the same energy levels, its energy remains constant and neither it absorb nor emit energy.
• ‎When an electron absorb energy, then its jumps to the next higher energy level called excited state. When electron jumps back to its original energy level called ground state then it emit energy in the form of radiation.
• ‎Energy absorbed or emitted by electron during the transition between two energy levels is equal to the energy difference between two energy thus change in energy = E2 - E1 where E1 is the energy of lower level and E2 is the energy of higher level. Energy of radiation is given by the equation Change in energy = hv.
• ‎Angular momentum (mvr) of an electron in each circular orbit is equal to nh/2pie where n is the number of orbit.

Electronegativity (E.N):-
Power of an atom to attract bond pair of electrons toward it self is called electronegativity.
When two atoms share their electrons to form a chemical bond then nucleus of each atom attract the bond pair of electrons toward itself, this attraction of an atom on the electrons of a bond is called electronegativity.
Generally matels have low value of electronegativity and non metal have high value of electronegativity. In periodic table value of E.N decreases in a group from top to bottom and increases from left to right in a Period. E.N of noble gasses is zero because noble gasses does not share their electrons to form a bond.