Structure of Atom

 Introduction

 John Dalton

John Dalton said in 1808 that matter is made up of indivisible particle called atoms. Electron and proton are essential part of matter.
 The name electron was proposed by stoney.

 Other Scientists 

Various experiment done by J.J Thomson , GoldStein, Millikan confirmed the divisibility of atom. Millikan, in charge electron. 

Thomson, Model of atom.

 In 1904, J.J Thomson suggested "Plum pudding" model. According to this model an atom consists of a sphere of positive charge in which negative charged particles known as electrons are embedded. The electrons are embedded like the plum in a pudding, as shown in fig(2.1). But this model colud not satisfy the experimental facts and it had to be discarded. 

Rurtherford,s Atomic Model and Discovery of Nucleus.

 Lord Rutherford was a promising physicist of the time. He bombarded a thin sheet of gold of thickness 0.00004 cm with a_ particles. An a_particle is a helium (He) nucleus (He2+). 

A_particles were obtained from a radioactive element radium. A thin lead (Pb) plate with a hole cut in it, served to form a beam of a_particles. A ciricular screen coated with zinc sulphide (ZnS) was placed on the other side of the foil. It was observed by Rutherford that most of the a_particles passed straight through the gold foil. These a_particles caused tiny flashes on the zinc sulphide screen. Very few a_particles one out of ten thousands bounced back in the direction from which they came. Some were deflected at some angles after passing through the gold foil. It was concluded from this experiment that most of the space in an atom is empty. The scatterig of a_particles in all directions further revealed that there is a heavy positive charge at the centre of the atom which causes repulsion. 

 Conclusions of Rutherford,s Model of atom. 

(1) An atom is consisted of two parts I.e., nucleus and extra- nuclear part.
 (2) Nucleus is an extremely small positively charged it is situated at the centre of an atom.
 (3) The electrons which balance the positive charge are distributed in space around the nucleus.

 Defect of Rutherford model

 Rutherford planet like picture was defective and unstisfactory because the moving electron must be accelerated towards the nucleus.
 Therefore, the radius of the orbiting electron should become smaller and smaller and electron should fall into the nucleus . Thus an atomic structure as proposed by Rutherford would collapse. 

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