ATOMIC STRUCTURE INTRODUCTION ONLY.pptx

ATOMIC STRUCTURE INTRODUCTION ONLY

1. 2.0 INTRODUCTION Atom is a Greek word and its meaning is Indivisible i.e. an ultimate particle which cannot be further subdivided. John Dalton considered that " all matter was composed of smallest indivisible particle called atom. Daltons Atomic Theory : This theory is based on law of mass conservation and law of definite proportions. The salient features of this theory are :- Every element is composed of extremely small particles called atoms. (2) Atoms of a particular element are like but differ from atoms of other element. (3) Atom of each element is an ultimate particle and it has a characteristic mass but is structureless (4) Atoms are indestructible i.e. they can neither be created nor be destroyed. (5) Atoms of different elements take part in chemical reaction to form molecule KEY POINTS Particles carrying negative charge were called negatrons by Thomson . The name negatron was changed to 'electron' by Stoney .

2. 2.1 ATOMIC MODELS Thomson's Model of Atom [1904] 1. Thomson was the first to propose a detailed model of the atom. 2. Thomson proposed that an atom consists of a uniform sphere of positive charge in which the electrons are distributed more or less uniformly 3. This model of atom is known as "Plum-Pudding model" or "Raisin Pudding Model" or "Water Melon Model". Drawbacks : 1. An important drawback of this model is that the mass of the atoms is considered to be evenly spread over that atom. 2. It is a static model. It does not reflect the movement of electron.

3. Rutherford's Scattering Experiment Rutherford observed that – Most of the α-particles passed through the gold foil undeflected. A small fraction of the α-particles was deflected by small angles. A very few α-particles (~1 in 20,000) bounced back, that is, were deflected by nearly 180°