Monday, 27 November 2017

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Atomic Structure: Definition, History and theories

     Introduction of Atom

Introduction to atom

Define of Atom:-

  The Greek philosopher Democitus (460B.C – 370 B.C) was among the first to suggest the existence of Atoms (from the Greek word “atoms” means indivisible).

He believed that atoms were indivisible and indestructible. His ideas dis agree with later scientific theory, but did not explain chemical behaviour, and was not based on the scientific method, but just philosophy.

Dalton’s Method:-

 John Dalton took what was known about chemical reactions at his time and proposed the first atomic model.

Conservation of Mass

  • Law of Multiple Proportions
  • Law of Definite Composition

Billiard Ball Model:- 

Dalton combined the observation into one theory which stated that all matter was composed of small indivisible particles that he called atoms.

Demitri Mendeleev used this theory when he constructed the first working periodic table.

Dalton’s Atomic Theory (experiment based):-

  1. All elements are composed of tiny indivisible particle called atoms
  2. Atoms of the same element are identical. 
  3. Atoms of any one element are different from those of any other element.
  4. Atoms of different elements combine in simple whole-number  rations to form chemical compounds. E.g CO2

In chemical reactions, atoms are combined, separated, or rearranged, but never changed into atoms of another element.

Sizing up to the Atom:-

 Elements able to be subdivided into smaller and smaller particles, those are the atoms, and they still have properties of that element
If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1cm long.

Despite their small size, individual atoms are observable with instruments such as scanning tunnelling microscopes.

Cathode Ray:-

 Crookes worked in the areas of chemistry and physics. He had many accomplishments, one of which was the discovery of cathode rays.

Crookes Tube:-

A source of high potential difference was placed across the cathode of a glass tube that had gas at a very low pressure inside.
Noticed a glow coming from the negative terminal.

Properties of Cathode Ray:-

A wide variety of cathodes (different metals) were tested and all produced same results.

Magnetic fields deflected the rays.
The rays produced some chemical reactions similar to those produced by light.

The rays travelled in straight lines, perpendicular to the surface of the cathode.


Electron means “amber” in Greek
Properties discovered by the Greek Thales of Miletos 600 BC. Rubbed the mineral amber with cat fur and attracted feathers.

J. J. Thomson discovered the electron while experimenting with cathode rays. In 1897, J. J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: The Electron.

Cathode Ray:-  
Thompson showed that the production of the cathode ray was not depend on the type of gas in the tube, or the type of metal used for the electrodes. He conludes that these particles were part of every atom.

Cathode ray

Thomson’s Charge to Mass Ratio:- It was noticed that the beam of electrons be bent by a magnetic field. This means that: Fnet = Fm
mv2 = Bqvr
so, q/m = v/Br.


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