Chemistry: Atomic Structure

This chapter teaches about how various components make an atom and how they behave and what is the secret behind such structure of atoms.

• In 1804, John Dalton provided Atom's Postulations and it is called Dalton's Atomic Theory
• The word 'Atom is derived from the Greek word which means 'indivisible'. That means it cannot be further broken down into ["atomos"]

Postulates of Dalton's Atomic Theory

1. All matters are composed of very minute particles which are called atoms.
2. Atom can neither be created nor be destroyed by any physical and chemical method.
3. The atom of the same elements is identical in all respect. That means their shape, size,... are same.
4. Atoms of different elements are different in all respect.
5. Atoms of same or different elements combines in a fixed ration or simple whole number to give compound atom.

Discovery of Electron

Cathode Ray Experiment

Towards the end of the 18th century, JJ Thompson performed cathode rays experiment with the discharge tube. The discharge tube is a cylindrical hard glass tube connected with two metal electrodes and provided side tubes to evacuate the gas. Then he passed 1000V across the metal electrodes. Then, he started to evacuate the gas. When the pressure reaches to 0.0001 mm of Hg, greenish color rays appeared from the cathode which is cathode rays.

Properties of Cathode Rays

1. When the substance is kept in front of cathode rays, it casts a shadow which cathode rays travel in a straight line.
2. When the paddle wheel is kept in front of cathode rays, it rotates which shows that cathode rays consist of particles called electrons.
3. When electrical field is applied on cathode rays, they deflected towards positive plate/anode. That shows electron posses negative charge.