Structure Of Atom

Introduction

What is this section about?

This section discusses the historical development of the idea of atoms, from ancient philosophical concepts to the discovery of subatomic particles.

Simple Explanation:

The idea of atoms, as the basic building blocks of matter, is very old. It goes back to ancient Indian and Greek philosophers around 400 B.C. They believed that if you kept dividing matter into smaller and smaller pieces, you would eventually reach a point where you couldn't divide it anymore. They called these indivisible particles "atoms," which comes from the Greek word "a-tomio" meaning "uncuttable" or "non-divisible."

However, these early ideas were just guesses. There was no scientific way to test them. So, the idea of atoms remained mostly dormant for many centuries.

In 1808, John Dalton, a British school teacher, brought the atomic theory back into science with a solid scientific basis. Dalton's atomic theory stated that atoms were the smallest particles of matter. His theory was successful in explaining some important laws of chemistry:

• Law of conservation of mass: Matter cannot be created or destroyed in a chemical reaction.
• Law of constant composition: A chemical compound always contains the same elements in the same proportions by mass.
• Law of multiple proportions: ¹ When two elements form more than one compound, the ratios of the masses of one element that combine with a fixed mass of the other element ² are in small whole numbers.

However, Dalton's theory wasn't perfect. It couldn't explain some experimental observations, like how rubbing certain materials together creates static electricity (electrical charges).

Towards the end of the 19th century and the beginning of the 20th century, new experiments showed that atoms are not indivisible after all. They are made up of even smaller particles called subatomic particles:

• Electrons
• Protons
• Neutrons

This discovery completely changed the understanding of atoms from Dalton's time.

Simple Conclusion:

The idea of atoms as the fundamental building blocks of matter originated with ancient philosophers. John Dalton revived the atomic theory with a scientific basis, but his theory couldn't explain everything. Later experiments revealed that atoms are made of subatomic particles (electrons, protons, and neutrons), showing that atoms are not indivisible as originally thought.

Discovery of Sub-atomic Particles 

An insight into the structure of atom was obtained from the experiments on electrical discharge through gases. Before we discuss these results we need to keep in mind a basic rule regarding the behaviour of charged particles : “Like charges repel each other and unlike charges attract each other”.  (from NCERT)

Discovery Of Electrons

What is this section about?

This section explains how scientists discovered the electron using cathode ray tubes.

Simple Explanation:

In the mid-1850s, scientists were experimenting with electrical discharges in partially empty glass tubes called cathode ray tubes. These tubes had two metal pieces called electrodes sealed inside.

Here's how the experiments worked:

1. The air was partially removed from the tube to create a low-pressure environment.
2. A very high voltage was applied across the two electrodes.
3. A stream of particles was observed moving from the negative electrode (cathode) to the positive electrode (anode). These streams were called cathode rays.

To make these invisible rays visible, scientists made a hole in the anode and coated the back of the tube with a material called zinc sulfide. When the cathode rays hit the zinc sulfide, it glowed, creating a bright spot.

The experiments showed the following about cathode rays:

1. They travel from the cathode (negative electrode) to the anode (positive electrode).
2. They are invisible, but they can make certain materials glow (fluoresce or phosphoresce). This is how old TVs (with cathode ray tubes) worked.
3. They travel in straight lines unless a magnetic or electric field is present.
4. When a magnetic or electric field is applied, the rays bend in a way that shows they are made of negatively charged particles. These negatively charged particles are called electrons.
5. The type of metal used for the electrodes and the type of gas in the tube didn't change the properties of the cathode rays (electrons).

This led to the important conclusion that electrons are a fundamental part of all atoms.

Simple Conclusion:

Experiments with cathode ray tubes showed that when a high voltage is applied across two electrodes in a partially empty tube, a stream of negatively charged particles called electrons is emitted from the negative electrode (cathode). These electrons are a basic component of all atoms.