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Structure of Atom

Every substance around us looks solid and continuous. But science proved long ago that matter is not continuous. It has an internal structure.

Understanding the structure of the atom is important because it explains:

Why atoms combine
Why elements show different properties
Why chemical reactions happen

This chapter did not appear suddenly. It developed step by step through experiments. Each experiment corrected the previous idea.

Let us follow this journey logically.

Why Scientists Studied the Structure of Atom

Dalton’s atomic theory said that atoms were indivisible. This idea worked well for chemical reactions but failed to explain electricity, radiation, and particle behaviour.

When scientists began studying electricity in gases, they discovered that atoms contain smaller particles. This led to experiments with cathode rays and anode rays.

Cathode Rays: Evidence of Subatomic Particles

Cathode rays were discovered while studying the conduction of electricity through gases at low pressure.
When a high voltage was applied across electrodes in a discharge tube, rays were observed travelling from the cathode (negative electrode) to the anode. These rays were called cathode rays.

Properties of Cathode Rays

Experiments showed that cathode rays:

Travel in straight lines
Produce fluorescence on striking glass
Are deflected by electric and magnetic fields
Are attracted towards the positive plate

These observations proved that cathode rays are made of negatively charged particles. These particles were later named electrons.

Importance of Cathode Rays

Cathode rays proved that:

Atoms are divisible
Atoms contain negatively charged particles
Electrons are present in all atoms

This discovery completely changed Dalton’s idea of indivisible atoms.

Anode Rays: Discovery of Positive Particles

After cathode rays, scientists studied rays moving in the opposite direction. When holes were made in the cathode, rays were observed travelling from the anode towards the cathode.

These rays were called anode rays or canal rays.

Properties of Anode Rays

Anode rays:

Travel in straight lines
Are deflected by electric and magnetic fields
Carry positive charge
Have mass much greater than electrons

Unlike electrons, anode rays depend on the gas present in the discharge tube.

Significance of Anode Rays

Anode rays proved that:

Atoms contain positive particles
Positive charge is associated with the mass of the atom

These positive particles were later identified as protons.

Thomson’s Atomic Model: The First Structural Model

After discovering electrons, J. J. Thomson proposed a model to explain atomic structure.

According to Thomson’s model:

An atom is a positively charged sphere
Electrons are embedded in it
The total positive charge equals total negative charge

This model is often compared to a plum pudding.

What Thomson’s Model Explained

Electrical neutrality of atoms
Presence of electrons inside atoms

Limitations of Thomson’s Model

Thomson’s model could not explain:

How positive charge is distributed
Results of alpha-particle scattering experiment

To test this model, a new experiment was performed.

Alpha-Scattering Experiment: A Turning Point

Ernest Rutherford conducted the alpha-particle scattering experiment to test Thomson’s model. He bombarded a thin gold foil with fast-moving alpha particles.

Observations of the Experiment

Most alpha particles passed straight through the foil. 
Some were deflected slightly. 
A very few were deflected at large angles. 
One or two even bounced back.

These observations were surprising.

Conclusions from the Experiment

From these results, Rutherford concluded:

Most of the atom is empty space.
Positive charge and mass are concentrated at the centre.
The central region is very small but very dense.

This central region was named the nucleus.

Rutherford’s Atomic Model

Based on the experiment, Rutherford proposed a new atomic model.

According to Rutherford’s model:

Atom has a small, dense, positively charged nucleus
Electrons revolve around the nucleus
Most of the atom is empty space

Success of Rutherford’s Model

Explained alpha-particle scattering results
Established existence of nucleus

Limitations of Rutherford’s Model

Rutherford’s model failed to explain:

Why electrons do not fall into the nucleus
Stability of the atom
Atomic spectra

According to classical physics, revolving electrons should lose energy.
To solve this, a new model was proposed.

Bohr’s Atomic Model: Stability Explained

Niels Bohr modified Rutherford’s model using quantum ideas.
According to Bohr’s model:

Electrons revolve in fixed circular paths called orbits
Each orbit has fixed energy
Electrons do not lose energy while revolving in allowed orbits
Energy is absorbed or released when electrons jump between orbits

Importance of Bohr’s Model

Bohr’s model explained:

Stability of atoms
Atomic spectra of hydrogen
Energy levels of electrons

Limitations of Bohr’s Model

Bohr’s model works well only for:

Hydrogen atom
Hydrogen-like species

It fails for multi-electron atoms.

Discovery of Neutron: Completing the Atom

Even after discovering protons and electrons, atomic mass could not be explained fully.
In 1932, James Chadwick discovered a neutral particle inside the nucleus.
This particle was called the neutron.

Properties of Neutron

No charge
Mass nearly equal to proton
Present in the nucleus

Importance of Neutron Discovery

Neutrons explained:

Atomic mass properly
Existence of isotopes
Nuclear stability

With this, the basic structure of atom became clear.

How These Discoveries Fit Together

Cathode rays → electrons
Anode rays → protons
Alpha scattering → nucleus
Rutherford → nuclear model
Bohr → stable orbits
Neutron → mass explanation

Each step corrected the previous understanding.