Atom
1. What is an atom?
An atom is the smallest unit of matter that still keeps the properties of an element. Everything around us — air, water, plants, metals, and even our bodies — is made of atoms.
Atoms are extremely small and cannot be seen with the naked eye.
2. Structure of an atom (basic idea)
An atom has two main parts:
Nucleus (center of the atom)
Contains protons (positive charge)
Contains neutrons (no charge)
Almost all the mass of the atom is here
Electrons (around the nucleus)
Have negative charge
Move around the nucleus in the surrounding space
3. What is an atom like?
Most of an atom is empty space
The nucleus is very small but very dense
Electrons are much lighter than protons
Atoms are electrically neutral, meaning:
Number of protons = number of electrons
4. How did scientists come up with models of the atom?
Scientists could not see atoms directly, so they:
Performed experiments
Observed results
Created models to explain atomic behavior
Each new experiment improved the atomic model.
Rutherford’s Gold Foil Experiment
Scientist
The experiment was performed by Ernest Rutherford in 1911. He was studying the structure of the atom and wanted to test whether J. J. Thomson’s Plum Pudding Model was correct.
Aim of the Experiment
The main aim of Rutherford’s experiment was to understand how positive charge and mass are distributed inside an atom.
Materials and Apparatus Used
A radioactive source (such as radium or polonium) that emitted alpha particles
A lead block with a narrow slit to form a thin beam
A very thin sheet of gold foil
A fluorescent zinc sulfide screen surrounding the gold foil
A microscope to observe flashes on the screen
What are Alpha Particles?
Alpha particles are positively charged
They are heavy and fast-moving
They have high energy
They were chosen because they can penetrate thin metal sheets
Procedure of the Experiment
Alpha particles were released from the radioactive source.
The lead slit focused them into a narrow beam.
This beam was directed toward the thin gold foil.
The particles struck the gold foil.
After passing through the foil, the particles hit the zinc sulfide screen.
When alpha particles hit the screen, small flashes of light were seen.
Observations of the Experiment
Rutherford observed three main results:
Most alpha particles passed straight through the foilThis showed that most of the atom is empty space.
Some alpha particles were deflected through small anglesThis showed that there is a positive charge inside the atom.
A very few alpha particles were deflected backThis showed that the positive charge and mass are concentrated in a very small and dense region.
Conclusions Drawn by Rutherford
From these observations, Rutherford concluded:
An atom is mostly empty space
All the positive charge of the atom is concentrated in a tiny central nucleus
The nucleus is very small but very dense
Almost all the mass of the atom is in the nucleus
Electrons move around the nucleus
Why Gold Foil Was Used
Gold is highly malleable
It can be beaten into extremely thin sheets
Thin foil allows alpha particles to pass through easily
Rutherford’s Atomic Model (Result of the Experiment)
Based on the experiment, Rutherford proposed:
A small, positively charged nucleus at the center
Electrons revolving around the nucleus
Large empty space between nucleus and electrons
This model is called the Nuclear Model of the Atom.
Limitations of Rutherford’s Experiment and Model
Could not explain why electrons do not fall into the nucleus
Could not explain atomic spectra
Needed improvement, which later came from Bohr’s model
Importance of the Experiment
Proved that Thomson’s model was incorrect
Discovered the nucleus
Changed the understanding of atomic structure
Laid the foundation for modern atomic theory
J. J. Thomson’s Atomic Model
(Plum Pudding Model)
Background
After discovering the electron in 1897, J. J. Thomson realized that atoms were not indivisible. Since electrons carry a negative charge, he concluded that there must also be some positive charge inside the atom to balance it.
To explain this, he proposed a new atomic model.
Name of the Model
Thomson’s model is called the Plum Pudding Model.
In some books, it is also compared to a raisin cake model
The name comes from a popular English dessert called plum pudding
Description of Thomson’s Model
According to J. J. Thomson:
The atom is a uniform sphere of positive charge
The positive charge is spread evenly throughout the atom
Electrons (negative charges) are embedded inside this positive sphere
The total positive charge is equal to the total negative charge
Because of this balance, the atom is electrically neutral.
Explanation Using the Plum Pudding Analogy
The positive charge is like the soft pudding
The electrons are like plums or raisins
The electrons are fixed in position, not moving freely
The atom has no empty space according to this model
Important Features of Thomson’s Model
Atom is spherical in shape
Positive charge is spread uniformly
Electrons are scattered throughout the atom
No nucleus is present
Atom as a whole is electrically neutral
Why Thomson Proposed This Model
To explain the presence of electrons inside atoms
To explain how atoms remain neutral
To improve the idea of atoms being indivisible solid spheres
Significance of Thomson’s Model
First atomic model to include subatomic particles
Proved atoms have internal structure
Important step in the development of atomic theory
Limitations of Thomson’s Model
Thomson’s model failed to explain several observations:
Could not explain Rutherford’s Gold Foil Experiment
Failed to explain the concentration of positive charge
Did not explain atomic stability correctly
No explanation for the nucleus
Because of these limitations, Thomson’s model was later replaced by Rutherford’s model.
Final Short Summary
J. J. Thomson discovered the electron
He proposed the Plum Pudding Model
Atom = positive sphere + embedded electrons
Model explained neutrality but failed in experiments
Led to the discovery of the nucleus
J. J. Thomson’s Experiment
(Cathode Ray Tube Experiment)
Aim of the Experiment
The aim of J. J. Thomson’s experiment was to study the nature of cathode rays and to find out whether atoms contain smaller particles.
Apparatus Used
A glass discharge tube
Two metal electrodes:
Cathode (negative electrode)
Anode (positive electrode)
A high-voltage power supply
A vacuum pump (to remove air from the tube)
Electric and magnetic fields
Description of the Cathode Ray Tube
The tube was sealed at both ends
Most of the air inside the tube was removed
Low pressure allowed electricity to pass through the gas
Electrodes were connected to a high voltage source
Procedure of the Experiment
A very high voltage was applied across the electrodes.
When electricity passed through the tube, rays were produced from the cathode.
These rays traveled in straight lines toward the anode.
When the rays hit the glass wall, a greenish glow appeared.
These rays were called cathode rays.
Observations Made by J. J. Thomson
Cathode rays always originated from the cathode.
They traveled in straight lines.
They produced fluorescence when they struck the glass.
When an electric field was applied:
Rays bent toward the positive plate.
When a magnetic field was applied:
Rays were deflected again.
The behavior of the rays was the same:
For different gases
For different electrode materials
Conclusions of the Experiment
From these observations, J. J. Thomson concluded:
Cathode rays are made of negatively charged particles
These particles are present in all atoms
These particles are much smaller than atoms
These particles have a very small mass
He named these particles electrons.
Importance of Thomson’s Experiment
Proved that atoms are divisible
Discovered the first subatomic particle
Changed the idea of atoms being indivisible
Laid the foundation for modern atomic theory
8. Limitations of Thomson’s model
Could not explain:
How positive charge is arranged
Results of later experiments
No nucleus included
Because of this, a new experiment was needed.
9. Rutherford’s Gold Foil Experiment
Scientist:
Ernest Rutherford (1911)
Experiment setup:
Thin sheet of gold foil
Beam of alpha particles directed at the foil
Screen around foil to detect particles
10. Observations of the experiment
Most alpha particles passed straight through
Some were deflected slightly
A very few were reflected back
11. Conclusions from the experiment
Rutherford concluded:
Atom is mostly empty space
Positive charge and mass are concentrated in a tiny nucleus
Electrons move around the nucleus
12. Rutherford’s Atomic Model
Nucleus at the center (positive, dense)
Electrons revolve around the nucleus
Replaced Thomson’s model