How did Rutherford Discover the Nucleus?

Table of Contents

In the early 18th century, the atom was considered indivisible. But we know that atoms can be further divided into protons, neutrons, and electrons.

JJ Thomson and Ernst Rutherford proved that the atom is divisible by their discovery of the electron and the nucleus. So, how did Rutherford discover the nucleus?

By alpha scattering experiment, popularly known as the gold foil experiment.

Tracking the composition of matter
Tracking the composition of matter

Matter, Molecules, and Atoms

All living and non-living things in the Universe is made of matter. Anything that has mass and volume is matter.

Matter is of different size and shape. So, is there a fundamental unit of matter?

For example, all living things are made of cells. What is all matter made of?

Matter can be broken down into molecules, which are further made of atoms. Atom is the fundamental particle of matter and is made of protons, neutrons, and electrons.

Molecules of water
Matter to molecules, the amount of matter can change but its properties remain the same


A bucket, mug, puddle, or drop of water. Will they taste the same? Will water of different volumes boil at 100ᵒC? They most certainly will.

The volume of water changes but the properties remain the same. This is because they are all made of the same molecule – water.

Science Snippets: One drop of water has approximately quintillion (1,000,000,000,000,000,000,000) molecules of water

Let us get inside a molecule. We take a single water molecule from the quintillion molecules in a drop of water. What do we have? One water molecule is made of two hydrogen atoms and one oxygen atom.

Why do the properties remain the same? It is because they are made of the same kind of molecules.

H2O

What is an atom?

An atom is a fundamental unit of matter which has a central dense region called the nucleus. The nucleus has positively charged protons and neutrons with no charges. The negatively charged electrons revolve around the nucleus. The total number of protons in an atom is always equal to the total number of electrons. This makes an atom electrically neutral.

What is the fundamental unit of matter? Atoms

The structure of an atom

It took over 100 years of research and five models to arrive at the structure of an atom. The discovery of the electron and the nucleus helped in the structure of an atom. We shall discuss some of the models and the key experiments that led the journey to the structure of an atom.

Why is the atom electrically neutral? Because the number of protons is equal to the number of electrons

structure of an atom

Dalton atomic model

This is also called the Billiard ball model and initiated the research on atomic theory. According to this theory, all matter is made of small indivisible particles called atoms. Atoms can be neither created nor destroyed.

JJ Thomson’s atomic model

This is also called the Plum pudding model. JJ Thomson discovered electrons through the cathode ray tube experiment.

Cathode_ray_tube
Cathode ray tube, Credit: Sharon Bewick

Discovery of electrons by JJ Thomson

When a high voltage was applied between two electrodes in a cathode ray tube, a beam of particles traveled from the cathode to the anode.  Thomson named them cathode rays as they started from the cathode.

What are cathode rays?

Thomson wanted to determine the properties of cathode rays.  He placed positive and negatively charged plates in the path of the cathode rays. He observed that the cathode ray particles moved away from the negative plate and moved toward the positive plate. We know ‘Like charges repel each other, and unlike charges attract each other’. As cathode rays moved toward the positive plate, they must be negatively charged.  This led to the discovery of electrons by JJ Thomson.

What are the particles in cathode rays? Electrons

J. J. Thomson’s atomic model

This is also called the Plum pudding model. According to this model, an atom is a sphere of positive charge in which the electrons were embedded. The model failed with the discovery of the nucleus by Ernst Rutherford.            

J. J. Thomson’s atomic model

How did Rutherford discover the nucleus?

Rutherford and his colleagues Marsden and Geiger performed the alpha scattering experiment as follows:

1. Alpha particles with a positive charge equivalent to a helium atom, and a mass that was
7000 times greater than that of an electron was generated from radium.

2. A gold foil surrounded by a screen coated with zinc sulfide was placed in the path of the alpha particle.

3. When an alpha particle struck the screen, it would emit a light that can be detected.

4. A narrow beam of alpha particles was bombarded onto the gold foil placed in its path.

Rutherford’s gold foil
Gold foil experiment

Rutherford anticipated the alpha particles to pass straight through the gold foil and strike the zinc sulfide screen. Yes, most particles did pass through the foil without deflection. Some particles bounced slightly as they encountered the electrons.

Geiger-Marsden_experiment_expectation_and_result.svg
Geiger-Marsden experiment expectation and result, Credit: Kurzon

The most interesting observation was that one in 20,000 alpha particles bounced back at angles greater than 45 degrees.  Some of them almost bounced back to the radiation source. Rutherford suggested that this was possible when their path is blocked by a dense and heavy particle, This led to discovery of the nucleus. Rutherford proposed a new model for the atom.

Rutherford_atomic_planetary_model.svg
Rutherford atomic planetary model, Credit: Bensteele1995

According to this model

1. The atom is composed mostly of empty space.
2. The atom has a central, small, dense body called the nucleus.
3. The nucleus composed of positive charges.
4. The electrons revolve around the nucleus in orbits.

The Rutherford’s model was later replaced Bohr’s atomic model.

Conclusion:

The discovery of nucleus led to the structure of an atom. Ernst Rutherford won the Nobel prize for his discovery of the nucleus.

Flesch Kincaid Grade Level: 7.0

Flesch Kincaid Reading Ease: 64.2

References

Glossary

1. Atom: the smallest fundamental particle of an element that has all chemical properties of that element. They are composed of electrons, protons, and neutrons.
2. Nucleus: the core of the atom, where most of its mass and all of its positive charge is concentrated
3. Proton: a component of an atomic nucleus with a mass defined as 1 and a charge of +1
4. Electron: a particle that carries a negative electrical charge
5. Matter: anything that has mass and occupies space
6. Molecule: the simplest structural unit of a substance that still keeps the properties of that substance

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  • Sai Sudha, Ph.D.

    Dr. Sudha Purushothaman with a Ph.D. in Biochemistry has been fascinated by writing for a long, long time. She researches metabolism and its role in pluripotency. Her commitment to research kept her away from writing. Finally, during the pandemic she became a full-time science writer. She strongly believes that curiosity-driven learning harnesses the cognitive skills. She considers that writing is a way of leaving your fossil behind. She enjoys creativity in writing and believes that every article must have a storyboard. She needs her daily dose of yoga and recommends yoga and meditation for neural connectivity. The prime attraction in writing for Smore is the target audience, youngsters who need to be inspired to seek STEM as their career.