The History of Understanding Atoms

A brief history of our understanding of...

ATOMS

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For MOST of human history, people had NO IDEA what made up all matter.

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This is quite understandable since what makes up matter is way too tiny to see with the human eye! They did not have the tools needed to solve the mystery.

What am I made of? What is that rock made of? What is my cat made of?

Continue on the cat to continue.

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This timeline shows some of the ideas and discoveries about what makes up matter.

J.J. Thomson discovers electrons

Democritus' Atomos

Dalton's Atomic Theory of Matter

Many more discoveries about the atom!

Belief in 4 Elements

Rutherford's model of the atom

Today

OVER 2,000 years pass without any significant discoveries

1803

1897

~400 BCE

1911

Click on the image along the timeline that shows what many people living before 400 BCE believed made up all matter.

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Yes! About 2,500 years ago, many people believed an idea by a philosopher named Empedocles. He suggested that all matter was made of different combinations of earth, water, fire, and air.

The Four Elements

Earth

Water

Fire

Air

That rabbit is about half fire and half water.

Click on me to continue.

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WAY before anyone had a way to verify what makes up matter, a guy named Democritus challenged the idea that everything was made of earth, air, water, and fire.

I have a different idea!

Click on Democritus to continue.

He didn't have the tools and technology to find out, so all he could do was think about it. But, his thinking was on the right track!

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What would be left if I kept halfing this rock until it couldn't be divided anymore?

Democritus wondered what would be left if an object was continually cut in half until it could not be cut in half anymore. He thought that there must be a basic unit of matter.

Democritus concluded that all matter was made up of indivisible particles that are too small to see. He called these particle "atomos," which means indivisible.

What does the term "atomos" mean?

tiny

matter

indivisible

Democritus - painting by Diego Velazquez (1640)

Atoms can be broken down into even smaller particles.

That's right. "Atomos" means indivisible.

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Democritus' ideas about atoms were not totally correct.Atoms can be broken down into even smaller particles.

BUT, his idea that all matter is made up of extremely small particles was correct!

Who's awesome? This guy!

All matter is made up of tiny particles called _________.

atoms

pixels

mobs

Democritus - painting by Diego Velazquez (1640)

After Democritus' ideas, it took over 2,000 years for people to figure out more about atoms!

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J.J. Thomson discovers electrons

Democritus' Atomos

Dalton's Atomic Theory of Matter

Many more discoveries about the atom!

Belief in 4 Elements

Rutherford's model of the atom

Today

OVER 2,000 years pass without any significant discoveries

1803

1897

~400 BCE

1911

Click on the part of the timeline that shows when people started to make progress in understanding atoms again after Democritus.

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In 1803, John Dalton proved that all matter is made from atoms by observing the volumes and masses of gases formed in chemical reactions. Like Democritus, he incorrectly believed atoms couldn't be broken down into smaller parts. But, Dalton did correctly conclude that there are different kinds of atoms, called elements and that all atoms of a given element have identical properties.

Atoms are like ice cream flavors- there are many kinds...each with their own properties!

John Dalton

Click here to open an interactive about the experiments that led to Dalton's Theory.

Charles Turner, Public domain, via Wikimedia Commons. This image has been altered.

Each kind of atom is called ____________.

a flavor

an element

an enderman

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Dalton's discoveries were very exciting!

Yes! And there are different kinds of atoms. Each kind of atom is an element that has its own unique properites!

Hey! Did you guys hear? Dalton and his team figured it out! We, and everything around us, are made of atoms!

I'm told this is what we think atoms probably look like. Indestructible solid spheres!

Click on the image that shows what people thought atoms probably looked like after Dalton's discoveries.

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What did Dalton mean when he suggested there are different kinds of atoms?

Click anywhere to check out 4 kinds of atoms (elements).

Each of these images shows a pure unique element. Each element is made from only one kind of atom that has unique properties.

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• A shiny metal
• Liquid at room temperature
• Very toxic!

Mercury

• Can form coal, graphite, or diamonds
• Can be found all living things!

Carbon

• Light gas
• Unreactive (noble gas)
• Clear

Helium

• Yellow solid
• Nonmetal
• Oderless but can smell like rotten eggs when it bonds with hydrogen to make a gas

Sulfur

Mercury

Carbon

Sulfur

Helium

All of the atoms in this pure sample are sulfur atoms.

Click on the element that is a clear and very light gas.

That's correct! At normal temperatures, helium is a clear and very light gas.

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• A shiny metal
• Liquid at room temperature
• Very toxic!

Mercury

• Can form coal, graphite, or diamonds
• Can be found all living things!

Carbon

• Light gas
• Unreactive (noble gas)
• Clear

Helium

• Yellow solid
• Nonmetal
• Oderless but can smell like rotten eggs when it bonds with hydrogen to make a gas

Sulfur

Mercury

Carbon

Sulfur

Helium

Click on the element that is a yellow solid.

You got it! Sulfur is a yellow solid.

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• A shiny metal
• Liquid at room temperature
• Very toxic!

Mercury

• Can form coal, graphite, or diamonds
• Can be found all living things!

Carbon

• Light gas
• Unreactive (noble gas)
• Clear

Helium

• Yellow solid
• Nonmetal
• Oderless but can smell like rotten eggs when it bonds with hydrogen to make a gas

Sulfur

Mercury

Carbon

Sulfur

Helium

Click on the element that is a shiny liquid at room temperature.

Yes! Mercury is a shiny liquid at room temperature.

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• A shiny metal
• Liquid at room temperature
• Very toxic!

Mercury

• Can form coal, graphite, or diamonds
• Can be found all living things!

Carbon

• Light gas
• Unreactive (noble gas)
• Clear

Helium

• Yellow solid
• Nonmetal
• Oderless but can smell like rotten eggs when it bonds with hydrogen to make a gas

Sulfur

Mercury

Carbon

Sulfur

Helium

Click on the element that can be found as coal, graphite, or diamonds.

That's right! Carbon is a kind of atom that can be found in the form of coal, graphite (inside pencils), or diamonds.

Back

• A shiny metal
• Liquid at room temperature
• Very toxic!

Mercury

• Can form coal, graphite, or diamonds
• Can be found all living things!

Carbon

• Light gas
• Unreactive (noble gas)
• Clear

Helium

• Yellow solid
• Nonmetal
• Oderless but can smell like rotten eggs when it bonds with hydrogen to make a gas

Sulfur

Mercury

Carbon

Sulfur

Helium

Dalton was right, there are many kinds of atoms, each with their own properties!

Of the four elements shown, click on the one you like the most.

Almost 100 years after Dalton's atomic theory, J.J. Thomson proved that atoms are divisible afterall! Using a cathode ray tube, he discovered negatively charged particles within atoms. He called these tiny particles electrons. He thought these electrons were floating around inside the atom within a positively charged area. It made him think of how plums are spread throughout plum pudding. So, he came up with a new model of the atom, which he called the plum pudding model!

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Click here for a video about how Thomson discovered electrons using a cathode ray tube!

I didn't think science could get any better...but now we've added dessert!

Plum Pudding Model of the Atom

JJ Thomson

The electrons are like the plums and the positively charged area is like the cake part.

Public domain, via Wikimedia Commons

Click on one of the negatively charged electrons in the diagram to continue.

Click here to cntinue.

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Most of the alpha particles did go straight through the gold atoms. BUT....a very small percentage of them bounced back or were deflected. This would not happen if

Click here to learn about Rutherford's gold foil experiment.

Woah, defintely NOT what I expected!

Ernest Rutherford

A thin sheet of gold atoms

Source of alpha particles

Where alpha particles land

Rutherford's Gold Foil Experiment

Click on the red circle that respresents where MOST of the alpha particles landed after traveling through the gold atoms.

Click here to learn about Rutherford's gold foil experiment.

In 1911, Earnest Rutherford aimed positively charged alpha particles through a thin sheet of gold atoms. He expected ALL of the bigger and heavier alpha particles to push straight through the protons and electons in the gold atoms...like how you would easily push past a room filled with balloons.

What will happen if I blast alpha particles at atoms?

Ernest Rutherford

A thin sheet of gold atoms

Source of alpha particles

Where alpha particles land

Rutherford's Gold Foil Experiment

Click on the red circle that respresents where MOST of the alpha particles landed after traveling through the gold atoms.

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Thomson's discovery of electrons was surprising!

Yeah, now we know there are particles with a negative CHARGE inside atoms! They are called electrons.

Woah! Woah! Woah! So atoms are NOT the smallest particle?

I'm told this is now what we think atoms probably look like ....like plum pudding.

Click on the image that shows what people thought atoms probably looked like after Thomson's discoveries.

Then in 1911, Earnest Rutherford developed a new model of the atom based on experiments that aimed alpha particles at a thin sheet of gold foil. He concluded that an atom is more than 99.999% empty space with a tiny, dense, positively-charged nucleus.

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Click here to learn about Rutherford's gold foil experiment.

I like dessert too...but my gold foil experiment disproves Thomson's plum pudding model.

Ernest Rutherford

Not to scale! The nucleus is way too tiny to show in a diagram easily.

Rutherford Model

Plum Pudding Model

Click on the postively-charged nucleus in the Rutherford Model of the atom.

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Not to scale! The nucleus is way too tiny to show in a diagram easily.

Rutherford Model

Plum Pudding Model

The plum pudding model of the atom is not correct. We know this because when Rutherford and others shot alpha particles through the thin sheet of atoms, ______________

some repelled away from the tiny dense nuclei.

they all traveled straight through the atoms.

Play with this PhET simulation. Compare what happens when positively charged alpha particles are shot through the two atom models.

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Rutherford's findings were mind-blowing!

So...all the mass of an atom in is the very tiny nucleus? And the rest is essentially ...empty space?

This is crazy! Apparently all matter is almost completely EMPTY SPACE!

I'm told this is now what we think atoms kind of look like .....except not to scale.

Click on the image that shows what people thought atoms probably looked like after Rutherford's gold foil experiment.

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What does it mean when we say atoms are almost completely empty space?

Click on the person to find out.

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If you magnified an atom to the size of a football stadium, the nucleus would be the size of a marble.Essentially all of the mass of the atom is in this tiny dense nucleus in the center of the atom.

Click on the marble to continue.

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But what about the electrons? Don't they have mass?

Click one of the electrons to find out.

Electrons have an extremely tiny mass compared to the particles in the nucleus.

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If the particles in an atom's nucleus (protons and neutrons) each had the mass of a baseball....

...the electrons would each have the mass of a pea.

Click on the pea to continue.

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This picture is definitely NOT to scale....not even close.

We draw the nucleus big in diagrams. If we drew it to the right scale, you wouldn't be able to see it.

All of the significant mass of the atom is in its tiny nucleus....which would be too small to show to scale in a diagram. The electrons are outside the nucleus and essentially have no mass.

Click on the nucleus in the diagram.

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Our understanding of matter has changed a lot. Let's do a quick review of the contributions covered in this lesson. Click on Democritus' spot on the timeline.

J.J. Thomson discovers electrons

Democritus' Atomos

Dalton's Atomic Theory of Matter

Many more discoveries about the atom!

Belief in 4 Elements

Rutherford's model of the atom

Today

OVER 2,000 years pass without any significant discoveries

1803

1897

~400 BCE

1911

Great work so far on learning about 4 major advancements in the understanding of matter!

Back

J.J. Thomson discovers electrons

Democritus' Atomos

Dalton's Atomic Theory of Matter

Many more discoveries about the atom!

Belief in 4 Elements

Rutherford's model of the atom

Today

OVER 2,000 years pass without any significant discoveries

1803

1897

~400 BCE

1911

Click here to breifly check out some other work that has been done or is being done to better understand matter.

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In 1913, Niels Bohr discovered that electrons travel in separate orbits around the nucleus and that the number of electrons in the outer orbit (valence electrons) determines the properties of an element.

Bohr Model of the Atom

Click on one of the outer orbit (valence) electrons in the diagram.

Schrödinger developed an equation that describes the movement of electrons by treating them as both particles and waves. He is also famous for this thought experiment called "Schrödinger’s cat" that has contributed the understanding of particles. This understanding has led to the development of computer chips... like the ones you are using to do this lesson!

Click on Schrödinger's cat .

Today, many scientists and engineers work at colliders, like the Large Hadron Collider in Switzerland and France . The scientists ram particles into each other at incredible speeds to try to learn more about smaller and smaller particles!

Continue

the existence of a nuclear atom - a small, positively-charged nucleus surrounded by empty space and then a layer of electron

As you can see, the work on understanding matter continues! For the purposes of this lesson, let's quickly review the work of Democritus, Dalton, Thomson, and Rutherfod.

J.J. Thomson discovers electrons

Democritus' Atomos

Dalton's Atomic Theory of Matter

Many more discoveries about the atom!

Belief in 4 Elements

Rutherford's model of the atom

Today

OVER 2,000 years pass without any significant discoveries

1803

1897

~400 BCE

1911

Continue

Back

Democritus ~400 BCE

Democritus' ideas were not widely accepted and progress stopped for thousands of years.

Democritus challenged the thinking of his time by suggesting that all matter was made up tiny __________ he called "atomos."

drops of water

spheres of air

pieces of dirt

indivisible particles

Everything is made of "atomos"...not Earth, Air, Fire, and Water!

The understanding of matter has changed so much!

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Dalton 1803

Democritus ~400 BCE

Dalton used experiments to show that everything is made of different kinds of atoms!

Democritus' ideas were not widely accepted and progress stopped for thousands of years.

Over 2,000 years later, Dalton used experiments to show that all atoms of a given ______ are identical in mass and properties.

location

element

color

state

Everything is made of "atomos"...not Earth, Air, Fire, and Water!

Atoms were thought of as solid spheres that did not contain any smaller particles.

At the end of the 1800s, J.J. Thomson showed that atoms contained smaller _________ particles called electrons.

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negatively- charged

delicious

colorful

sweet

Dalton 1803

Democritus ~400 BCE

Thomson 1897

Dalton used experiments to show that everything is made of different kinds of atoms!

Democritus' ideas were not widely accepted and progress stopped for thousands of year.

Thomson discovered the electron and created the Plum Pudding Model of the atom.

Plum Pudding Model of the Atom

Everything is made of "atomos"...not Earth, Air, Fire, and Water!

Atoms were thought of as solid spheres that did not contain any smaller particles.

The understanding of matter has changed so much!

About 14 years later, Rutherford disproved Thomson's Plum Pudding Model of the atom by showing that atoms are almost completely _______ space.

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fluid-filled

empty

charged

rock-solid

Dalton 1803

Democritus ~400 BCE

Thomson 1897

Rutherford 1911

Rutherford showedthat an atom has a dense, postivitely-charged nucleus and is almost completely empty space.

Dalton used experiments to show that everything is made of different kinds of atoms!

Democritus' ideas were not widely accepted and progress stopped for thousands of year.

Thomson discovered the electron and created the Plum Pudding Model of the atom.

Everything is made of "atomos"...not Earth, Air, Fire, and Water!

Atoms were thought of as solid spheres that did not contain any smaller particles.

Negative electrons embedded within a positively charged area

Tiny, dense positive nucleus

Rutherford's Model of Atom

Plum Pudding Model of Atom

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Of course, many other scientists have contributed and continue to contribute to our understanding of matter.

Great work on learning about some of the major contributions which help us know that:

• Matter is made from atoms.
• Elements are different kinds of atoms, each with unique properties.
• Atoms contain smaller charged particles.
• Atoms have a dense positive nucleus.
• Atoms are mostly empty space.

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Everything is made of ATOMOS!

Democritus

Dalton's experiments showed that all matter is made of different kinds of atoms. Each kind of atom (element) has unique properties.

Atoms are the building blocks of all matter!

Thomson's experiments showed that atoms contained small particles that had charge! He discovered the electron.

Plum Pudding Model of Atom

Rutherford showed that an atom is mostly empty space and its mass is in its postively charged nucleus.

Dalton's experiments showed that all matter is made of different kinds of atoms. Each kind of atom (element) has unique properties.

An atom was thought to be like a solid ball.

the existence of a nuclear atom - a small, positively-charged nucleus surrounded by empty space and then a layer of electron

Democritus came up with the idea of atoms over 2,500 years ago.

J.J. Thompson discovers electrons

Democritus' Atomos

Many more discoveries about the atom!

Dalton's Atomic Theory of Matter

Belief in 4 Elements

Rutherford discovers positive nucleus and that atom is mostly empty space

Today

OVER 2,000 years pass without any significant discoveries

1803

1897

~400 BCE

1905

Click on the part of the timeline that shows when people started to make progress in understanding atoms again after Democritus.