The History of Atomic Chemistry: Crash Course Chemistry #37

Summary

This script explores the evolution of atomic theory, from the ancient Greek philosophers Leucippus and Democritus who first proposed the concept of indivisible 'atomos', through to modern quantum theory. It highlights key scientific figures like Dalton, Lavoisier, Thompson, Rutherford, Bohr, and Heisenberg, each contributing to our understanding of atomic structure. The narrative underscores the scientific process of discovery, emphasizing that while our current models are sophisticated, the quest for knowledge is ongoing, encouraging viewers to stay curious and engaged in science.

Takeaways

• 😲 The concept of atoms was first introduced by Leucippus and Democritus around 2500 years ago, who thought that matter is composed of indivisible particles they called 'atomos'.
• 🔬 Over time, atomic theory evolved through the contributions of many scientists, with significant advancements coming from Antoine Lavoisier's law of conservation of mass and James Dalton's atomic theory.
• 🌌 The discovery of the electron by J.J. Thompson using cathode-ray tubes marked a major step in understanding atomic structure, leading to the 'plum pudding model' of the atom.
• 💥 Ernest Rutherford's gold foil experiment revealed the existence of a concentrated positive charge in a small area, which he called 'the nucleus', and indicated that most of the atom is empty space.
• ⚛️ Rutherford's experiments also led to the discovery of the proton, a fundamental particle within the atomic nucleus.
• 🌐 Niels Bohr developed a model of the atom where electrons orbit a central nucleus in specific energy levels, which was a significant but flawed step towards the modern understanding of atomic structure.
• 🚀 Werner Heisenberg's uncertainty principle fundamentally changed the way we understand electrons, suggesting that their position and momentum cannot both be precisely known, leading to the quantum theory of the atom.
• 🌀 The modern quantum model of the atom describes electrons in terms of probability, with regions of higher probability known as 'orbitals', which are crucial for understanding chemical bonding.
• 🔮 Atomic theory, as we know it today, is the result of thousands of insights and experiments, and while it is highly sophisticated, it may not be entirely correct, emphasizing the importance of continued inquiry and experimentation.
• 📚 The script highlights the importance of learning from the history of scientific discovery, the process of building upon previous knowledge, and the role of mathematics in developing and understanding atomic models.
• 🔬 The script encourages viewers to pay attention to their studies in chemistry and physics, as these fields are essential for furthering our understanding of the atomic structure and its implications.

Q & A

Who were the first philosophers to propose the concept of atoms?

The first philosophers to propose the concept of atoms were Leucippus and his pupil Democritus, Greek philosophers from around 2500 years ago.

What is the meaning of the word 'atomos'?

The word 'atomos' means uncuttable or indivisible, reflecting the early belief that atoms were the smallest particles of matter that could not be divided further.

What was the significance of Antoine Lavoisier's contribution to atomic theory?

Antoine Lavoisier, a French chemist, proposed the law of conservation of mass, which states that the mass of matter remains constant even when it changes shape or form.

Who is credited with determining that elements exist as discrete packets of matter?

James Dalton, an English teacher, is credited with determining that elements exist as discrete packets of matter.

What did J.J. Thompson discover about the cathode rays in his experiments with discharge tubes?

J.J. Thompson discovered that cathode rays were not rays or waves, but were actually very light, very small negatively-charged particles, which he called 'corpuscles' and are now known as 'electrons'.

What model of the atom did J.J. Thompson propose, and what was its basis?

J.J. Thompson proposed the 'plum pudding model' of the atom, which suggested that negatively charged electrons were distributed randomly in a positively charged matrix, similar to fruit in a cake.

What was Ernest Rutherford's experiment that led to the discovery of the atomic nucleus?

Ernest Rutherford conducted an experiment using a thin sheet of gold foil and a screen coated with zinc sulfide, bombarding the sheet with alpha particles. The deflection of some particles at large angles indicated the presence of a concentrated positive charge in a small area, which he called 'the nucleus'.

What fundamental particles did Rutherford discover when he bombarded nitrogen with alpha particles?

When Rutherford bombarded nitrogen with alpha particles, he discovered the creation of hydrogen ions, which he correctly identified as protons, fundamental particles with a positive charge.

What was Niels Bohr's contribution to the understanding of atomic structure?

Niels Bohr applied mathematical principles from Max Planck and Albert Einstein to Rutherford's atomic model, predicting the most likely positions of electrons within the atom and proposing the planetary model, which depicted electrons in orbits around a central nucleus.

What is the Heisenberg Uncertainty Principle, and how does it relate to the understanding of atomic structure?

The Heisenberg Uncertainty Principle states that it is impossible to know with certainty both the momentum and the exact position of an electron or any subatomic particle. This principle led to the quantum theory, which describes the arrangement of electrons around a nucleus in terms of probability, rather than fixed orbits.

What is the modern understanding of atomic structure, and how is it represented visually?

The modern understanding of atomic structure is based on the quantum theory, which describes electrons as having properties of both particles and waves. This is represented visually as 'orbitals' or 'cloud models', where the intensity of color indicates the probability of finding an electron in a particular position.

Outlines

00:00

🔬 The Evolution of Atomic Theory

This paragraph traces the evolution of atomic theory, beginning with the ancient Greek philosophers Leucippus and Democritus, who first proposed that matter is composed of indivisible particles called 'atomos.' It highlights how their ideas, though primitive and speculative, laid the groundwork for future scientific inquiry. Over time, this initial concept of atoms evolved through rigorous experimentation and theoretical developments, eventually leading to the sophisticated understanding of atomic structure we have today.

05:04

🧪 Early Experimental Discoveries

This section delves into key developments in atomic theory during the 19th and early 20th centuries. It starts with Antoine Lavoisier's law of conservation of mass and James Dalton's atomic theory, which established the existence of discrete packets of matter called elements. The narrative then moves to the experimental discoveries using discharge tubes that led to the identification of cathode rays (electrons) by J.J. Thompson and the later discovery of the positively charged proton by Eugen Goldstein and others. These discoveries marked significant progress in understanding atomic structure.

🍮 The Plum Pudding Model and Beyond

This paragraph describes J.J. Thompson's 'plum pudding' model of the atom, where electrons were thought to be scattered within a positively charged matrix, much like plums in a pudding. This model, while pioneering, was later challenged by Ernest Rutherford's gold foil experiment, which revealed that atoms have a small, dense nucleus surrounded by mostly empty space. Rutherford's work paved the way for a more accurate understanding of atomic structure, setting the stage for subsequent discoveries.

🌌 Bohr's Planetary Model and Heisenberg's Uncertainty

Here, the focus shifts to Niels Bohr's contribution to atomic theory, particularly his planetary model of the atom, which depicted electrons orbiting a central nucleus. The paragraph explains how Bohr applied mathematical principles from Max Planck and Albert Einstein to refine Rutherford's model, leading to a deeper understanding of electron positions and energy levels. It also introduces Werner Heisenberg's uncertainty principle, which challenged the notion of fixed electron orbits and led to the development of quantum theory, where electrons are described in terms of probabilities rather than specific paths.

☁️ The Quantum Model of the Atom

The final paragraph discusses the modern quantum model of the atom, which is based on the principles of quantum mechanics introduced by Heisenberg and others. It describes how electrons are now understood to exist in probabilistic orbitals rather than fixed orbits, and how this model is visualized as a cloud representing the likelihood of finding an electron in a given region. The text emphasizes the collaborative nature of scientific discovery, with each generation of scientists building on the work of their predecessors, and encourages ongoing inquiry to further refine our understanding of atomic theory.

Keywords

💡Atom

An atom is the basic unit of matter and the defining structure of elements. It consists of protons, neutrons, and electrons. In the video, the concept of atoms is central to understanding the historical development of atomic theory, from the early ideas of indivisible particles by Leucippus and Democritus to the modern quantum model.

💡Atomic Theory

Atomic theory refers to the scientific theories that explain the nature of atoms and their interactions. The video discusses the evolution of atomic theory from the early Greek philosophers to the quantum model, highlighting key developments and scientists who contributed to our current understanding of atomic structure.

💡Leucippus and Democritus

Leucippus and Democritus were ancient Greek philosophers who first proposed the concept of atoms. They believed that all matter was composed of indivisible particles, which they called 'atomos.' Their idea is foundational to the video's narrative on the history of atomic theory.

💡Electron

Electrons are subatomic particles that carry a negative electric charge. Discovered by J.J. Thompson, electrons were initially referred to as 'corpuscles.' The video explains the significance of electrons in the structure of atoms and their discovery as a key moment in the development of atomic theory.

💡Cathode Ray

A cathode ray is a stream of electrons observed in experiments with discharge tubes. In the script, the discovery of cathode rays led to the understanding that atoms have both negative and positive components, which was a significant step in the study of atomic structure.

💡Plum Pudding Model

The plum pudding model, proposed by J.J. Thompson, visualized the atom as a positively charged 'cake' with negatively charged 'plums' (electrons) scattered throughout. This model was a step towards understanding atomic structure, although it was later superseded by more accurate models.

💡Eugen Goldstein

Eugen Goldstein was a German physicist who discovered that positive rays, or anode rays, were emitted from the positive electrode in discharge tubes. His discovery contributed to the understanding of atomic structure by revealing the presence of both positive and negative charges within atoms.

💡Ernest Rutherford

Ernest Rutherford was a New Zealander physicist known for his gold foil experiment, which led to the discovery of the atomic nucleus. His work is central to the video's discussion on the structure of the atom and the development of the nuclear model of the atom.

💡Niels Bohr

Niels Bohr was a physicist who proposed a model of the atom with electrons orbiting a central nucleus. His model, sometimes called the planetary model, was a significant advancement in atomic theory and is discussed in the video as a step towards the modern understanding of atomic structure.

💡Quantum Theory

Quantum theory, as introduced by Werner Heisenberg and others, describes the behavior of electrons and other subatomic particles in terms of probability rather than definite orbits. The video explains how quantum theory revolutionized the understanding of atomic structure by introducing the concept of electron orbitals.

💡Orbitals

Orbitals are regions around the nucleus where electrons are most likely to be found, as described by quantum theory. The concept of orbitals is central to the modern model of the atom and is discussed in the video as the culmination of the development of atomic theory.

Highlights

The concept of atoms was first proposed by Leucippus and Democritus 2500 years ago, with the term 'atomos' meaning uncuttable or indivisible.

Early atomic theory attributed properties of substances to the shapes of atoms, such as hard iron atoms with hooks and soft clay atoms with ball and socket joints.

Atomic theory has evolved through centuries, combining insights from numerous scientists and rigorous experimentation.

Antoine Lavoisier's law of conservation of mass and James Dalton's atomic theory contributed to understanding atomic behavior in the 1800s.

Eugen Goldstein's discovery of rays from the positive electrode in discharge tubes indicated the presence of positive charges in matter.

J.J. Thompson's experiments with cathode rays led to the discovery of electrons, very light and negatively charged particles.

Thompson's 'plum pudding model' visualized atoms as a positively charged matrix with randomly distributed electrons.

Ernest Rutherford's gold foil experiment revealed the existence of a concentrated positive charge in a small area, the nucleus.

Rutherford's findings showed that most of an atom is empty space, with electrons orbiting a central nucleus.

Niels Bohr applied quantum theory to the atomic model, predicting electron positions and introducing the planetary model of the atom.

Bohr's model, with electrons in defined orbits, laid the groundwork for understanding energy levels and orbitals.

Werner Heisenberg's uncertainty principle challenged the notion of electrons having fixed positions and momenta.

Quantum theory introduced the concept of probability in describing electron arrangements, leading to the idea of orbitals.

The modern quantum model of the atom describes electrons as existing in 'clouds' of probability rather than fixed orbits.

The development of atomic theory is an ongoing process, with the quantum model being the current but possibly not final understanding.

The history of atomic theory demonstrates the importance of continuous questioning and experimentation in scientific advancement.

The Crash Course Chemistry episode emphasizes the collaborative nature of scientific discovery and the value of ongoing education in the field.

Transcripts

How do you picture an atom in your mind – like this, or like this, or maybe one of these?

If you understand enough about atoms to visualize any of those things,

then you know more about atomic theory than scientists did just 100 years ago.

And, like, WAY more than they thought they knew 2500 years ago.

That's when Greek philosopher Leucippus and his pupil Democritus

first came up with the idea that matter is composed of tiny particles.

No one knows how they developed this concept, but they didn't think the particles were particularly special –

they just thought that if you cut something in half enough times,

eventually you'll reach a particle that can't be cut anymore.

They gave these particles the name "atomos," which means uncuttable or indivisible.

So basically, they thought that iron was made up of iron particles

and clay was made up of clay particles and cheese was made up of cheese particles.

And they attributed properties of each substance to the forms of the atoms.

So, they thought that iron atoms were hard and stuck together with hooks,

clay atoms were softer and attached by ball and socket joints that made them flexible,

and cheese atoms were squishy and delicious.

Now this makes a certain amount of sense if you don't happen to have access to electron microscopes

or cathode-ray tubes or the work of generations of previous scientists.

Because the fact is atomic theory as we know it today is the product of hundreds,

if not thousands, of different insights.

Some models, like that of Leucippus, were just blind guesses.

As time went on, many more were the result of rigorous experimentation.

But, as has been the case in all science, each scientist built on what had been learned before.

We've been talking a lot about the fine details of chemistry in recent weeks,

and we're gonna keep doing that as we move on to nuclear chemistry and then to the basics of organic chemistry,

but we do, I wanted to set aside some time to explain how we know what we know about the atom today,

and how we know that we're not quite done figuring it out.

[Theme Music]

Now you might think that once Leucippus and Democritus came up with the general idea of atoms,

it'd be pretty easy for someone else to take that little, indivisible ball and run with it.

But you'd be wrong.

The next major developments in atomic theory didn't come along for nearly 2300 YEARS.

I've already told you, for instance,

about the French chemist Antoine Lavoisier, who proposed the law of conservation of mass,

which states that even if matter changes shape or form, its mass stays the same.

And you should remember the English teacher James Dalton

who determined that elements exist as discreet packets of matter.

Thanks to these, and other great minds, by the 1800s we had a better grip on the general behavior of atoms.

The next logical question was "Why? Why do they behave the way they do?"

This led to the investigation of atomic structure.

In the 1870s, scientists began probing what stuff was made of using discharge tubes,

basically gas-filled tubes with electrodes in each end,

which emit light when an electrical current passes through them – basically, what a neon light is.

Because this light was originally produced by a negative electrode, or cathode,

it was called a cathode ray, and it had a negative charge.

But in 1886, German physicist Eugen Goldstein found that the tubes also emitted light from the positive electrode,

basically, a ray headed in the opposite direction, which meant that there must also be a positive charge in matter.

Goldstein didn't fully understand what he'd discovered here –

I mean, scientists still hadn't figured out what was responsible for the negative charge in the rays either.

Then, English physicist J.J. Thompson took the discharge tube research further:

by measuring how much heat the cathode rays generated,

how much they could be bent by magnets and other things,

he was able to estimate the mass of the rays.

And the mass was about 1000 times lighter than a hydrogen, the smallest bit of matter known at the time.

He concluded that the cathode "rays" weren't rays or waves at all, but were, in fact,

very light, very small negatively-charged particles.

He called them "corpuscles;" we call them "electrons."

So even though we didn't understand what shapes they took,

we knew that there were both negative and positive components to matter.

The next question was "How were they arranged in the atom?"

Thompson knew that the atom overall had a neutral charge,

so he imagined that the negatively charged electrons must be distributed randomly in a positively charged matrix.

And the very English Thompson visualized this model as a familiar English dessert:

plum pudding, the positive matrix being the cake, and the electrons the random, floating bits of fruit within it.

Even today, Thompson's model of the atom continues to be called the "plum pudding model."

And while a single electron's motion is random, the overall distribution of them is not.

The next big step was taken by New Zealander Ernest Rutherford in 1909.

He designed an experiment using an extremely thin sheet of gold foil and a screen coated with zinc sulfide.

He bombarded the sheet with alpha particles, which he didn't really know what they were,

just that they were produced by the decay of radium, they were positive charged, and they were really, really small.

He expected them to just fly right through the foil, with no deflection, and many of them did just that.

But as it turned out, some of the particles were deflected at large angles and sometimes almost straight backward.

The only explanation for this was that the entire positive charge of an atom,

the charge that would repel an alpha particle,

must be concentrated in a very small area, an area that he called "the nucleus."

Because most of the alpha particles passed right through the atoms undeterred,

Rutherford concluded that most of the atom is empty space, and he was correct.

Rutherford would later discover that if he bombarded nitrogen with alpha particles,

it created a bunch of hydrogen ions.

Now he correctly surmised that these tiny positively charged ions were themselves fundamental particles:

Protons. Now we're getting close to reality.

So these chemists had a fairly good idea of the structure of the atom,

they just needed to figure out what exactly the electrons were doing.

Enter Niels Bohr.

In 1911, the same year the results of Rutherford's gold foil experiment were published,

Bohr traveled to England to study with Rutherford.

And as a physicist, he was also interested in the mathematical model set forth by

German physicists Max Planck and Albert Einstein to explain the behavior of electromagnetic energy.

Over time, Bohr came to realize these mathematical principles could be applied to Rutherford's atomic model.

His analysis of the gold foil experiment and calculations

based on the proportion of the alpha particles that went straight through,

those that were slightly deflected, and those that bounced almost completely backward,

allowed him to predict the most likely positions of electrons within the atom.

Bohr's resulting model, sometimes called the planetary model, is still familiar to most people, probably including you.

It represents the electrons in orbits around a small central nucleus.

Each orbit can have a specific number of electrons,

which correlates to the energy levels and orbitals in the modern model of an atom.

And while it's definitely flawed, Bohr's model is very close to reality in some important ways.

But like everyone I've mentioned in the past couple of minutes, Bohr was at once fantastically right and way off.

The problem was those pesky electrons.

It was the German theoretical physicist Werner Heisenberg

who got everyone to understand just how huge and mind-blowing this electron problem was.

But he was also the one who helped tie the whole mess up into a neat little bundle.

Using his wicked math chops, Heisenberg discovered that it is impossible to know with certainty

both the momentum of an electron or any subatomic particle and its exact position.

And the more you know about one of those two variables, the harder it gets to measure the other one.

So if you can't measure the position or momentum of an electron,

you obviously can't say with certainty that the electrons in an atom are all neatly aligned in circular orbits.

So he and a new wave of physicists and chemists proposed a new theory: a quantum theory,

which proposes that electrons weren't particles or waves, instead, they had properties of both and neither.

By this thinking, the arrangement of electrons around a nucleus could only be described in terms of probability.

In other words, there are certain regions where an electron is much more likely to be found.

We call these regions "orbitals."

You know, the very same orbital that you and I have been talking about –

the ones that go by the names "s and d and p and f" and that form sigma and pi bonds –

those are the things that Heisenberg's theory predicts.

And that's the modern understanding of atoms.

Because it's based on probability, quantum atoms are often drawn as

clouds with the intensity of color representing not individual electrons

but the probability of finding an electron in any particular position.

For this reason, the quantum model is often called the cloud model of the atom.

And now ya know!

All the people I've mentioned and many others put their heads together over time to build this current and

– I might say – quite elegant understanding of atomic theory.

Now, after 2500 years, even though we can't see them, we can know what they're like and how they work,

because a long succession of scientists contributed bits and pieces to the whole fantastic picture.

But it's also important to recognize that we still may not be quite all the way right.

Thompson's contemporaries were sure that the plum pudding model was right;

scientists in Bohr's day fully believed that the planetary model was right,

and today we're extremely confident that the quantum model is correct.

But it may not be all the way correct, and that's where you come in:

the only way we can go on being sure is to keep asking questions and conducting experiments.

And that's why you're taking chemistry and physics. Pay attention!

Thank you for watching this episode of Crash Course Chemistry.

If you paid attention, you learned that Leucippus and Democritus

originated the idea of atoms nearly 2500 years ago,

but that the real work didn't really begin until

both protons and electrons were discovered by experimenting with discharge tubes,

and how Ernest Rutherford figured out what and where the nucleus is.

You also learned that chemistry can sometimes be done with just math,

like how Bohr figured out his model

or the way that Heisenberg used math to usher in the quantum theory of the atom.

This episode written by Edi Gonzales and edited by Blake de Pastino.

Our chemistry consultant is Dr. Heiko Langner, and it was filmed, edited and directed by Nicholas Jenkins.

The script supervisor was Katherine Green, Michael Aranda is our sound designer,

and Thought Cafe is our graphics team.