2.1 History of the Atom

Summary

This educational video script explores the history of atomic theory, starting with Dalton's billiard ball model, moving through Thompson's plum pudding model, and Rutherford's discovery of the nucleus. It highlights the contributions of scientists like Chadwick and Bohr, who identified neutrons and defined electron energy levels. The script uses engaging language to simplify complex concepts, making the progression of atomic theory accessible and fascinating.

Takeaways

• 🔬 **John Dalton's Atomic Theory**: Dalton proposed that matter is composed of small, indivisible particles called atoms, which are the same for a given element and different between elements.
• 🎱 **Billiard Ball Model**: Dalton visualized atoms as solid spheres, like billiard balls, which was a significant leap from the Greeks' concept of atoms 2000 years prior.
• 🚫 **Indestructibility of Atoms**: Dalton's theory stated that atoms cannot be created or destroyed, aligning with the modern law of conservation of matter.
• 🔗 **Whole Number Ratios**: Dalton also suggested that atoms combine in whole number ratios, which was a precursor to the understanding of chemical reactions.
• 🔄 **Chemical Reactions**: He noted that in chemical reactions, atoms can combine, separate, and rearrange, forming the basis for modern chemistry.
• 🌀 **J.J. Thomson's Plum Pudding Model**: Thomson's experiment with cathode rays led to the discovery of electrons, suggesting a different atomic structure with a positively charged 'dough' and negatively charged 'plums'.
• 💥 **Rutherford's Gold Foil Experiment**: Rutherford's experiment with alpha particles and gold foil led to the discovery of the nucleus, challenging Thomson's model and suggesting a central, dense, positively charged core.
• 🧲 **Atomic Nucleus**: Rutherford's findings indicated that the positive charge and most of the mass of an atom are concentrated in a tiny nucleus, with electrons orbiting around it.
• 🥚 **Neutrons Discovery**: James Chadwick discovered neutrons, which are neutral particles in the nucleus, after noticing discrepancies between the mass of atoms and the mass of their protons and electrons.
• 🌌 **Bohr's Model and Electron Shells**: Niels Bohr introduced the concept of electron shells or energy levels, explaining the discrete energy states of electrons and their role in chemical reactions and spectral lines.

Q & A

What was John Dalton's view of the atom?

John Dalton proposed that atoms were like solid spheres, similar to billiard balls, and that they were the smallest indivisible units of matter.

What were the key points of Dalton's atomic theory?

Dalton's key points included: matter is composed of small particles called atoms, atoms of the same element are identical, different atoms have different properties, atoms can't be created or destroyed, atoms combine in whole number ratios, and in chemical reactions, atoms combine, separate, and rearrange.

How did J.J. Thomson's experiment with cathode rays contribute to the understanding of the atom?

J.J. Thomson's experiment with cathode rays led to the discovery of the electron, proving that atoms were not solid spheres but contained smaller, negatively charged particles within them.

What was the significance of Ernest Rutherford's gold foil experiment?

Rutherford's gold foil experiment led to the discovery of the atomic nucleus. It showed that most of the atom's volume is empty space and that a very small, dense, positively charged nucleus exists at the center.

What did Rutherford's experiment reveal about the distribution of charge within the atom?

Rutherford's experiment revealed that the positive charge and most of the mass of the atom are concentrated in a small central region called the nucleus, while the electrons are distributed around it.

Who discovered the neutron, and how did this discovery impact the understanding of atomic structure?

James Chadwick discovered the neutron. This discovery completed the understanding of the atom's nucleus, which consists of both positively charged protons and neutral neutrons.

What was Niels Bohr's contribution to the model of the atom?

Niels Bohr proposed that electrons orbit the nucleus in specific energy levels or shells. He suggested that electrons could only occupy these quantized energy levels and that they could move between them by absorbing or emitting energy.

How did Bohr's model explain the emission spectra of elements?

Bohr's model explained the emission spectra by suggesting that the lines seen in the spectra correspond to the energy differences between the quantized energy levels of electrons in atoms.

What is the significance of the plum pudding model in the history of atomic theory?

The plum pudding model, proposed by J.J. Thomson, was a significant step in understanding the atom's structure. It suggested that atoms were composed of a positive 'soup' with negatively charged 'plums' (electrons) scattered within it.

How did the understanding of atoms evolve from Dalton's solid sphere model to the modern atomic model?

The understanding of atoms evolved through a series of experiments and theories. From Dalton's solid sphere model, to Thomson's plum pudding model, to Rutherford's discovery of the nucleus, and finally to Bohr's model with electrons in quantized energy levels.

What is the importance of understanding the history of atomic theory in modern chemistry and physics?

Understanding the history of atomic theory is important because it provides context for the development of modern scientific theories and highlights the incremental nature of scientific discovery. It also helps to illustrate the scientific method and the importance of experimentation and observation.

Outlines

00:00

🔬 John Dalton's Atomic Theory

The paragraph introduces John Dalton's perspective on atoms, marking him as a pioneer in atomic theory. Dalton posited that atoms were like billiard balls, solid and indivisible. He is credited with establishing the fundamental concept that matter is composed of small particles called atoms. He also proposed that atoms of the same element are identical and that different atoms have distinct properties. Additionally, Dalton theorized that atoms could neither be created nor destroyed, a principle that aligns with the law of conservation of matter. His final points were that atoms combine in whole number ratios and that in chemical reactions, they can combine, separate, and rearrange.

05:01

🌌 J.J. Thomson's Plum Pudding Model

The second paragraph discusses J.J. Thomson's experiment with cathode rays, which led to the discovery that atoms contain negatively charged particles. Thomson's experiment involved a cathode ray tube where he observed that a beam of particles was deflected by an electric field, indicating they were negatively charged. This finding contradicted the idea of atoms being solid spheres and led to the 'plum pudding' model of the atom, where positive and negative charges were thought to be interspersed within the atom. This model suggested that atoms were composed of a positively charged 'dough' with negatively charged 'plums' (electrons) scattered throughout.

10:02

💥 Rutherford's Nuclear Model of the Atom

The third paragraph details Rutherford's gold foil experiment, which resulted in the discovery of the atomic nucleus. By firing alpha particles at a thin gold foil, Rutherford observed that while most particles passed through, a few were deflected at large angles. This unexpected result indicated the presence of a small, dense, positively charged nucleus at the atom's center, with electrons distributed around it. This contradicted Thomson's plum pudding model and led to the Rutherford model of the atom, which features a central nucleus with electrons orbiting around it.

Keywords

💡Atom

An atom is the basic unit of matter that defines the elements in the periodic table. In the video, the historical progression of understanding atoms is discussed, starting from the billiard ball model by John Dalton to the plum pudding model by J.J. Thomson, and finally to the nuclear model by Ernest Rutherford. The atom is central to the video's theme of exploring the history of atomic theory.

💡John Dalton

John Dalton was a chemist and physicist who is historically recognized for his significant contributions to the atomic theory. In the video, Dalton's view of the atom as solid, indivisible spheres is discussed, which was a foundational step towards understanding the atom's structure.

💡J.J. Thomson

J.J. Thomson is credited with the discovery of the electron and the plum pudding model of the atom. The video describes his experiment where electrons were found to be components of atoms, which led to the idea that atoms are not solid but have a diffuse structure with electrons embedded within.

💡Plum Pudding Model

The plum pudding model was a theoretical atomic model proposed by J.J. Thomson. It suggested that atoms are composed of a sphere of positive charge with electrons embedded within, similar to raisins in a plum pudding. This model was an improvement over Dalton's but was later proven incorrect by Rutherford's gold foil experiment.

💡Ernest Rutherford

Ernest Rutherford was a physicist who conducted the gold foil experiment that led to the discovery of the atomic nucleus. The video explains how his experiment contradicted Thomson's model and established the concept of a central, dense nucleus with electrons orbiting around it.

💡Nucleus

The nucleus is the central part of an atom, containing most of its mass and consisting of protons and neutrons. The video describes how Rutherford's experiment led to the realization that atoms have a small, dense nucleus, which was a significant departure from previous models.

💡Alpha Particles

Alpha particles are positively charged particles that consist of two protons and two neutrons. In the video, Rutherford's gold foil experiment is discussed, where alpha particles were used to probe the structure of the atom, leading to the discovery of the nucleus.

💡Neutrons

Neutrons are neutral particles found in the nucleus of an atom, alongside protons. The video mentions James Chadwick's discovery of the neutron, which explained the discrepancy between the mass of atoms and the mass of their protons and electrons.

💡Niels Bohr

Niels Bohr was a physicist who developed a model of the atom with electrons in quantized orbits around the nucleus. The video discusses how Bohr's model introduced the concept of energy levels or shells, where electrons reside and from which they can absorb or emit energy by jumping to different levels.

💡Energy Levels

Energy levels, or shells, are specific regions around the nucleus where electrons can exist in an atom. The video explains that Bohr's model introduced the idea that electrons occupy these discrete energy levels and can move between them by gaining or losing energy.

💡Spectrum

A spectrum is the range of light emitted by electrons as they transition between energy levels in an atom. The video mentions how the observation of spectral lines provided evidence for the existence of quantized energy levels and helped in understanding the structure of the atom.

Highlights

Introduction to the concept of atoms and their historical significance.

John Dalton's atomic theory and his view of atoms as solid, indivisible spheres.

Dalton's postulates on the composition of matter, identical atoms of the same element, and different properties of different atoms.

The law of conservation of matter stating atoms cannot be created or destroyed.

Dalton's theory on atoms combining in whole number ratios.

Dalton's view on chemical reactions involving atoms combining, separating, and rearranging.

J.J. Thomson's experiment with cathode rays leading to the discovery of electrons.

Thomson's plum pudding model of the atom with electrons scattered within a positive 'dough'.

Ernest Rutherford's gold foil experiment and the discovery of the atomic nucleus.

Rutherford's model of the atom with a central nucleus and electrons outside.

The realization that atoms must contain neutral particles, later identified as neutrons.

James Chadwick's discovery of the neutron and its role in atomic mass.

Niels Bohr's model of the atom with electrons in distinct energy levels or shells.

Bohr's explanation of electron movement between energy levels and the emission of light.

The significance of atomic spectra in understanding electron energy levels.

Bohr's postulate of seven energy levels in an atom and the distribution of electrons.

Summary of the progression from Dalton's solid spheres to Bohr's model with electrons in energy levels.

Transcripts

he Mr it's great to be back with

you uh it's great to see you again here

let's uh let's learn some stuff yeah

yeah some really cool stuff today we're

gonna start learning about like Adams

and Adams are like crazy cool and I know

you're going to help us understand like

the history of the atom so why don't you

take it

away yeah so um one of the things that I

want you to keep in mind here when we're

doing all of this this is that these

guys uh these brilliant scientists they

were trying to figure out what the atom

looks like and what you're taught in

grade school that is way more than they

knew so have a little mercy so Mr

Demitri uh John Dalton was the first guy

he's the first dead white guy that we're

gonna look at tell me about his view of

the adom so he essentially said well the

fact that he thought they were Adams is

kind of a big deal because the Greeks

said they were Adams 2,000 years ago and

then nobody touched them because nobody

could see them and then D came along and

said yeah there are Adams and he thought

they look like these billiard balls that

we have drawn here on the

screen and and you know he was kind of

right because they do like look like

billiard balls but well but let's kind

of talk about the things that he learned

about the atom I think this is like

1600s maybe 1700s he's doing this work

yeah um again he he he came up with some

statements and when you see these

statements you're going to be like oh

come on man but need to understand that

no one knew anything about the Adam and

if you picture in your mind like this

Amphitheater full of people and

everything that he said all these

statements that Mr Bergman is going to

write down right now they were mind

shattering so picture like aborus

applause for each one of these but uh

let's go through these points one by one

Mr Bergman will you write out the first

point for us

please so it says here that matter is

composed of small particles called atoms

so why that sort of seems obvious to me

right but yeah again we we as a as in

our elementary school we know this but

you need to realize that people didn't

think that that they knew anything about

what was around them so him saying that

everything around us was made up of

Adams is kind of a big deal yeah number

two he said

this Adam the same element are identical

and different atoms have different

properties now that's mindboggling that

he said this it seems like we sort of

know this but why was this Earth

shattering in the in in his day well I

mean if you take a look at how he

thought the atom looked he thought it

looked like a big ball right and so

where other atoms of different things

different they look different so him

coming up with the idea like and again

for us we're GNA be like really but like

a copper atom it's going to be the same

as other copper or gold Adams as other

gold Adams him saying that out

loud ATS actually different for

different things so that's kind of a big

deal so it's as if like the you've got

big spheres and small spheres so like a

gold at would be a big sphere and a

little hydrogen atam would be a little

sphere but they're still all spherical

if you will all right the third one is

this atoms can't be created or

destroyed now that that one probably

caused some controversy because people

the day were probably like whoa whoa

whoa whoa wait I can light things on

fire and it destroys them but the idea

that everything's made up of atoms and

the individual particles couldn't be

destroyed was kind of a big deal in fact

there's a law of the universe and

there's very few of them that says

matter cannot be created nor destroyed

it can change forms and since matter is

made up of atoms yeah big deal that's

weird like you burn something it seems

like it's destroyed but actually the

atoms just move around number four is

this atoms combine in whole number

ratios this is kind of a big deal as

well right because people didn't even

know atoms existed and now he's saying

oh by the way we can put them together

but I think the key part of this is that

they're in whole number ratios you can't

have part of an atom right Mr Bergman no

no you only have a whole atom or another

whole atom you could

yeah all right and then the last one is

this in chemical reactions atoms combine

separate and

rearrange so I guess the question comes

up like how do Adams interact and they

didn't know like yeah sure Adams exist

Bravo Mr uh Dalton but what do they

actually do and so he said oh by the way

they can connect with each other and

they can actually form new things and

that's essentially what the last point

of this says correct Mr Bergman yeah

yeah that's exactly right so MRT now I

think we've got an interesting thing is

that Dalton basically saw the atom as

spheres and then a more Advanced version

came from this dude JJ Thompson tell me

about

it well I love the fact you called this

guy that looks like this a dude uh

that's super awesome um the atom

essentially it evolves over time the

views of it and JJ Thompson's like hey

maybe it is a solid sphere maybe it

isn't and he came up with this really

ingenious experiment so essentially what

he did is he took uh a piece of metal

right here and he put in this cathode

rate tube and then what he did is he

noticed that something shot out from

that tube and he discovered that when he

changed the charge on the outside of the

tube it caused that beam to shift so if

you take a look right here you see how

there's a negative plate down below here

and you see that this negative plate is

causing the beam to push up well that's

a big de let's let's take a look at what

that might actually really look like

let's let's watch this this this guy do

it let's do it

[Music]

[Music]

now that was super cool I could see how

that light got uh moved by that uh by

that that magnet that's really super

awesome so but what did that tell us

about the atom well so that must mean

this that this beam that was shut out

and you saw it in the video if it gets

bent by negative charge it must mean

this beam is negatively charged where

did that beam come from it came from the

metal that has a bunch of atoms in it

well what does that mean to us well

therefore if the metal is made up of

atoms and the atom have negatives in

them that means that our atoms can't be

these solid balls also Mr Berman aren't

atoms neutral so there I think it's

telling us two things right because if

an atom is normally neutral has no

charge and something negative shoots out

that means there's got to be positive

parts to the atom so it gives us the

fact that the adom is made of of both

negative and positive right so he still

like the sphere thing so he still say

says atoms are this spherical thing but

they're made up of some kind of a

particles that have

charge yeah and so the new view and

again you need to understand where he's

coming from he came up with idea and you

can see this here on the on on the board

he came with the idea of the plum

pudding model which by the way this is

what Plum Pudding actually looks like I

think Mr Berman you have a better uh

better way of doing it a better thing if

I have an addiction it's it's too

chocolate chip cookies and think of

chocolate chip cookie dough and the

chips represent the the electrons are

the negatively charged particles and

then the D is positive so you probably

remember that there's things called

protons you understand it from your atom

but the reality is this Thompson didn't

know that so he thought the dough was

positive if you will and the chips were

negative so in essence the positive

negatives were just interspersed all the

way around and what's kind of cool about

this is that while it's a big

improvement over the salt

billiard ball model not quite right is

it Mr Bergman right that leads us to

another dead white guy Ernest Rutherford

he was like some sir guy from from

England right and so tell us he did a

really weird experiment tell us about

that experiment yeah this is actually

Uber cool trying to relate to you with

the language here what he did is he took

a really really really thin piece of

gold foil and so the gold foil was about

10,000 atoms thick and you may think to

yourself 10,000 atoms that's huge but it

isn't 10,000 atoms is about would look

at a piece of your hair and be like oh

that's big so get a picture of very very

finely thin piece of gold foil and then

he shot these alpha particles which are

essentially these big fat positives at

the gold foil now what did he expect was

going to happen here Mr bman Well he

kind of expected him to like bounce and

stuff like that right to go through it

actually because it was so thin so try

that one again here said go straight

through yeah so because it was so thin

because he thought it was so thin he

thought I was just gonna shoot right

through it yeah and so he at the first

he he sees this go fall and how he

couldn't see Adam so what he did is he

put this fluorescent screen around it

that would light up wherever it hit and

so over and over again this positive

particle hitting the wall hitting the

wall hitting the wall and one out of

every 3,000 times or approximately um

actually I think it's one every 6,000

but I have to verify that you'd have

this

random hit to side like this right here

and he's like that can't possibly be the

case in fact I think he said something

crazy like this as if I shot a

cannonball at a piece of tissue paper

and it came back and it hit me that's

yeah that that's crazy right so he said

that the idea of the atom being these

positives and negatives randomly

interspersed this is my impression of

this positive negative spersed couldn't

be the case because this big fat

positive would just roll right through

but he said if instead all the positives

were really really Compact and in the

center if that alpha particle flew

through here and hit that it could

actually Ricochet off so so this this is

the discovery of the nucleus guys what

he discovered he discovered a very small

dense positively charged thing that we

now know as the nucleus of the atom and

the electrons are interspersed on the

outside so so Thompson was wrong he had

the idea positives and negatives but he

didn't realize that the positives were

all concentrated on this little itty

bitty spot in the middle of the atom so

when you take a look at the our our

interpretation of the atom now see this

big fat positive in the center so the

alpha particles come through here now

the majority of the time they don't and

in the they go straight through and they

don't hit and we're going to watch in

our next video we're going to see how

small the atom actually is and it'll

make a little more sense but Mr Bergman

this is actually very close to the view

of the atom we have now positive Center

and negatives on the outside but there's

one piece of the subatomic particle

trilogy protons electrons and there's

one more thing that they not account for

and you probably remember this from

middle school science guys it was called

neutrons right and so it took this next

dead white guy uh Chadwick to figure

that out and tell me about Mr how did he

figure out the whole Neutron piece well

again I This falls on the category of uh

beyond the scope of what we need to know

which I I know is not great but in

essence he was able to figure out the

masses of the actual atoms and again

beyond the scope we to know and every

time with just protons and electrons it

didn't match up with the masses that he

was getting so there had to be something

else and he knew it couldn't be more

positive or negative because that would

make an atom charged so he came up with

this essentially dead weight balast if

you would these neutrons and all these

neutrons essentially do is make things

heavier why does that matter well

heavier things move differently um Mr

Bergman and myself I would wager to say

right now that I move a little bit

differently than you do is that

Fair probably so fat shaming on myself

that's okay to do um but it is a big

deal that we understand how neutrons

work and how they they're in the nucleus

because that's gon to allow us to set up

when we start talking about the rest of

the atom right and the next part is

there's one more guy we got to talk

about here and then we'll close is this

guy named Neils bore so what Neils did

is so we've got the central dense

nucleus and the electrons kind of on the

outside but he was was able to Define

where on the outside the electrons were

and so much of chemistry is really about

the electrons on the outside so we had

to talk about bore so what what what's

B's addition to this whole Saga of the

history of the atom so you need to

understand that again we're progressing

along the way and so Rutherford said hey

there's a positive nucleus and electron

somewhere on the outside right um which

begs the question by the way we're going

to answer that a little bit is why

weren't the negative charges remember

opposite trct getting sucked in the

nucleus we'll talk about that a bit

later but bore said you know what these

electrons are actually in these energy

levels surrounding the nucleus now he

came up with the idea of these energy

levels or shells essentially circles and

uh they would go in those circles and if

the electrons gained or lost energy they

could move up or they could move down in

these energy levels and we'll get a lot

of detail about that later on in the

course and what that does but what

proved how did he how was he able to

figure that out that the electron levels

uh exist in these shells so what

essentially was able to do is this and

you guys will talk about this a lot more

but when electrons jump up in an atom

and then they fall back down they give

off energy and that energy can come in

the form of light and so if you're

looking at an atom and you excite the

electrons give it energy and the

electrons jump up and fall back down

you're going to see lines and every

single line represents an energy fall

down now how do that look exactly well

we're going to play around with this in

a bit but do you see how here these are

these Spectrum you see how there these

four different lines here in for

hydrogen these four different lines they

repres repr energy fall Downs that

electrons make from a higher energy

level to a lower energy level so if you

see for hydrogen and helium there's a

certain number of energy levels and he

came up with the idea Mr Burgman that

aren't there seven energy levels in the

atom yep so like seven energy levels and

there's certain number of electrons in

each of them and we're going to talk

about that later so guys if we're going

to summarize the first model of the atom

was a sphere or like different size

spheres that's do and then Thompson

right he's got the spherical chocolate

chck cookie model and then Rutherford

says he's got the central D nucleus and

the electrons on the outside and then

bore he still got the central D nucleus

and he now has the electrons existing in

these shells on the outside and that's

kind of where we're at there's a little

bit more to it but I think that's a good

progression of what we've been learning

in this video