11AS01 - History of the atom - WACE Chemistry Year 11

Summary

This video introduces Year 11 students to the history of atomic structure, covering key scientists and their contributions. It starts with Dalton's ideas about atoms and compounds, followed by Thomson's discovery of electrons using the cathode ray tube. Rutherford's gold foil experiment reveals the dense atomic nucleus, while Bohr introduces electron orbits and energy levels. Lastly, Chadwick discovers the neutron, completing the modern atomic model. The video emphasizes the importance of understanding these concepts for grasping atomic structure in chemistry.

Takeaways

• 🧑‍🏫 Dalton introduced the idea that all matter is made up of atoms, indivisible units with identical mass and properties within each element.
• ⚖️ Dalton also proposed that compounds are combinations of atoms in fixed ratios, leading to different substances with unique properties.
• 🔄 Chemical reactions, according to Dalton, are simply rearrangements of atoms, adhering to the law of conservation of mass and constant composition.
• 🔬 Thompson's cathode ray experiment led to the discovery of the electron, a small negatively charged particle, part of every element.
• 🍪 Thompson developed the 'plum pudding' model, where electrons are embedded in a positively charged 'soup,' balancing out the atom.
• 🏹 Rutherford's gold foil experiment revealed that atoms are mostly empty space, with a dense, positively charged nucleus at the center.
• 🌍 Bohr expanded the model, suggesting that electrons orbit the nucleus in defined paths and jump between orbits when energy is absorbed or emitted.
• 💡 Bohr's model also explained that electrons emit specific wavelengths of light when transitioning between energy levels.
• ⚛️ Chadwick discovered the neutron, a neutral particle within the nucleus, explaining why atomic mass didn't match the number of protons alone.
• 📜 The video emphasizes that understanding the history of atomic theory is key but not a major part of exams, advising students to focus on grasping the core concepts.

Q & A

What was John Dalton's main contribution to atomic theory?

Dalton proposed that all matter is made of indivisible atoms, and that atoms of a given element have the same mass and properties. He also introduced the idea that compounds are combinations of atoms in fixed ratios.

What are the two key laws Dalton's atomic theory is based on?

Dalton's theory is based on the Law of Conservation of Mass, which states that mass is conserved in chemical reactions, and the Law of Constant Composition, which says that compounds have a consistent composition throughout.

How did J.J. Thomson's cathode ray experiment change the understanding of the atom?

Thomson discovered the electron, a small, negatively charged subatomic particle, through his cathode ray experiment. This led to the understanding that atoms are divisible and contain smaller particles.

What was the significance of Thomson's plum pudding model?

Thomson's plum pudding model suggested that atoms consist of negatively charged electrons embedded within a positively charged 'soup,' similar to plums in a pudding, which explained the overall neutrality of atoms.

What did Rutherford's gold foil experiment reveal about the atom's structure?

Rutherford's experiment showed that atoms have a dense, positively charged nucleus at their center. This was inferred when some alpha particles bounced back after hitting the nucleus, while most passed through the atom's mostly empty space.

How did Bohr's model of the atom differ from previous models?

Bohr's model proposed that electrons orbit the nucleus in distinct energy levels or shells. When electrons gain energy, they move to higher energy levels, and when they return to lower levels, they emit light of specific wavelengths.

What is the concept of energy quantization in Bohr’s atomic model?

In Bohr’s model, electrons can only occupy specific energy levels. The energy released or absorbed by an electron when moving between these levels is quantized, meaning it comes in fixed amounts corresponding to specific wavelengths of light.

What discovery did James Chadwick make about the atom?

Chadwick discovered the neutron, a neutral subatomic particle found in the nucleus of atoms. This helped explain why the mass of atoms did not correspond directly to the number of protons, as neutrons contribute to atomic mass but have no charge.

How did Chadwick’s discovery solve the issue of mismatched atomic mass?

Chadwick's discovery of the neutron explained why the atomic mass was higher than the mass of just the protons, as neutrons add mass without affecting the atom’s charge.

What role do neutrons play in the atomic nucleus?

Neutrons help stabilize the nucleus by providing a buffer between the positively charged protons, which would otherwise repel each other due to their like charges.

Outlines

00:00

📚 Introduction to Atomic Structure

The speaker introduces the lesson for Year 11 students about the history of atomic structure. He emphasizes the importance of learning intentions and success criteria, which help students understand what they should achieve by the end of the video. The lesson covers the experiments and ideas that led to the modern understanding of the atom, starting with Dalton's early theories about atomic indivisibility and the formation of compounds.

05:01

🔬 Dalton's Atomic Theory

Dalton is discussed as a key figure in the development of atomic theory. He theorized that atoms are indivisible, and all atoms of a given element have the same mass and properties. Compounds are formed when atoms combine in fixed ratios, resulting in new substances with unique properties. Dalton’s theory also introduced the concepts of chemical reactions as rearrangements of atoms, supported by the laws of conservation of mass and constant composition.

10:02

⚡ Thompson's Cathode Ray Tube Experiment

Thompson’s experiment with a cathode ray tube revealed the existence of subatomic particles, later identified as electrons. By placing charged metal plates inside a glass tube and applying a voltage, he observed that negatively charged particles (electrons) were drawn toward the positively charged plate. These particles had a very small mass, much smaller than atoms, indicating that atoms contained even smaller components. This led to Thompson's 'plum pudding' model, where electrons are embedded in a positively charged matrix.

15:03

✨ Rutherford's Gold Foil Experiment

Rutherford’s gold foil experiment challenged the existing atomic model by showing that atoms consist mostly of empty space. When alpha particles were shot at thin gold foil, most passed through, but some were deflected, indicating the presence of a small, dense nucleus. This experiment led to the discovery that atoms have a dense central nucleus surrounded by a cloud of electrons, revolutionizing the understanding of atomic structure.

🌟 Bohr's Quantum Mechanical Model

Bohr built upon Rutherford's findings by proposing that electrons orbit the nucleus in distinct layers or energy levels. He observed that when electrons are energized, they move to higher energy levels and emit light when they return to lower levels. This discovery of specific energy emissions supported the idea of well-defined electron orbits, leading to the development of the quantum mechanical model of the atom.

🧪 Chadwick's Discovery of the Neutron

Chadwick resolved the remaining mysteries about atomic structure by discovering the neutron. His experiments with alpha particles and beryllium revealed neutral particles within the nucleus, which were unaffected by electric or magnetic fields. These neutrons accounted for the additional mass in the nucleus that could not be explained by protons alone, completing the modern understanding of atomic structure.

Keywords

💡Atomic Structure

Atomic structure refers to the composition and arrangement of atoms, including the nucleus, protons, neutrons, and electrons. In the video, the history of atomic structure is discussed through various scientific discoveries that shaped our current understanding of atoms. The video covers different models, from Dalton's solid sphere model to the more complex quantum mechanical model.

💡Dalton's Atomic Theory

Dalton's Atomic Theory was an early scientific theory that proposed atoms as indivisible particles that make up all matter. Dalton believed that all atoms of a particular element have the same mass and properties. This theory introduced the concept of compounds formed by combinations of atoms in fixed ratios. In the video, Dalton's theory is presented as the starting point of modern atomic theory.

💡Law of Conservation of Mass

The Law of Conservation of Mass states that mass cannot be created or destroyed in a chemical reaction. This principle is used to explain that the mass of the products in a reaction equals the mass of the reactants. In the video, this law supports Dalton’s theory that atoms are rearranged during chemical reactions, but their total mass remains constant.

💡Thomson’s Cathode Ray Experiment

Thomson’s Cathode Ray Experiment was pivotal in the discovery of the electron. In this experiment, a stream of negatively charged particles (electrons) was emitted from a cathode and attracted to a positively charged plate. Thomson's experiment demonstrated that atoms contain smaller subatomic particles, which led to the 'plum pudding model' of the atom. The video uses this experiment to illustrate the discovery of electrons as part of atomic structure.

💡Electron

An electron is a subatomic particle with a negative charge, first discovered through Thomson's Cathode Ray Experiment. Electrons are found outside the nucleus and play a crucial role in chemical reactions and bonding. In the video, electrons are shown to be much smaller than atoms and are discussed as being embedded within the atom in early models, such as Thomson's 'plum pudding model'.

💡Rutherford’s Gold Foil Experiment

Rutherford’s Gold Foil Experiment involved firing alpha particles at a thin sheet of gold foil. Most particles passed through, but some were deflected, suggesting the existence of a small, dense nucleus at the center of the atom. This experiment disproved the 'plum pudding model' and introduced the nuclear model of the atom, which is discussed in detail in the video.

💡Nucleus

The nucleus is the dense center of an atom, containing protons and neutrons. It was first identified by Rutherford during his gold foil experiment. The video explains how the nucleus is responsible for most of the atom's mass and how this discovery shifted scientists' understanding of atomic structure, from a uniform 'plum pudding' model to one with a central nucleus.

💡Bohr Model

The Bohr Model is a theory proposed by Niels Bohr that describes electrons as orbiting the nucleus in defined energy levels or orbits. When electrons jump between these orbits, they absorb or emit energy in specific quantities. The video explains how this model built upon Rutherford’s discovery by incorporating the behavior of electrons, leading to a better understanding of atomic structure and quantum mechanics.

💡Quantum Mechanical Model

The Quantum Mechanical Model is the most advanced model of the atom, where electrons exist in 'clouds' or probability regions rather than fixed orbits. The video touches on how this model further refines the Bohr Model by considering the wave-like behavior of electrons. This model is central to modern chemistry and physics, explaining complex phenomena like chemical bonding and atomic behavior.

💡Neutron

The neutron is a subatomic particle with no electrical charge, discovered by James Chadwick. Neutrons are located in the nucleus alongside protons and contribute to the atom's mass. In the video, Chadwick’s discovery of the neutron resolved the issue of missing atomic mass that could not be explained by protons alone, thus completing the understanding of the atom’s internal structure.

Highlights

Introduction to the history of atomic structure and key scientific contributors.

Dalton's atomic theory: atoms are indivisible, all atoms of an element have the same mass and properties.

Dalton introduces the idea that compounds are combinations of atoms in fixed ratios.

Law of conservation of mass: in a chemical reaction, the mass of reactants and products remains the same.

Thomson's cathode ray tube experiment leads to the discovery of electrons as negatively charged particles.

Thomson proposes the plum pudding model: negatively charged electrons are embedded in a positively charged 'soup'.

Rutherford's gold foil experiment shows that atoms have a dense nucleus, with most of the atom being empty space.

Rutherford concludes that the mass of the atom is concentrated in a small, dense nucleus.

Bohr introduces the quantum mechanical model: electrons orbit the nucleus in distinct energy levels.

Bohr explains that electrons emit light of specific wavelengths when they transition between energy levels.

Bohr's model demonstrates that electrons exist only in specific orbits, not in between.

Chadwick discovers the neutron, a neutral particle within the nucleus, which explains the discrepancy in atomic mass.

Chadwick’s experiment shows that neutrons have a similar mass to protons and are not affected by electric or magnetic fields.

The history of atomic structure helps explain the key developments and models leading to our current understanding of atoms.

Final note: understanding these contributions is important but not critical for exams; focus on the basics of atomic structure.

Transcripts

all right hi you're 11. this is mr lim

here and this

is our very first video for year 11

about the history of atomic structure so

let's get started

okay so we're going to be every time you

watch one of these videos you're going

to have the learning intentions there

and the success criteria so

just knowing what you should be able to

do you should probably skip back to this

um

part of the video at the end of it just

to make sure that you can

remember that you can what you're

supposed to do and be able to actually

diverse things

all right so today we're going to be

learning about

the history of the atom which means that

we're going to be learning about all the

experiments

and ideas that were um contribute to how

we understand the atom as it is

today all right so this first scientist

called dalton all right

he was wasn't the first person to think

about atoms as

that make up everything right atoms

being something that is

uh indivisible which means that you

can't divide it anymore

right he wasn't the first one to work it

out

but he was the one to start thinking

about it and

start giving explanations about it all

right so he thought that all atoms of an

element had the same mass and the same

properties right and then when you were

to combine

certain atoms of an element in a fixed

ratio they gave you a different

substance a compound he called them and

these

compounds they have different properties

to their individual elements

right and it's due to the fact that

you've got them

together as a rearrangement of those

atoms of those elements

that gives them the different properties

okay so it's the idea that

compounds actually i guess

all matter is made up of atoms that's

important all atoms

have of the same element have the same

mass and the same properties

okay compounds are combinations of atoms

right two or more different elements

in fixed ratio that's important that

means that you've got like

uh if you've got one of one type of atom

then you've got two of the other type

and you've got that

all the way through the um the substance

all right

chemical reaction is a rearrangement of

atoms so the idea is that you're

rearranging

how they're attached to each other

and it's based on the law of

conservation of mass

and the law of constant composition okay

so

law of conservation of mass hopefully

you remember from year i don't know

9 or 10 or something like that the idea

that when you have a chemical reaction

the mass of all the products and the

massive over reactants have to be equal

so no matter is lost in the chemical

reaction

right and law of constant composition is

the idea that compounds will have

the the same composition throughout

their

uh their substance which means that that

idea of one atom

of one type and two atoms of the other

type all the way through the um

substance

all right so the idea is that when a

chemical reaction occurs

between two elements they change their

arrangement

which changes their properties okay

but when you do another chemical

reaction to bring them back to their

original elemental state which means

that they're back in their elements you

change the arrangement again

and then the properties of the atoms do

not change they go back to what they

were before

right the arrangement is the is the

thing that gives them

the different properties from each other

okay so the idea is that here we have

two different elements the red ones and

the green red ones and the blue ones

when you change them and in a chemical

reaction to a different arrangement

they have different properties but if

you were to bring them back to their

original states

they would go back to having their

original properties because

the atoms have the same properties

uh it's when they've got the same

arrangement all right

it's the arrangement that gives them

their properties unless you keep them

in their pure form right so that's the

idea of dalton

law of conservation of mass and law of

constant composition were his things

and the idea that defined atoms of an

element and defined compounds

all right next was a dude called

thompson

all right he built a cathode ray tube

all right what's a cathode ray tube look

at that

so catheter red tube is a tube of glass

right which was emptied of air

and then a large charge is placed on two

metal plates

inside that tube of a glass

all right here's a little bit of it here

all right i haven't drawn the glass

around it but

yeah there'd be a big glass tube in

there right

and so you've got your very strongly

negatively charged plate and very

strongly charged positively charged

plate

right so two charges

placed in the metal plates inside the

tube this causes

charged particles to come out of the

cathode which is one of the metal plates

and towards the anodes which is at the

other plate

and then these particles are allowed to

pass through a small slit

and create a beam okay so let's have a

look here

right here we have the two charged

plates in black

right and the anode is the positively

charged one so that's this one here

okay that's the anode in black so

that anode in black is positively

charged and

negative particles seem to come out of

the negative plate and towards the

positive plate because you know negative

things are attached to positive things

and if you leave a slit in the middle

which is that thing there

right negatively charged particles can

fly through it so you've got the

stream of yellow and the stream of green

negative particles right

so these negative particles

are then affected by other charged

plates or magnetic fields

the negative particles are affected by

other charge plates or magnetic fields

right and the negative particles were

found to move towards the positive

charges

uh charge plate meaning that they were

negatively charged

right so just then obviously they're

oppositely charged so that's why

those stream of positive and negative

particles they're moving towards the

positive plate here okay that positive

plate there that's the negative plate

there

the positive plate is attracting them

making them want to go

that in that direction and so they kind

of curve

along that line right

now what they found was that

the negative particles found to be moved

very easily by the charge plates of

magnetic field

which means that they have a very small

mass because if you think about it

the the charged plates that are

affecting their flight

are like is like the wind and things

that are very light are more affected by

wind than things that are very heavy

so the smaller and so this very small

mass it was much smaller than the mass

of any atoms that they had worked out

already

okay meaning that they had to be

subatomic which means smaller than

atoms particles right so if you look

here

um this one here

uh is lighter than this one here

because it is more affected by the thing

and the idea is that they

um they worked out that the very very

light

these things are very very light which

makes them much lighter than the atoms

that they were

uh supposed to be inside and so

therefore they were

small parts of that atom okay

um he ran the experiment with all kinds

of oops

they didn't like that he ran the

experiment with all kinds of

elements so he found that they did it

over and over again

right so they all acted the same and

they all had very small mass meaning

that all

elements had these very small negatively

charged particles

okay so after a while they decided to

call them electrons

and so these and so they thought that

these small negative particles were

embedded

in the center of the atom like chop

chips in a chocolate chip ice cream or

chocolate cookie

right but he called it a plum pudding

model because you know

plum puddings were apparently popular

back then but the idea is that they're

embedded within the thing

right so and that would be what they

look like so

here is the atom okay this entire

thing is the atom right and within that

atom

is the little bits of a negative charge

and everything else in that atom is all

positively charged

right they didn't know what it was yet

but they they they knew it had to be

positively charged to balance out to

make it neutral

all right then next was this dude called

rutherford all right who did the gold

foil experiment

so he made a super thin piece of gold

foil and then shot alpha particles at

them

okay so at the time they thought that

the size of the atom was much

larger and thus didn't have a very high

density so like a beach ball

right so they thought with this low

density the alpha particles was going to

go right through it

or the idea that they would kind of push

them out of the way because of their

such low density

okay so if you think about it there's

the there's the

model that um what's his name made

thompson

and then these are a whole bunch of them

in a row

okay now imagine these are like beach

balls and you're shooting a bullet at

them

if the beach balls didn't um you know

explode

the beach ball would just simply get

pushed out the way because it's so

low density because they thought well

the size of the atom we can work out

the mass of the work atom we can work

out but they seem

to be then very low density all right so

he's expecting it to go right through it

and he's expecting a lot of them to go

right through it right

however this is what happened a lot of

them did go

through the substance but

every so often one of them bounced

back as if it hit something very hard

and very high density

right so the idea is that if it's

hitting something that's high density

and he did it over and over again to

make sure that it was right

if something's hit if it's setting

something high density

there must be something high density

within those these atoms

right and so they came up with the idea

that if things are still going through

there must be like

empty space which that the particles can

go

right through okay but because there

must be something that's very high

density

maybe all of the mass of the atom is

focused

in one very very very small

part in the center right meaning that

there's large gaps between them

here and here so the idea that

they have a very small very dense

nucleus comes from that idea that well

a whole bunch of stuff went through but

one but every so often it hit something

very dense and that dense thing would be

the nucleus

okay and then finally after

uh rather third was bore okay

actually no not even finally i've still

got two guys to go all right

so boar he worked out the quantum

mechanical model so the idea is that if

the center is very dense

then and separate from the negatively

charged electrons

because you could cause the negative

electrons to separate from the

materials but not the positive charge

then what were these electrons doing

okay so if it worked out it's got a

small base small

center part with very high density the

electrons are somewhere there it must be

around somewhere

so since these are negative and positive

charge they thought that they're going

to be attracting to each other

and not being able to see what the

electrons are doing they assume

that the electrons are in orbit because

you know attracting things like the

earth and the sun

they orbit around the center which is

the nucleus right

however what they did was they also saw

that if you give electrons energy

okay so if you give them energy of some

sort they will emit light of a specific

wavelength okay um

and the like that being emitted uh as

the electrons move from one orbit to

another meant that there were multiple

orbit paths for these electrons

but not anything in between okay and

they

meant that they created well-defined

paths so that's a bit

we're going to be going through this a

little bit later on but this is the idea

all right the idea is that when you gave

these little blue electrons

energy right they would move up

and then they would come down when they

came down they released

light and these lights were exactly

specific

amounts of energy and because of these

specific

amounts of energy or portions of energy

they must be in distinct layers because

if you think about it right

if they went to here if they went to

here in this

kind of empty space and then came back

down

you would have you wouldn't have exactly

100 units of energy every single time

you would have like you know

and then like you know some of them went

up to here at some point down here some

of them only went up a little bit

someone

went down a little bit if they could go

everywhere

then you wouldn't have these specific

portions of energy but because you have

these specific portions of energy

they only go up to certain levels

all right it's like saying oh i'm going

to the first floor or to the second

floor of the third floor but there's no

such thing as going to the 2.6 floor

all right because you're just going to

be floating in the middle of the air

that just doesn't work

so the idea is that they would have

certain levels and only they could they

could only exist

in those levels and it's because of the

light that they gave off was in specific

amounts

all right and we'll be learning about

that later on as well

and then finally this guy chadwick which

i'm sorry chadwick but i gave you the

wrong

i didn't change your name sorry right

chadwick

so there was still confusion about the

center of the atom because the number of

protons did not match the mass of the

atom

okay the idea is that if you've got

positive stuff in the middle

uh what's holding all those positives

together and um

the masses just didn't match so what

they did they ended up shooting some

alpha particles at beryllium as you do

i'm sure they just shot alpha particles

and everything to see what happens they

found out that the emit

and but they found out this that the

brilliant particles emitted particles

which were highly penetrating

but were not affected by magnetic or

electric fields

and if they're not affected by magnetic

or electric fields it means that they

were neutral

right so the idea is this um

shoot some alpha particles out some

beryllium and something came out

right these little green things came out

these little green things that came out

were not affected by the magnetic field

or electric fields

right so remember how electrons are

affected by electric fields positive

charges would also be affected by

electric fields

but these things were not affected by

electric fields which means that they

must be

neutral or at least not positive or

negatively charged

however those things that they shot at

them

when they hit a peripheral film which is

like you know some of something else

they knocked off protons off those uh

atoms

and those protons were affected by a

magnetic field

okay because these things are being

knocked off by something and these

protons

are affected by a magnetic field but

being drawn towards the negative

charged plate right and so therefore

these things that came out

must be within these beryllium things

right

of this beryllium atoms and what they

worked out was that if they're coming

out of this

stuff and you know uh they must have

done it for other

elements as well uh they knocked off

proton whoops

that's not what i wanted yeah

change okay

they're knock so when you shoot it at

the paraffin film it knocks off protons

um which and then be detected because

they move um

around in the electric fields that's

suggested that there is a neutral

particle within the pro

within the neutron about the same sorry

not

within the nucleus about the same mass

that was

uh of the proton right

so that's the history of the atom sorry

that took so long

there you know there might be one or two

multiple choice questions of this

in the exam it's not a very large part

so if you're just watching this

and you've got all the way to the end

don't really have to memorize all of it

but it's a good idea to just kind of

have an understanding of why there is

and we're going to be going through some

of this stuff later on as well all right

that's all

adios