11AS01 - History of the atom - WACE Chemistry Year 11
Summary
TLDRThis 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
📚 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.
🔬 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.
⚡ 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.
✨ 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.
Mindmap
Keywords
💡Atomic Structure
💡Dalton's Atomic Theory
💡Law of Conservation of Mass
💡Thomson’s Cathode Ray Experiment
💡Electron
💡Rutherford’s Gold Foil Experiment
💡Nucleus
💡Bohr Model
💡Quantum Mechanical Model
💡Neutron
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
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