Fun Video: Rutherfords Atomic Model Video Lecture - Class 11

in this video we're going to look at how
scientists discover that atoms have a
nucleus in them this discovery was made
by Ernest Rutherford as well as his
colleagues in 1911 and they made this
discovery with their famous gold foil
experiment and we're going to talk about
that here so atoms this is sort of how
we think about atoms today with
electrons on the outside and then in the
middle a dense hard nucleus that's where
the protons and neutrons live but it
wasn't always like this before
Ernest Rutherford and his co-workers did
their experiment this is what scientists
thought the atom looked like they called
it the plum pudding atom now plum
pudding is kind of a British thing so if
you're American it might be better to
call this the blueberry muffin model
think about this you take a blueberry
muffin and break it in half what do you
see on the inside of that muffin well
you'll see blueberries and the
blueberries are just kind of randomly
stuck into dough and that is exactly
what scientists thought the atom looked
like at a very small scale that
electrons were sort of like blueberries
and they were stuck in this dough kind
of that was positively charged so you
got this positively charged dough with
electrons stuck randomly in it this is
the plum pudding model but of course it
turned out not to be true
here's how Rutherford and his co-workers
showed that the atom had a nucleus and
the plum pudding model wasn't right they
started out with a piece of gold foil
which is like tin foil or aluminum foil
except it's made of gold I mean wouldn't
this be awesome to like wrap your
sandwiches in for lunch um anyway I
don't know maybe if you're really rich
you use gold foil to like prepare your
lunch anyway they took this piece of
gold foil and they shot tiny little
things at it what they shot at it were
called alpha particles which are tiny
pause
we charge particles that are much much
much smaller than a nap so take the gold
foil
they Bam Bam Bam shoot alpha particles
out now they want to know what happens
to the alpha particles after they hit
the gold foil so here's what they do
they take a material that flashes when
it gets hit by alpha particles and they
take this material and they bend it
around into a circle like this so
wherever the alpha particles hit they'll
see a flash they take this material and
then they take their piece of gold foil
and they put it right here in the middle
of this circle so now they shoot alpha
particles into the gold foil and they'll
see a flash somewhere on this circle
that will tell them where the alpha
particle ended up after it hit the gold
foil now here's what they see most of
the time they shoot alpha particles bam
bam bam bam here at the gold foil
straight on and they see flashes right
here behind the gold foil so this would
indicate that the alpha particles are
going right through the atoms of the
gold foil and are ending up over here so
that's the first thing they notice the
second thing is a really huge surprise
when they shoot the alpha particles
every once in a while they don't see a
flash here instead they see a flash over
here or a flash over here or sometimes a
flash here or here which makes them
think that the alpha particles are
hitting the gold foil and are veering
off to the side once in a while or
sometimes they're hitting a gold foil
and they're bouncing back completely all
the way back so we can kind of summarize
what they see using a diagram like this
here we've got our circular detector
screen that flashes here's an alpha
particle emitter which is just a fancy
word for an alpha particle gun something
that shooting alpha particles most of
the time these alpha particles hit the
gold foil and they go right through and
we see flashes on the screen on the
other
but every once in a while an alpha
particle hit the gold foil
zum-zum and will veer off to the side
and flash over here and every once in a
while it will hit the gold foil and will
bounce back
almost 180 degrees flash here and flash
here so based on these two pieces of
information Rutherford and his team are
able to make some conclusions about what
the atom actually looks like here's the
conclusions that they come to let's
start with this gold foil okay
the gold foil that they use is
incredibly incredibly thin in fact it's
so thin that these guys reason that
there are only a few atoms in its
thickness it's only a few atoms thick so
let's look at it on its side here and
look at a magnified view of the gold
foil if you want to think about gold
atoms as being sort of circles or balls
this is what the view from the side of
the gold foil might look like just two
atoms thick here now let's think about
the results that they got from the gold
foil experiment
okay the first thing that they saw is
that most of the time the alpha
particles seemed to have gone straight
through the gold foil so based on this
information they're able to reason that
most of the atom is empty space or if
there's something there it's very light
it's not very dense and it's really easy
for the alpha particles to shoot right
through so most of the atom is probably
empty space but now the second thing
they see is that every once in a while
an alpha particle zoom veers off to the
side or it's like it hits the gold foil
and bounces back and so based on this
information the reason that while most
of the atom is probably empty space they
think that there must be something very
small
but very hard inside each at and here's
a drawing here of course you know that
this is the nucleus now but they didn't
know that the nucleus existed so they
had to discover here and so they think
there must be something really really
hard and when an alpha particle hits it
head-on
BAM that alpha particles gonna ricochet
back and they also notice as we said
that sometimes the alpha particle veers
to the side they say well maybe it's
because it hits the nucleus on the side
of it so it goes off like that or they
also said maybe since alpha particles
are positively charged
maybe the nucleus is positively charged
so that if the alpha particle comes
close to it remember things that have
the same charge repel each other so
you've got this this alpha particle
coming close to a positively charged
nucleus then zoom it's going to veer off
to the side because they're pushing away
from each other
so regardless based on this information
they reason that there is something
small and hard in the atoms that once a
while the alpha particles are bang it
into and that's what's causing them to
bounce back or to veer off course and
they also reason that whatever is may is
whatever this nucleus is made up it's
probably positively charged because of
the way it looks like the alpha
particles are sometimes getting repelled
and pushed away so if you take these two
pieces of information together we can
come up with a model for the atom that
looks like this where this is just a
combination of those two things I saw
where we've got these gold atoms lined
up and most of the time if alpha
particles pass through so they don't hit
any nucleus here they just go straight
on through but if an alpha particle hits
this hard dense nucleus or is close
enough to be repelled by its charge it
veers off or it bounces back so here's
how the gold foil experiment changes how
scientists think about the app okay so
as we said before
here's a plum-pudding atom with these
electrons stuck in a positively charged
kind of pudding but Rutherford's
experiment shows that this isn't quite
true
instead he comes up with a model like
this where there is a hard dense nucleus
in the middle and that is where the
positive charges so the positive charge
isn't like this kind of blueberry muffin
dough that spread throughout the atom
but instead the positive charge
everything that I've drawn in red here
the positive charge is concentrated
right here in the middle of the atom and
then there might be some electrons
floating around on the outside but
they're certainly not held in place
because of the positively charged oh now
this is just one piece in a puzzle of
figuring out what the atom actually
looks like so shortly thereafter
Rutherford comes up with this model for
the atom but then scientists realize
that the electrons probably aren't just
stuck in place and instead they think
that the electrons are spinning around
the outside of the atom and you still
have this hard dense nucleus in the
middle and then only a short while after
this it's discovered that the nucleus
isn't just a hard piece of positive
charge but that the nucleus is actually
made of protons each one of which has a
positive charge and then the electrons
are spinning around the outside now this
is what we call the nuclear atom because
it has a nucleus so we got the nucleus
of protons and then electrons spinning
around the outside but of course you
probably know this still isn't an
accurate representation of exactly what
the atom looks like in fact they're
totally missing the neutrons it was
until the 1930s that James Chadwick and
his colleagues discovered that there are
neutrons neutrally charged subatomic
particles in the nucleus of an atom and
then as I've alluded to before it's not
that the electrons are really spinning
around in circular orbits but it's more
like they're just buzzing around like
crazy hyperactive flies all over the
atom but we'll get to all that later for
right now all you need to know is that
Ernest Rutherford's gold foil experiment
showed the atom was mostly empty space
but that it had something hard something
dense and something positively charged
that the alpha particle sometimes hit
and bounced back now I want to talk
about one more thing that's can be
confusing but it's really important and
it has to do with the predictions that
Rutherford made what he was expecting to
see okay now when people first learn
about the gold foil experiment they're
often really shocked that the alpha
particles went straight through the gold
foil and I think that Rutherford and his
co-workers must have been really shocked
as well when they found this out but it
turns out that Rutherford was actually
expecting the alpha particles to go
through the gold foil he thought that if
atoms were the plum pudding model that
the alpha particles would have gone
straight through because he thought that
this positively charged dough was so
light that the alpha particles would
just be able to pierce right through it
like bullets through a giant blueberry
muffin and sure there are some electrons
there but compared to an alpha particle
an electron is so tiny it's like a poppy
seed compared to a bullet and so he
thought that that bullet would just fly
right through the plum pudding atoms now
if you want to think of a model of the
atom that would have made the alpha
particles bounce away it would have been
Dalton's model which was one of the
first models of the atom Dalton thought
that atoms were hard indestructible
balls and so atoms had really been like
Dalton thought sure the alpha particles
would have hit them and would have
bounced away
but if the atoms were the plum pudding
model because this positively charged
dough is so light Rutherford thought
that the alpha particles would go
straight through so when it turned out
that the alpha particles went straight
through the gold foil it wasn't really
that much of a shock but what was a
shock was when sometimes the alpha
particles would veer off to the side or
would bounce back this is what was
really shocking and it told Rutherford
that instead of the positive charge
being distributed throughout in this
kind of dough that the positive charge
was concentrated here in the nucleus so
that's what led them to the model of the
nuclear atom with a nucleus so it might
even be better to draw the nuclear atom
like this where instead of having this
pink positively charged dough there
really isn't anything here at all and
it's just all this positive charge is
focused right here in the middle of the
atom now it's almost not even good to
draw these atoms as if they're shells
with a nucleus in the middle because
it's not like they have shells a more
accurate way to talk about this is to
say that the shells what I'm drawing as
a shells are really just the outer
limits of where the electrons go okay
and so what's between what I'm drawing
is a shell and the nucleus is really
just where the electrons live but
they're tiny and they're moving around
all the time so there's really not much
there so in fact maybe one of the best
ways to draw the nuclear atom would to
be like this where there isn't a shell
but there's a nucleus in the middle of
each atom and then you can kind of see a
circle here which is the outer reaches
of where the electrons go but then of
course this isn't a perfect view of
electrons because instead of going
around in circles
they sort of buzz around in various
shapes but again that's getting a little
ahead of ourselves
what the main important points are here
is
Rutherford's gold foil experiment
disproved the plum pudding model of the
atom it wasn't that there was positive
charge all throughout the atom like the
dough and a blueberry muffin but that it
was the positive charge was heavily
concentrated in a very hard very dense
nucleus in the middle of the atom and
then the electrons were on the outside
so that is how Ernest Rutherford Hans
Geiger and Ernest Marsden discover the
nucleus with their gold foil experiment