Composition of Nucleus Video Lecture | Physics for JEE Main & Advanced

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hi it's mr. Andersen and this is ap
physics essentials video 3 it's on the
atomic nucleus which was discovered by
this man Ernest Rutherford JJ Thompson
had already discovered the electron
which they knew had a negative charge
but they have viewed the atom as a plum
pudding and this is what plum pudding
looks like it's got little plums in it
and then it's got the pudding portion so
they thought they thought the atom
looked like this it was this amorphous
positive charge inside the atom and then
these little negative charges were
interspersed and so what Rutherford did
is he shot alpha particles at it those
have a positive charge so he assumed
they would just go right through since
there was no positive centering of that
positive charge that they were just kind
of moved through in a straight line but
what he found is that these positive
charges went straight on through but
occasionally would come shooting right
back at him it would be bent in some
form and so he said it was like shooting
a shell at a kleenex in that shell just
came shooting back at him and so what he
discovered was this really dense tightly
packed positive nucleus which we know
now is made up of positive protons but
also these neutral neutrons now the
protons are important because the number
of protons tell us what the element is
in other words if it's got six protons
we know that it's going to be carbon if
it's got one proton then we know it's
going to be hydrogen now you can have
something that's the same element but
varying amounts of neutrons and so if we
add up the number of protons and
neutrons and they're different in an
given element we call those isotopes of
that element and so for example
carbon-12 is going to have six protons
so is carbon-14 because they're both
carbon but carbon-12 is going to have
six additional neutrons and carbon-14 is
going to have eight additional neutrons
and so we're gonna have these different
isotopes and we'll find that for all of
the different elements some of these are
radioactive and what that means is
they're unstable and they have a
potential to decay or to fall apart they
give off radiation when they do that and
the rate at which they do that is known
as their half-life and so again the
atomic nucleus is made up of two
subatomic particles we call those
protons which have a positive charge and
neutrons which have
no charge you could count the number of
positive charges right here and figure
out what the element is if we organize
the protons in a certain fashion we get
the periodic table and so we know that
hydrogen has one proton that's where the
atomic number means we know that iron
has 26 we know that gold has 79 it tells
us what the element is but you can have
atoms of the same element and varying
numbers of neutrons and when we do that
we create something called an isotope
and so if we look low on the periodic
table we find uranium 92 that means it
has 92 protons but where there are going
to be three naturally occurring isotopes
on our planet uranium 238 it's gonna
have a hundred and forty six neutrons
uranium 235 will have 143 and uranium
234 will have 142 where did I come up
with those numbers if I take 238 which
is the sum of neutrons and protons and I
subtract 92 that tells me the number of
neutrons and so on our planet we're
gonna have varying amounts of that and
the average of that gives us the average
atomic mass now if we look at this graph
right here this is graphing the number
of protons along the x axis and the
number of neutrons along the y axis and
so this would be that virtual straight
line if the number of protons and the
number of neutrons are equal and you'll
find right here the average which is
this Jaggi line right here starts to
drift towards the neutron side what does
that mean as our atoms get larger and
larger and larger and as our atomic
nucleus gets larger and larger and
larger you have to have more neutrons to
maintain the stability of the atomic
nucleus and we'll talk about that in
later videos but what you create and
what the colors on this graph are are
different types of decay or isotopes
breaking down or they're giving off
what's called radiation so radioactive
decay is when the atomic nuclei breaks
down and we could summarize that in
three different types of radiation you
have an alpha particle and that's two
protons and two neutrons that are given
off you could get a beta particle and
that's either going to be an electron or
a positron and then you have this
high-energy high-level gamma radiation
it's electromagnetic radiation so this
would be an alpha particle given off by
it by a nucleus and so it's become a
different element so if we talk about an
example of that uranium 238 naturally
occurs on our planet but it's gonna
undergo decay it will lose a alpha
particle and so as it loses that it's
losing four of these nucleons these
parts inside the nucleus and so you can
see that the mass number has changed but
it's also become a new element since
you've lost two of these protons it's
not uranium anymore its thorium it could
undergo then beta decay so we lose a
beta particle and it comes proact inium
234 it can lose another beta our beta
particle and it could become uranium 234
and so each of these have a probability
of occurring and that probability is
going to be in the atom itself and so if
we take a sample of 238 we can create a
curve of what it's what's called its
half-life which is the ability for half
of the atoms in that sample to decay or
to break down and so if we look at
uranium 238 at the beginning of time so
time is graphed along the x axis at x 0
we're gonna have a hundred percent of
that uranium 238 in one half-life that's
what this one on the x axis stands for
we're going to have 50% of that uranium
238 decay we're going to lose those
alpha particles now what's the half-life
of uranium 238 it's a ridiculously large
number it's 4.5 billion years which is
about the same age of our planet and so
we would expect in that first 4.5
billion years we're going to have half
of that uranium 238 decay in the next
4.5 billion years we'll go from 50% to
25% and then to 12 and 1/2 and it'll
keep following that at a known rate
because there's a known probability of
each of these atoms decane during that
period of time now what's cool about
this is scientists can find a sample of
uranium 238 we could figure out how much
of it has decayed and we could find it
along this line and that would tell us
how long ago that that uranium 238 let's
say a rock for example had actually
formed and so did you learn to describe
how the internal structure
a nucleus or an atomic nucleus relates
to the properties of that atom remember
the protons tell us what it is and if we
add the protons and neutrons we get the
isotopes which can be stable or unstable
and can break down over time and I hope
that was helpful
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