this video sponsor is Squarespace the all-in-one website building 2 ion 56 is
one of the most stable elements in the
universe but the big question is why I
mean the most common answer I get in the
textbook is that it has very high
binding energy per nucleon but that
doesn't explain anything in reality
what's going on is that if you were to
draw a graph of the stability of the
nuclei versus the number of protons and
neutrons you find in this nuclei you
will see that initially as the nuclei
become heavier the stability increases
but then after a point the stability
decreases and eventually nuclear tends
to become very unstable and since the
stability increases and then decreases
there must be nuclei in between which
have the maximum stability one of them
happens to be ion 56 but the big
question is now why does the graph turn
out to be that way why do nucle
initially become more stable as it
becomes heavier and then becomes less
stable what's going on this is not just
a curious question turns out that we owe
our life to this graph and I'm not
exaggerating over here think about it in
this region because heavier nuclei are
more stable than lighter ones this
motivates the light nuclei to fuse into
heavier nucleus this is literally what
powers of the Stars allowing them to
manufacture heavier and heavier elements
but what's important is that once they
they reach ion they can no longer fuse
ion because fusing ion will make
elements more unstable right and that's
where once they reach ion the fusion
stops and these very super massive stars
then they will collapse under their own
gravity and then explode and that
unlocks all the elements they had
manufactured and these elements can
eventually find their way into your and
my bodies that's how we exist okay but
there are so much more this is also the
reason why unstable elements undergo
radioactive decape this is one of the
reason why the earth's core is still
very hot after 4 and a half billion
years since its formation and this hot
core is what allows us to have a
protective magnetic field without which
again we wouldn't survive and nuclear
fusion also happens because of this but
the big question is why does the nuclear
stability work this way why initially
the nuclei become more stable as they
become heavier but then they become less
stable as they become even more heavier
what's going on over here for a long
time I had no intuition for this until I
met POI so the goal of this video is to
understand the nuclei slightly more
deeply so that we can intuitively
ReDiscover this particular graph and
appreciate our existence so if you're
ready for this
let's begin so po Where Do We Begin well
po asks mahes what keeps an atomic
nucleus stable and I like that's easy
neutrons Atomic nuclear have a lot of
protons which tend to blow each other
apart because of their Mutual repulsion
repulsion which makes the whole
thing unstable and that's where you have
to put neutrons if you put enough
neutrons in them the whole thing acts
like a glue and makes it whole makes the
whole thing stable that's their one job
so just put a lot of neutrons and the
nucleus becomes stable or does it say
the this is an oxygen nucleus which has
eight protons and eight neutrons the
number of protons are written over here
and the total number of protons and
neutrons are written over here so you
can see 8+ 8 is 16 okay now we know that
this is stable right so prob ask mes
according to your logic if we imagine a
nucleus we had which had just two
protons and 14 neutrons then this should
be super duper stable right because
there are only two protons trying to
repel each other and 14 neutrons gluing
the whole thing right and I'm like yeah
that should be super stable but it's not
it's super unstable and that's why
helium 16 does not exist why not mahes
and I'm like that's an interesting
question Po in fact the broader question
over here is if neutrons make the whole
thing stable shouldn't our world be
filled with isotopes with lots and lots
of neutrons yeah that's a great question
why why is that so unstable PO says
mahes the mistake you're making is your
only thinking in terms of forces at the
quantum level to stand stability you
need to think in terms of energy levels
for example you probably know that
electrons inside an atom they don't have
any energies they want they have some
discrete energy levels right if the
electrons are at higher energy level
they tend to be unstable and they want
to jump down to the lowest energy level
but that does not mean all electrons can
occupy the lowest energy level why not
Po's Exclusion Principle it says that no
two electrons can have the same Quantum
state so if you have the lowest energy
level you can occupy the two electrons
can occupy that because they can have
the opposite spin but if a third
electron occupies that same level then
look these two will have the same
Quantum State and POI says that's not
allowed it's a great question as to why
not something a topic probably for
another video but because it's not
allowed this electron is forced to go to
the next higher energy level and that's
how the electrons fill it up now to be
technically correct I shouldn't use the
word energy level I should use the word
orbital but let's not worry too much
about technicalities over here intuition
is what matters
but the big thing is the same thing
applies to protons and neutrons inside
the nucleus they too obey Po's Exclusion
Principle and they too have energy
levels so let's now look at our
hypothetical helium 16 this doesn't
exist but if it did what does the energy
levels of protons and neutrons look like
well let's draw them just like with the
electrons they also have protons and
neutrons can also occupy discrete energy
levels now if you put since this has
only two protons they can occupy those
lowest energy State because they would
have opposite spins I've not shown the
spins just to make sure that there's not
much clutter over here but now what
about the neutrons where would they go
and I'm like okay the neutron should
occupy the higher energy level and no
says FY see protons and neutrons are
different particles which means they
will have their own energy levels so
even neutrons will start occupying from
here and then they'll start going up so
this is what it would look like but now
do you see the problem over here these
neutrons are occupying these high energy
levels when there are these juicy lower
energy levels up for grabs but the
neutrons can't go and grab them because
these are only meant for protons what a
shame nature is now thinking what if she
could convert these high energy neutrons
into protons then those protons could
occupy lower energy levels and the
system will be more stabilized and
that's exactly what she does nature has
stck uper sleeves using which she can
convert a proton a neutron into a proton
and for that matter a proton into a
neutron po asks mes do you identify this
and I'm like that's the better DK that's
the radioactive DK we've learned in
schools right of course this is not a
complete reaction because the charge is
not conserved actually when a neutron
converts into a proton it spits out an
electron so the charge gets convert
conserved over here and a tiny neutral
particle called as an anti-neutrino this
is why this is called a beta minus DK
because of the electron and when a
proton gets converted to Neutron well it
spits out a positron an evil twin of
electron but just positive and that's
why this is called a beta plus DK but
the details are not important for us
what's important is that nature has a
way to convert protons and neutrons into
each other so if we go back over here do
you think nature will accept this no she
would not accept these high energy
neutrons she would rather just convert
them into protons using better DK and
then make sure the protons occupy those
juicy lower levels and the whole thing
now has much lower energy the whole
thing is stabilized this is why helium
16 would never ever exist and this is
why Oxygen 16 is stable now similarly if
you had to add even more neutrons to
this oxygen nucleus do you think this
would be stable no it won't be because
again the problem is the same there are
so many neutrons at high energy level
and there are so many juicy energy
levels available nature would might as
well convert them to protons using beta
DK and make sure the lower energies are
occupied and therefore magnesium 12 for
example is also highly stable and so on
and so forth so from this we can see why
nuclei tend to be stable when the number
of protons equals the number of neutrons
because if you had too many protons or
too many neutrons then nature would just
better Decay them and convert them into
each other to occupy the lower energy
states and stabilize the whole nucleus
doesn't this make intuitive sense that
it also helps me understand why beta DK
even exists in nature what's the
motivation for the beta DK it makes
perfect sense so po this means all the
stable nuclei should have similar number
of neutrons and protons but do they if
you look at the lighter elements then
look you can see that this number is
double of this number meaning the number
of protons is pretty much the same as
the number of neutrons but if you look
at the heavier elements that's no longer
true look this number is no longer
double of this this is more than double
of this that means there are way more
neutrons over here compared to protons
and the same is the case in very heavy
elements what's going on over here po
why is it to build a stable heavy nucle
say we need more neutrons and PO says
make them occupy the lower energy State what's going on PO well PO says mahes
let's now talk about the force that
actually binds the nucleus together it's
called the strong nuclear force the
strongest force in the nature right but
it has a weakness it is is short ranged
so if you were to look at a hypothetical
nucleus like this what you would find is
that each Proton or Neutron can only
exert a strong nuclear attraction on its
neighbors so if you look at this proton
it's only experiencing a strong nuclear
attraction due to its neighboring
particles but kum's repulsion is long
ranged it has almost infinite range
which means it's experting repulsion due
to all the other protons so even though
the strong interaction is much stronger
than the kum's repulsion because kum's
repulsion has infinite range if you have
too many protons and the nucleus becomes
big enough then the protons can
experience a much larger kums repulsion
it can overwhelm the nuclear attraction
but wait a second why doesn't this
affect the the lighter nuclei well when
you consider the lighter nuclei they are
much smaller and because they are
smaller most of them are within the
nuclear range so if you consider this
particular proton look all the other
neutrons and protons are in its
neighborhood and so they will all exert
a very nice strong nuclear force and it
is only experiencing a repulsion due to
these two protons so the strong nuclear
force is very very dominant over here we
don't have to worry about the repulsion
at all now if we add more protons and
neutrons look the neighbors the amount
of neighbors increases and because the
amount of neighbors increases the strong
nuclear attraction actually increases
and so the whole thing gets bound
together even more tightly and more
strongly which means as we add more
protons and more neutrons as the
neighborhood as the neighboring
particles increases the stability
increases that's why initially as you
add more protons and neutrons the
nucleus starts becoming more and more
stable it gets more bound tightly but
then after a point the number of
neighbors don't increase anymore right I
mean you can only have enough Neighbors
after a point point the particles will
no longer be your neighbors so imagine
an extreme case where we have these many
protons and neutrons now you can
immediately see most of these particles
are now outside of the nuclear range so
this proton is only experiencing a
nuclear attraction due to its neighbors
but it's experiencing a repulsion kums
repulsion due to all the other protons
at some point that will overwhelm the
colum's repulsion will win and therefore
a nucleus like this will be highly
unstable so how do you stabilize a
nucleus with a lot of protons well you
need to make sure that each proton is
surrounded by a lot more neutrons that's
the only way and if you want to do that
you want to make sure that there are way
more neutrons compared to protons that's
the only way you can make these nuclei
stable but wait a second po nature
doesn't like these hydrogen neutrons
right why doesn't she just convert them
into protons like she did earlier and
then occupy those lower energy State and
PO says mahes yes nature hates these
high energy neutrons but she doesn't
have a choice right now for example
there's just enough balance between you
know kum's repulsion and nuclear
attraction because of these extra
neutrons if she converts them into
protons that balance is lost and the
whole thing becomes unstable so this is
actually unfavorable so so she doesn't
have a choice she has to make do with
this high energy neutrons and because
she hates those high energy neutrons
this is why she's unhappy this is the
reason why this nucleus tends to be less
stable than the lighter ones because
they have to have more neutrons which
occupy higher energy levels the heavier
it gets now the more neutrons it needs
and the less happy she becomes and the
less stable the nucleus gets all because
of these high energy neutrons but after
a point nature says enough is enough I
cannot accept these high energy neutrons
but what can she do she can't convert
them into protons but she has another
trick up her sleeves something that we
can again talk about in a future video
as to how exactly it happens but she can
throw out these protons and neutrons as
alpha particles as helium nuclei this is
called Alpha DK and this is actually
much more nuanced because throwing them
out requires some Quantum processes you
just cannot classically do it again
story for another video if you're
interested but she will throw them out
in doing so look she has reduced the
number of protons which is great reduce
kum's repulsion and she has reduced the
energy level of the high energy neutrons
which is more awesome and that's how
that's how she stabilizes the whole
thing and that's what explains
radioactivity that's why after a
particular Point elements just just
cannot be stable because nature will not
allow it she will just keep throwing out
more and more alpha particles until the
whole energy is lowered to a manageable
state so now if you put it all together
can we understand the stability of the
nuclei initially if you consider the
light nuclei as you add more and more
protons and neutrons to it you are
increasing its neighbors as you increase
the neighbors the nuclear force tends to
increase
making it bind together much more
tightly making it more stable and that's
why initially as you have more and more
protons and neutrons the stability
increases but after a point you can only
have so many neighbors that's when
saturation for nuclear force is reached
now if you add more protons and more
neutrons particles will no longer
experience more attraction it's
saturated but protons will start
experiencing more repulsion because of
the kum's infinite range and and that's
why the stability after a point tends to
decrease so this happens because of the
short range and of course it's
manageable by having more neutrons at
high energy nature doesn't like it the
whole idea behind why it becomes less
and less stable but after a point nature
basically gives up and says no that's
not acceptable and so she just starts
throwing out particles giving you
radioactive DK so this explains why
Alpha DK happens we also understand how
better DK happens and all of this allows
life to exist protons and electrons get
all the Limelight because they decide
the elements and they are involved in
chemistry and electricity and whatnot
but the real stars are neutrons they are
not just there to support they are
making the nuclei stable in very
sophisticated ways and just to
appreciate that we named our pet Neutron
understanding of quantum super position
what it is and what it is not see you