when I started learning chemistry I
learned about this powerful rule called
the octed rule which says that any
element if it gets eight electron in its
outermost shell the valency shell it
tends to be super stable that's why
noble gases are all stable because all
of them have eight electrons in the
outermost shell and the elements that
don't have eight electrons well they
tend to somehow get it for example if
elements have one extra electron they
tend to lose it if elements have less
than eight electrons they tend to gain
an electron all because they want to
form eight electrons in his outermost
shell and this becomes the core
motivation for elements to form bonds
and then we go about learning different
kinds of bonds and so on and so forth
but for me the big question remained why
OCT it where does the eight number come
from why is it that if elements have
eight electrons in the autost Shell they
become stable what is it about that
number I never got an intuitive answer
for this question until now so the goal
of this video is like always pretend we
don't know anything about octed or noble
gases or nothing like that and then l
logically step by step try and
ReDiscover everything from first
principles and if I do it right spoilers
we will see that that number eight is
not some random number but it's
connected deeply to the fundamental
structure of the electron and the
universe itself so if you're ready for
this let's begin so fman how do we
logically predict which elements are
noble and which elements are reactive
fan says mahes chemistry at the heart of
it is all about transfer of electrons so
there are two things that we can think
about first if you take any element how
easy it is to pluck an electron from its
outermost shell in other words how much
energy is required to pull out an
electron we call this the ionization
energy now if an element has low
ionization energy that means it's very
easy to plug that electron which means
it's very easy to transfer electrons so
that element is going to be reactive so
that's one thing we can look for but
there's another thing just like when you
drop a ball towards the Earth the
potential energy reduces and gets
released as kinetic energy similarly if
I throw an electron towards a neutral
atom then as it falls down because of
the nuclear pull the potential energy
reduces and that gets converted into
kinetic energy which can be converted
into heat and so many other things so
energy will be released this energy is
what we call electron affinity now if an
element has very high electron affinity
that means a lot of energy is released
when it grabs on an electron and that
energy can be used to do more
interesting things like maybe pluck an
electron from some other atom which
means if an element has an high electron
affinity even then it tends to be
reactive because a lot of energy gets
available so look this means if an
element has low I energy or high
electron affinity it tends to be
reactive now in contrast mahes what if
an element has high ionization energy
and no Affinity at all to electrons then
it's very hard to pluck an electron and
it will not even try to grab an electron
because no energy will be released
then this element will be non-reactive
in other words the element will be Noble
that's how we can logically figure out
which elements are noble and that sounds
pretty interesting for me all right so
that makes sense but now the big
question is how do we logically figure
out the inan energies and the electron
affinities of different atoms and fman
says for that we need to now have a
model of an atom well the most intuitive
model is a planetary model but it does
not have a lot of predictive power in
contrast the most accurate model is the
quantum mechanical model here for
example um we do not we do not think of
electron as orbiting the proton in
hydrogen for example instead we like to
think that the electron is forming some
kind of a standing probability wave you
can kind of think about that in three
dimensions very hard to imagine that
however if you try to look at it or make
a measurement that whole wave collapses
to a particular point and now you
measure electron at a specific point
stop looking at it and again it evolves
to form a stationary wave look at it
again it collapses to a particular point
that collapse is random okay but if you
do it enough number of times you will
now see a map being formed a probability
map of you know where is the high high
chances of finding an electron this is
what we call an orbital and this is the
way quantum mechanics deals with the
model this is the quantum mechanical
model of the atom okay so this is what
the ground state looks like on the other
hand if I Supply some energy to it the
electron get excited and now it can have
different probability maps for example
in the first excited state it might look
like this or maybe like this and so on
and so forth and we've talked about
these different kinds of orbitals their
shapes and all about them in our
previous video but you don't have to
watch that video if you haven't already
okay what's necessary for the sake of
this video is that in the first energy
level you have only one orbital in the
second energy level you have two Square
1 2 3 four orbitals in the next allowed
excited state energy level you have
three Square nine orbitals and so on and
so forth fourth okay now the important
thing is that you can say this is just
for one electron this is just for the
hydrogen imagine you have two electrons
and more electrons oh my God things are
going to become super complicated super
fast so this is very complicated model
so what do we do well the best way to
move forward is to sort of use a model
in between in fact we're going to take
insights from our quantum mechanical
model and use it to upgrade our
planetary model I feel that's the best
way to go forward in in which we Ure
that there's still intuition intact and
it's still pretty accurate okay all
right so what insights do we need from
the quantum mechanical model the Insight
is that each orbital has a maximum
capacity of just two electrons this
comes from the PO Exclusion Principle
which is probably a topic for another
video okay as a result of that the first
energy level since only has one orbital
it can accommodate a maximum of just two
electrons the next energy level has four
orbitals so it can accommodate four * 2
eight electrons the next one has nine so
9 * 2 18 electrons and so on and so
forth and now let's use this to upgrade
our planetary model so we can now say in
our planetary model the first shell can
accommodate two electrons the next Shell
can accommodate eight electrons the
shell after that can accommodate 18
electrons and so on and so forth all
right so now that we have an upgraded
planetary model let's see if we can use
that to think about ion energies and
electron affinities so let's start with
hydrogen it has one proton and one
electron and we'll use hydrogen as our
Baseline we'll look at the numbers for
hydrogen and use that to predict the ion
energies and the electron affinities of
all the other elements okay all right so
what is the ion energy of hydrogen one
can actually mathematically calculate
this and it turns out to be somewhere
close to 14 electron Wars and
experimentally it's hard to ionize
hydrogen which means 14 electron is
pretty high value of high energy just to
get some feeling for numbers okay what
about its electron affinity well let's
just shoot an electron towards it and
see whether it can grab it well an
electron will experience an attractive
force from the proton and some repulsive
force from this of already present
electron but you can kind of see the
attractive force is larger so the
hydrogen will be able to grab onto an
electron and some energy will be
released and that energy released
happens to be about7 electron volt and
it turns out in nature we do find
hydrogen minus ions for example we do
find hydrides you know lithium hydrid
sodium hydrides so this means that 7 is
pretty decent value of electron affinity
okay so with these numbers in mind now
let's think about all the other elements
so let's go to the next element helium
helium has two protons and two electrons
so what changes well the two protons
will pull on the two electrons much more
than before and so they will be much
closer now because of the electron
repulsion removing electron becomes
slightly easier but because there are
two protons and we are so close to it
the nuclear pole is much stronger than
we had that than what we had in hydrogen
so we would expect overall the ion
energy to be much larger than hydrogen
and that's exactly what we get the ion
energy for helium happens to be about 24
electron hores super high okay what
about electron affinity well for that we
need to add an electron and guess what
the first shell is full it cannot
accommodate more than two electrons so
the next electron has to go into the
next Shell now this electron experiences
attraction from these two protons but
also experiences repulsion from these
two electrons and it's so far away we
can kind of model it and say that it
completely cancels out so we like to
think that these two electrons kind of
Shield the nuclear pull of the protons
so the two electrons and the two protons
their their attraction repulsion cancel
out and as a result this electron does
not feel anything which means helium
will not be able to grab onto that
electron at all in other words helium
should have no electron affinity and
that's exactly what we find it has zero
electron affinity and look right in
front of your eyes we have very high
anation energy no electron affinity that
is our Noble and that's why helium is
Noble amazing isn't it that's how we
figure things out and before moving on
t-shirts well I make my own you can buy them Link in the description below okay
all right moving on to lithium there are
three protons and three electrons but
wait a second fan in the second shell
there are S and P orbitals which orbital
does it go into well F says for now
let's not worry about that let's just
imagine that the electron just goes into
the next Shell let's not worry about
orbitals all right okay just like before
because of the stronger pole the the
first shell becomes even closer and just
like before the two electrons over here
will shield two protons which means this
electron effectively only experiences
the pole of just one proton and what we
now find is look this is just like the
hydrogen except the electron is much
farther away than what it was in the
hydrogen which means plucking it must be
easier than hydrogen so I would expect
now its ion energy to plummet it should
be much smaller than hydrogen itself
right and indeed that's what we find its
anation is Inc energy is incredibly
small small so it's very easy to pluck
it out and that's what makes lithium
super reactive what about his electron
affinity I don't care about it because I
already know the electron iish energy is
very low so it's very reactive but still
thinking about it just like with the
case of hydrogen it can hold on to
another electron but again we would
expect its Affinity to be smaller than
the hydrogen because it's much farther
away and that's exactly what we find its
Affinity is slightly smaller than the
hydrogen okay moving on berum has four
protons two are shielded and therefore
these two electrons will experience the
PO of two protons which means increased
pool the shell comes closer and just
like before as a result of this look the
inish energy we would expect it to be
higher than lithium because electrons
are experiencing double the pool twice
the pool they're closer and so on and
that's exactly what we find it's more
than what you get in lithium and
similarly the electron affinity should
also increase because there's a stronger
pull and in fact we can now go faster
the more protons we as we add go towards
Florine for example the number of
protons increases the number of
electrons increases the shell comes
closer and closer so we would expect
both the iish energy to Skyrocket and
also the electron affinity to Skyrocket
because of the higher pole it can grab
onto that electron much more easily and
if you look at the numbers that's
exactly what we fine look the ion energy
has increased and similarly electron
affinity should also increase at this
point I'm like wait a second fine one I
do see some anomalies over here and fman
says don't worry too much about that if
you blur your eyes you see a Trend that
the Affinity is increasing and the
ionization energy is increasing we'll
address this a little bit later okay now
we add one more proton and go to Neon
what happens well again the shell comes
closer we would expect the iish energy
to further increase very high and what
about its Affinity well now if I have to
add another electron the shell is full
it has to go to the next Shell and by
the time it goes to the next Shell all
these 10 protons are shielded by these
10 electrons so the electron will not
feel any pole and so you'll not be able
ble to grab onto it which means the
electron affinity of neon plummets and
look again we have our Noble very high
Antion Energy Zero Affinity that's what
makes neon noble so fan says mahes let's
pause and reflect for a while okay if
you go back to lithium why does it tend
to lose electrons well not because it
wants to lose electron it wants to be
like helium none of that it's because it
takes very little energy to plug that
electron it has low Ian energy and
that's why it tends to lose electrons
and that's that on the other end of the
spectrum when you come to Florine why
does it tend to gain electrons in
chemical reactions well again not
because it wants to fill its shell or it
wants to be like neon none of that the
fact is when you grab an electron a lot
of energy is released it has a very high
electron affinity and that's why it
tends to grab electrons and become
negative ions in chemical reactions but
what happens when you go to Neon well
iation energy increases because you have
one extra proton however because you
have now the next electron can only go
into the next Shell the electron
affinity plummets to zero and that's why
neon becomes Noble so look what I love
about this is that we are going away
from wants and desires and all that lose
language and concretely talking in terms
of IO energies and electron affinities I
love it and so when you have a filled
shell very high en Energy Zero affinity
and that's why you get a noble and so
now fan asks mahes when do you think
will you get the next Noble and I'm like
well the next Noble element is when the
next sh shell is completely filled and
the next Shell can accommodate 18
electrons so we would expect the next
Noble to be the element which has 2 + 8
that's is 10 10 + 18 28 electrons and
that is nickel wait
what nickel is not Noble I know that I
know argon is Noble but that doesn't
make any sense why is argon Noble argon
has only 18 electrons when mean which
means in its outermost shell it should
have only eight electrons so why would
argon be Noble argon I would expect to
have high high iation energy but I would
expect it to also have high electron
affinity because the shell can still
accommodate more electrons why is argan
Noble shouldn't nickel be Noble what's
going on fan and fan says mahes this can
only mean one thing it's time to upgrade
our planetary model just like how we can
also in the description all right now back to upgrading our model so the big
problem is we get the first noble gas
when the first shell is completely
filled we get the second noble gas when
the second shell is completely filled
however for some weird reason we don't
get the third noble gas when the third
shell is completely filled no no no we
get the third noble gas when the third
shell is partially filled how does that
make any sense fan and fan says well
mahes remember that these shells are
actually orbitals well let's think about
them now so if we go back to the
original question that you asked for
lithium where exactly will the third
electron be in the second shell will it
be in the 2s orbital or 2p orbital where
do you think will it be let's try to
answer that question now between you and
me I know that the S orbital gets filled
before the p orbital because we've taken
some chemistry course but remember we're
pretending we don't know anything this
is a complete rediscovery So my answer
to fan would be fan I don't know they
look exactly the same they have the same
energy level so how do we figure this
out and fan says well let's look at this
in a little bit more detail remember the
whole point we said that for the lithium
the third electron is in the outer shell
and as a result the two electrons over
here Shield the two protons and so the
electron the valence electron only FS
the pole of one one proton that was a
story but is that accurate well the
reason why that cannot be accurate is
because remember according to quantum
mechanical model electrons are not at a
fixed distance remember that they are
forming some kind of probability
standing waves so it's possible that
sometimes when you measure this electron
it can be found inside that Shield it's
completely possible remember it's all
about orbitals which means when that
happens that electron feels the full
force of all the three protons but next
time when we measure the electron maybe
it materializes outside the shield this
time it will only experience the pull of
one proton next time again it might be
you know it might collapse inside and
next time again it might collapse
outside so look on an aage that electron
is going to experience a pull of more
than one proton because there is a
chance that it can be found inside the
shield as well but I'm like all right
that kind of makes sense fan but how
much more fan says well that depends
upon its probability of being inside the
shield if there's a higher probability
of being inside the shield on average it
will have a higher pole if it has a
lower probability of being inside the
shield on average it'll experience a
lower pole that makes sense right so now
let's look at the SNP orbital and see in
which case there's a higher probability
of being inside the shield so this is
what the S orbital looks like and this
is what the p orbital looks like in
which case mahes there's a higher chance
of electron finding an electron inside
the shield and I'm like oh my God in the
p orbital we have a node here the
probability of finding the electron
somewhere close to the nucleus is zero
but in the S orbital the 2s orbital the
node is outside it's a radial node so
clearly just from the diagram you can
see there's a higher probability of
finding the electron inside the shield
in the S orbital compared to the p
orbital which means if you were to go
back to modeling this we could model
this electron we could say that hey this
electron experiences a pull of say 1 and
a half half you know uh protons on an
average but then what would be the pull
over here it would be less because
there's a less smaller chance of being
inside and so it would experience a pull
of let's say not one and a half just
1.2 protons as an example so look this
means in which case it's easier to plug
the electron in this case it's much
easier even though in both the cases
electron is in the same shell because
here it experienc is a less effective
nuclear pole it's much easier to plug
the electron from here compared to here
which means if you look at these two
energy levels the S orbital level must
be deeper compared to the p orbital p
orbital level must be slightly higher so
it's easier to pluck the electron from
the p orbital compared to plucking it
from the S orbital that's why 2s is
lower in energy compared to 2p because s
orbital can penetrate The Shield
better compared to p orbital does that
Mak sense and I'm like that makes sense
this is the reason why the S orbital
comes lower the 2s orbital comes lower
than 2p and that's why the 2s orbital
gets filled first it's amazing isn't it
but now the big question is fan does
this change anything about the elements
that we have uncovered so far and fan
says no Mahesh not really because look
the S and P orbitals are still very
close to each other so effectively our
story still holds as we go to heavier
and heavier elements the iish energy
will keep increasing the electron
affinity still keeps increasing until
the entire shell is filled when the
electron affinity plummets and we get a
noble gas but now what happens when you
go to the next element sodium which has
11 electron the 11th electron will now
enter the next Shell but where is it in
s orbital p orbital or D orbital well
again let's look at all the three cases
separately in our naive model we might
say that hey all the 10 electrons will
shield the 10 protons and therefore the
electron will effectively experience a
full of just one proton but we know now
that's not true because the electron can
penetrate The Shield but which orbital
penetrates the shield better well let's
look at it if you look at the three what
do you notice well again just like
before we see the S orbital penetrates
the shield way better compared to the p
orbital and the D orbital just
completely sucks look at that the D
orbital has absolutely no penetration at
all so in the S orbital because of the
penetration the effective nuclear pool
would be higher than one electron one
proton it turns out to be if you do the
math about 2 and a half protons in 3p
well again you can see it's lower than S
but the penetration is there so it will
turn out to be 2.2 protons let's say but
when it comes to D orbital look it
doesn't penetrate at all so for D
orbitals for all practical purposes we
could say that the electron experiences
of force of only one proton you know
effectively and so now when you compare
the three what do you notice just like
before the 3s and 3p are still very
close to each other but 3D look it only
experiences a force of one proton it's
much less tightly bound compared to
these two plucking this electron is way
easier it takes a little bit of energy
compared to these two in other words the
3D must be much higher compared to 3s
and 3p and so if we go back over here
the way our energy level diagram will
modify is that 3s and 3p are still close
to each other but 3D look how high it
has gone in fact now we can quickly look
at the next few elements if you look at
sodium it's the easiest to plug that
electron so you would expect very low
ion and energy and then as we go further
down the ion energy should keep
increasing until we reach chlorine very
high iation energy even very high
electron affinity but now when we go to
argon what's going to happen we have one
more proton and one more electron so the
iish energy further increases making it
super high but then what about Affinity
well earlier I thought that it should
have a very high Affinity because
there's still space in the third shell
but now you can see that if you add the
next electron the electron has to go in
3D in fact if you look at this carefully
3D is even higher than 4S and that is so
weird even though the fourth shell is
much farther away than the third shell
because the S can penetrate much better
than D the 4S orbital ends up at a lower
energy than
3D so when you add the next electron it
would effectively go to 4S before going
to 3D and if it goes to 4S just like
before it'll be very hard to hang on to
that all the electrons below are going
to Shield all the protons and that
electron will not be able to hang on to
it in other words it will have zero
affinity and that's why Argan becomes
Noble it's all because of the fact that
3D sucks at penetrating making its
energy level so high it's almost
inaccessible and that's why when Once 3s
and 3p gets filled the next electron has
to go it has no choice but it has to go
in the next Shell and that's why you get
a noble gas when the 3s and 3p got
filled now the same thing happens again
what when do you expect the next Noble
well we would expect the next Noble when
the 4S and 4p gets filled because again
just like before 4D would be much higher
so once 4S and 4p and of course 3D would
be filled by then gets filled we would
expect the next electron to come to 5S
very hard to hang on to it so once 4S
and 4B gets filled again we would expect
the element to have high ion energy low
electron affinity zero electron affinity
giving us a noble gas and so now fan
says mahes let's put it all together
when do we get a noble gas when the S
and the p orbital of a particular shell
gets filled and when that gets filled
look how many electrons we end up with
we end up with eight electrons and
that's why when you have eight electrons
in the outermost shell you end up
becoming super stable because the next
electron has to go in the next Shell
that explains the octed but fan what
about other elements why do they tend to
want to have octed and fan says they
don't if you look at for example the
next element the element that comes
after the noble gas over here for
example potassium then it's just that
the next electron is in the next Shell
it's so easy to pluck it and therefore
they tend to usually lose an electron
and when they do look they end up with
eight electrons in its outermost shell
and if you look at an element that was
just before an obal gas for example in
this case chlorine it has one less
electron and so it has a very high
electron affinity and that's why they
tend to gain an electron and when they
do look they end up having eight
electrons in its outermost shell it's
all about ion and electron affinities
they don't want or desire anything so
finally answer our big question where
does the eight electron rule comes from
why is it that if you have eight
electrons in the valence shell you end
up with a noble gas well that's because
you get a noble gas when the SN P get
filled and look since there are total of
four orbitals and each orbital has a
capacity of two electrons 4 * 2 is 8
that's where the eight comes from so we
could tie it back all the way to Po's
Exclusion Principle and the PO Exclusion
Principle actually comes from a deeper
fact that electrons are all identical
firion again fion are elements that have
half integral spins don't worry too much
about them but because electrons are
identical and they're fion that's why
each orbital can only accommodate two
electrons and therefore you get eight
hypothetically if poli's exclusion rule
did not hold good not did not hold and
if an orbital could accommodate more
than two electrons all bets are off it
would no longer be the magic number but
that's not all the second reason is
because we have exactly four orbitals
why do we have four orbitals well s is
spherically symmetric so you'll always
get one orbital but why do we have three
p orbitals well that's because we live
in threedimensional spatial Universe if
you had more or less spatial Dimensions
we would have more or less P orbitals
and again the number would be different
so it's also because of the fact that
our universe is spatially in three
dimensions but finally the final reason
is the fact that 3D electrons 3D
orbitals and higher F orbitals all of
them suck at penetrating and therefore
their energy level is much higher making
them inaccessible if they were
accessible then again it wouldn't be the
magic number but what makes them so
inaccessible because they cannot
penetrate the Shields and the shielding
effect comes at the heart of it comes
from the fact that electrons repel each
other and that's our reason number three
so if you put it all together what's
mindblowing is that the fact that
electrons are identical Fons that repel
each other and exist in a
three-dimensional spatial universe
literally directly gives us the octed
rule that is amazing what a connection
what a connection did I tell you it is
deeply connected to the fundamental
features of the electrons and our
universe itself oh man oh man now before
we end the video fan asks a bonus
question remember that we thought that
the electron affinities and the inos
energy should increase but there are
some exceptions now with the orbital
picture now the fact that s orbitals are
slightly lower than P orbitals we can
use that to explain these exceptions and
that's a homework for you but the
broader takeaway over here is I should
think that chemistry is full of rules
and exceptions and I can never make
intuitive sense of them but I was wrong
it's all about finding the right models
using the right model you can make
things intuitive and whenever you do
find an exception it just means that
your model is limited and once you
upgrade them the exceptions start making
sense I'll see you