said that there are several theories
which explain the bonding in molecules
and structure and one of them was
valence bond theory as we have covered
that and the second is molecular orbital
theory now as we said last time
molecular orbital Theory simply says
that the electrons in a molecule these
are the electrons which belongs to the
atoms that is in that molecule are found
in molecular orbitals just like in an
atom the electrons are found in atomic
orbital those are spdf orbitals and so
on but in the molecule the electron goes
in to the molecular orbitals now what
are these molecular orbitals and how we
construct those we said that the
molecular orbitals are simply obtained
by linear combinations of the atomic
orbitals of the atoms that present in
that molecule now the shape of these
molecular orbitals are different than
the atomic orbitals actually and they
expand overall molecule and when you
simply put the electrons into these
molecular orbitals
just like in Otto McQuarrie tools every
molecular orbital can accommodate two
electrons maximum and the spin off these
two electrons have to be opposite to
each other that means that the electrons
two electrons in a molecular orbital
will be paired spin paired
and any electron simply goes into the
Mallick bonding molecular orbital
stabilizes the molecule any electron
that goes into the anti-bonding
molecular orbitals DC stabilizes the
electron this stabilizes the molecule so
again there are two kinds of molecular
orbitals bonding molecular orbitals
anti-bonding molecular orbitals actually
when you combine linearly combine two
atomic orbitals in a molecule you will
get you will end up with two molecular
orbitals one will be binding the other
one will be anti-bonding any electron
again as it goes into the bonding
molecular orbital which is lower in
energy that simply stabilizes the
molecule means that it binds the atoms
in the molecule together
therefore the electrons which are in
bonding molecular orbitals are found
between the nuclei of the two atoms sort
of they simply concentrate increase
their intensity density between the
nuclei if any electron goes into
anti-bonding then they will be sort of
out of the inter nuclear volume they
will be out from there and they
destabilize actually as it says
anti-bonding destabilizes the overall
molecule okay as we showed last time if
we simply look at how these molecular
orbitals would look like in
three-dimensional space here the example
when you have hydrogen molecule so you
combine 1s orbitals of one hydrogen and
with the 1s orbitals of the other
hydrogen and when those two combines
linearly actually
two atomic orbitals we get two molecular
orbitals this is the bonding molecular
orbitals this is the nth abandoned
molecular orbitals so any electron that
goes into the bonding molecular orbiters
spend more time between the nuclei of
these two atoms between two hydrogen
nuclei that means that electron
corresponds to shared by the two atoms
and simply stabilizes the hydrogen
molecule or binds the hydrogen atoms
together in the molecule and an electron
goes into the anti-bonding which looks
like this again this is
three-dimensional remember and the
electron density more out of the inter
nuclear volume here sort of out here
there actually there is a plane where
the probability of finding the electron
is zero that's we call the nodal plane
in fact between the two nuclei now how
about the again here as we said last
time and when you consider the symmetry
of these two molecular orbitals
both have Sigma symmetry what does that
means again as we have seen it before
Sigma symmetry means that the orbital
have have rotational symmetry around the
bond axis bond taxes which binds the two
we call that Sigma type indicating just
the symmetry both of these are Sigma
type orbitals molecular orbitals and
this is called as Sigma 1 as simply just
the notation which tells us that one s
orbital combine to form this bond and
molecular orbitals and this is the
anti-bonding molecular orbital again
Sigma 1s but we put a star there and
that star means its anti-bonding now how
about the energies of these molecular
orbitals with respect to the energies of
the atomic orbitals and this is again
what we call molecular orbital energy
level
diagram indicates the molecular orbital
relative energy is off the molecular
orbital and this is the bonding
molecular orbital this is the end tie
bonding molecular orbital these are the
1s atomic orbitals and one SF atomic
orbitals again you see as we said last
time when you have hydrogen molecule you
form a hydrogen molecule from two
hydrogen atoms the electron will go from
this one s level to the Sigma 1s
molecular orbital binding molecular
orbital and this one will also go there
that simply means when you put the
electron from this level to there that
much energy will be released so if you
put two electrons there that means two
times of that much energy is released to
the surrounding means that much the
molecule become more stable right the
molecule become more stable so again as
we have described last time when we have
the molecular orbital energy level
diagram we can simply write down the
bond order for that molecule now
remember again when we say bond order
that is for the molecule it's not for
the atoms in this case since we have two
atoms we just that will be the bond
order for between for the two hydrogen
atoms actually but the bond order is for
the whole molecule once we have the
molecular orbital of a molecule that
means we know how many electron goes
into the bonding orbital how many
electron goes into the anti-bonding
orbital we can calculate the bond order
using this formula simply the number of
electrons in bonding molecular orbital
bonding simply means lower energy than
these two over here and minus the number
of electrons in anti-bonding divided by
two and that gives us the bond order one
in this case actually two minus zero one
now when we have helium molecule let's
consider the helium molecule remember
the helium molecule has two
helium atom has two electrons and the
electron structure is 1s2 this is the
simply electron configuration of helium
atom so when you combine two helium
atoms just like hydrogen atoms to form a
helium molecule and show the molecular
orbitals there again we will have the
bonding molecular orbital and
anti-bonding molecular orbital exactly
the same as the hydrogen molecule the
only difference is remember since the
helium has two protons hydrogen has one
proton the energies of these 1s orbital
in helium and hydrogen will be different
right and in the case of helium will be
more stable or less stable than the
hydrogen more stable because we have two
protons Z effective actually for those
electrons will be higher and more stable
that's the only difference we can simply
other than the number of electrons
between the helium atoms and the
hydrogen atoms so we have again 1s
orbital just the same shape similar
Sigma type simply as the hydrogen
molecule bonding molecular orbital Sigma
1s and anti-bonding molecular orbital
Sigma 1s
now since we have two electrons coming
from each helium atoms so we have to put
those electrons into the molecular
orbitals this is the molecule actually
these are separate atoms infinitely
apart so we put two Maalik two electrons
here in bonding molecular orbital and we
will put the other two in anti-bonding
molecular orbital now what will happen
energetically if you look at the energy
wise when you put two electrons to the
bonding molecular orbitals that means
that the helium molecules stabilised
twice as much energy here this twice as
that because that much energy is
released when you put one electron there
another elec
they're that much energy is released it
becomes stable but on the other side
when you take one electron and put it in
higher energy level put it in the Sigma
star Sigma 1s star that means we have to
give to the system that much energy and
again the second electron that much
energy what that simply means if the
energy of stabilizations of the bonding
molecular orbital is the same as the
energy of destabilization that means of
anti-bonding what i trying to say here
is this that means that this energy this
distance here is equal to that distance
there will be no energy gain when you
form a helium molecule by combining two
helium atoms no energy gain right
because when you put the two electrons
in bonding molecular orbitals the energy
will be released but when you have to
put the two electrons in anti-bonding
you have to give that much energy to the
molecule again there will be no net
energy gain actually if you calculate
the bond order for helium molecule it
will turn out to be like two electrons
in bonding molecular orbitals minus two
electrons in anti-bonding divided by two
equals to zero what does that means that
the bond order in helium molecule turns
out to be zero that will be interpreted
in molecular orbital theory that there's
no energetic reason for those two helium
atom to form a molecule right actually
in reality we know that helium molecule
do not exist no helium molecule but the
hydrogen molecule do exist actually so
you see again how the molecular orbital
explains that the helium molecule and
the other molecule some neon molecules
are band molecules will not exist
because energetically there will be no
gain or the other way to express that
the bond order becomes zero when bond
order becomes zero
that means there's no reason they have
to stay together as a matter of fact now
we have talked about we have shown you
how I've shown you how the 1s orbital
will combine to form a bonding molecular
orbital with the Sigma symmetry and
anti-bonding molecular orbital with
again Sigma symmetry star how about the
linear combinations of 2p orbitals to
form a molecular orbital
now here the examples we can combine the
2p orbitals
linearly head-on type that means
head-to-head fashion and then we will
simply get two molecular orbitals like
this this is the bonding molecular
orbitals and which has a sigma symmetry
again it has the rotational symmetry
with respect to the bond axis actually
and this is going to be the anti-bonding
molecular orbitals and this is simply
called Sigma 2p that means the symmetry
is Sigma and it is obtained by linear
combinations of these 2p orbitals and
this is the Sigma star now again you see
the electron in bonding orbital more
concentrates between the nuclei higher
electron density here but in
anti-bonding molecular orbital if there
is electron there the probability of
finding the electron right in the middle
here on this plane is 0 and they are
more concentrated out here off the inter
nuclear volume into nuclear space as a
matter of fact so these are again the
sigma bonding and sigma anti-bonding
molecular orbitals which is obtained by
linear combinations of 2p orbitals in
head-on fashion now the second example
shows how we simply combine the two
side-by-side simply P orbitals to get a
molecular orbital and here we have side
sort of linear combinations of two P
orbitals this is the bonding and this is
the anti-bonding molecular orbital now
again remember this these orbitals
bonding and antibonding molecular
orbitals do not have rotational symmetry
with respect to the bond axis therefore
they are PI type and we call them PI 2p
bonding and PI 2p star means it is
anti-bonding now you can simply combine
the other 2p orbitals in the same
fashion like this as a matter of fact
and this is going to be simply again PI
2p star and PI 2p just look in this
orbitals from the other side right you
just rotated 90 degrees as a matter of
fact now when you simply examine the
molecular orbitals in homonuclear
diatomic molecules homonuclear diatomic
molecules let's simply take an example
consider the oxygen molecule or let's
say nitrogen molecule like this so in a
nitrogen we have electron configuration
1 s 2 2 s 2 and T 2 P 3 electrons like
this so if I simply try to write down
the molecular orbital energy level
diagram for this it would be again as
follow the 1s orbital actually of this
hydrogen will combine with the other 1s
orbital of the other nitrogen actually
let me just put it over here this is the
energy so let's say this is the
1s orbital of the first nitrogen let's
say nitrogen a or just nitrogen and here
is I have the two s orbitals orbital and
here I have two P orbitals they are
degenerate right degenerate that means
they have the same energies this is the
energy order so here I have three
electrons this is the simply electron
configuration and also the relative
energies of these now the other nitrogen
here again will be the same here one s
let's call that B two electrons and two
s here two electrons same energy because
the same nitrogen atoms and the P
orbitals here like this and these are
the two P orbitals three of those
perpendicular to each other as a matter
of fact so how are we going to obtain
the molecular orbital constructive
molecular orbitals by combining the
atomic orbitals we can combine these two
and obtain bonding and anti-bonding and
this is going to be simply Sigma 1 s
this is going to be Sigma 1 s star and
now here combining these two I will have
simply bonding and anti-bonding same
things just the energies are different
and this is going to be Sigma 2 s and
this is going to be Sigma 2 s star like
this and then we will have the P
orbitals right the P orbitals now 2p
orbitals head-on will combine to form
put the little bit higher energy-wise
2p orbitals
so here I will have abundant molecular
orbital's
combined in two P orbitals head-on and
so this is going to be Sigma 2p you can
call that Z for example and now then I
will have here Sigma star 2p z the
anti-bonding molecular orbitals
these are hand on linear combinations to
form the Sigma symmetry bonding and
anti-bonding molecular orbitals now we
used one of the P orbitals here right to
get those one from there
there are two more and remember they are
perpendicular to this that means they
are this over here like this and now
they will combine sideways like and then
will we will get bonding molecular
orbitals with the PI symmetry and type
on the molecular orbital with the PI
symmetry so let's simply put it over
here bonding and anti-bonding like this
but now there are two more P orbitals on
the nitrogen which are perpendicular to
that there these two they will also
combine sideways like this to form
exactly the same symmetry pi symmetry
bonding and anti-bonding molecular
orbitals than v just just like these two
right so the energies of the bonding
molecular orbitals second one will be
same here and then over there like this
so I will simply combine I put it like
that now I will put the electrons here
and two electrons here three electrons
there two electron here two electrons
there and then I have simply one
electron there and then I will have one
electron there actually two
four six eight ten and
okay let's say put here 2 4 6 7
electrons 14 8 10 yes and then I have to
put one more here like this
it would be simply for and then I have
six there no no I got that over here all
right now I guess I get the correct
electron configurations 2 4 6 8 10 12 14
total number of electrons I put it there
okay
this becomes the energy level diagram
for nitrogen molecule let me put it over
here this is n 2 molecules according to
the molecular orbital theory let's
simply calculate the bound order and the
bond order for this case would be as
follow the number of electrons in
bonding molecular orbital two four and
six eight and ten no two four six eight
10 and minus the number of electrons in
anti-bonding molecular orbitals which
are the anti-bonding
this is the anti-bonding - this is the
anti-bonding for so let's put that for
there divided by 2 this becomes equal to
3 right so we say that the bond order in
nitrogen molecule is equal to 3 but
remember it doesn't mean that there are
triple bond between the nitrogen like in
the valence bond Theory it is just the
bond order is 3 that's what we say ok
now when we simply write the molecular
orbital energy level diagram for
molecules diatomic molecules or other
molecules we don't consider the core
electron linear
combinations the core electron will
simply linearly combine to form
molecular orbitals and bonding and
antibonding molecular orbitals but the
interaction of the core electrons will
be very very weakly so when we consider
the molecular orbitals in a molecule we
only show the molecular orbitals which
is obtained by the linear combinations
of the valence atomic orbitals valence
orbitals so simply these here is the
core electron core orbitals for the
nitrogen atoms so we don't really
consider that because this interactions
is very very weak interactions and these
are the valence simply electrons and
these are stronger interactions so this
is 2p set this will be again let's put a
PI 2p and this is going to be pi star
2py orbital and in this case I have PX &
2 py and let's call that px and 2 py 3 P
orbitals actually combine to form 6
molecular orbitals as a matter of fact
three bonding 3 anti bonding now this is
what it simply here this energy level
diagram just the same as this right we
have the Sigma 2's bonding Sigma star 2
as anti-bonding and then we have the
Sigma 2p you could call that - pizzette
actually if you call the inter nuclear
axis as z-axis and then we have the PI
2p bonding molecular orbiters two of
those I indicated them by dashes here
like 2 - represents two molecular
orbitals and anti bonding and
anti-bonding sigma anti-bonding PI
orbitals here so we can actually use
this diagram to
find out to calculate the bond order of
all second row diatomic molecules what
are they for example let's consider the
o2 molecule right co2 molecule in o 2
molecule we have the same electron
configuration here except we have one
more electron right in a P orbitals
there so that means instead of having 14
electrons we will have 16 electrons
right we will have 16 electrons so what
does that means if I just simply said
change this to night oxygen here this is
the oxygen atom this is the old true
molecule this is the other oxygen atom
then that means in addition to this
actually the energies will be different
definitely right I will have simply two
more electrons and those two electrons
will go where they will go to the pi
star 2py orbital right because these are
all full so now remember just like in
the atoms when we put two electrons we
have to put them both in the same spin
side and put one electron in each
orbital and the same thing applies here
so I will put one electron there and one
electron there so this is going to be
actually the molecular orbital energy
level diagram for oxygen molecule right
for oxygen molecule yes
we are over here
no again why we put those here because
remember just same as in Adam we feel
the atomic orbitals starting from the
lowest and we go to upper higher
energies right so we can't feel the
orbital upper orbital energy wise before
we feel the if there isn't
if the orbitals below energy wise is not
full so this is not full just we just
put those two electrons there there's a
point now consider the oxygen molecule
okay first of all let's try to calculate
the bond order bond order will be for
the oxygen again right 2 4 6 10 no eight
10 right this is 10 minus anti-bonding 2
4 6 6 and then we divided by 2 and this
becomes 2 so again you see the bond
order for oxygen molecule becomes true
right now if I simply write down the
lewis structure of nitrogen molecule and
the lewis structure of the oxygen
molecule the nitrogen molecule will
simply as follow right this is the
nitrogen molecule we say there is a
triple bond between them now the oxygen
molecule is this
and we say there's a double bond here
like there's that's the total number of
electrons but one thing here let me ask
you this by looking at these Lewis
structure would you think that the
oxygen molecule be paramagnetic or
diamagnetic oxygen molecule now remember
to be paramagnetic they should be
unpaired electron right must must be
unpaired electron from the Lewis
structures it seems like there's no
unpaired electron right now remember
these are in again hybrid orbitals you
can look at these as hybrid orbitals
three electrons in one orbital so they
will be paired up and so on that's not
so actually the valence bond Theory
according to the valence bond Theory the
oxygen molecule should be diamagnetic
that means should have no unpaired
electron but the molecular orbital
theory says that the oxygen molecule
should be para magnetic Y because I have
two unpaired electron according to
molecular orbital theory right now
according to the molecular orbital
Theory if you consider the nitrogen the
Lewis structure actually from the Lewis
structure you can say that it will be
diamagnetic nor electron and also with
the molecular orbital Theory remember
nitrogen molecule we have up to here all
electrons are paired so it should also
be diamagnetic no unpaired electron but
for the oxygen it has to be paramagnetic
is it paramagnetic yes
the oxygen molecule is paramagnetic in
fact the explanations of the para
magnetism of oxygen atom is only
possible with the molecular orbital
theory right just like here molecular
ology Theory actually before the
molecular orbital Theory people thought
that the actually the oxygen molecular
oxygen will be diamagnetic but it is
magnetic it has been measured and it is
part of that there is unpaired electrons
okay now let's see
consider this let's let's see n 2 plus
cation and n 2 minus ni and n 2 here
neutral ni now let's see let's say this
is Plus this is into minus let's
calculate the bond order for the n 2 and
2 plus that means one electron is
removed and also n 2 minus that means
one electron is added right so let's say
calculate this is we have find it to
right now in this case this n 2 plus
means that one electron will be removed
from the upper orbital there actually
right
that means remember for the nitrogen
cases we don't have these electrons so
the highest filled molecular orbital
which is called the highest occupied
homo we will take one electron from
there to make n2 plus now if I calculate
the bond order in that case the only
difference will be like here I had
simply 10 electrons in
in molecular orbitals so now I will have
nine electrons in bonding molecular
orbitals so nine minus four will be
simply 5/2 the bond order for this case
will be two point plus you see again we
have fractions of bond order two points
plus now how about in 2 minus cases now
in to minus cases again everything would
be the same here right but now I have
one more electrons simply where I will
have one more electron over here so that
means that instead of five I will have
six five instead of four I will have
five electrons there and the bond order
will be what 10 minus 5 divided by 2
again 2.5 now let's consider the all 2
molecule and let's see-oh - again all 2
plus and all 2 - the bond order will be
for this one we find it - for this one
again we will have what now let's think
about it
this is for the oxygen neutral oxygen
and that means that we take out that
electron we have to always take the
electron from the highest energy full
molecular orbital that was the easiest
actually right and so what will happen
this number will be not 6 but it will be
5 right so what will be the bond order
2.5 right so this is going to be 2.5
like this now how about oh - - now o2 -
cases we will put the electron up here
that means that the number of electron
in anti-bonding not will not be 6 but it
will be
seven so 10 minus 7 3 divided by 2 it
will be 1.5 right so the bond order here
is 1.5 now what does that means all of
these this simply means that if you take
an electron from the nitrogen molecule
and form an M 2 plus do you think that
the molecule is less stable or more
stable than the end - now remember again
if we use the bond order as criteria for
stability that means the higher the bond
order the more stable the molecule so
this simply means that the end 2 plus
cation will be less stable than the end
- that means removing an electron will
make the n 2 molecule less stable now
how about M 2 - this does the same
things
Edden an electron to the nitrogen
molecule will also destabilize it
because the bond order if you just take
the bond order as criteria for the
overall stability of the molecule so
that simply means that it will be again
less stable than the neutral but in this
case here for the oxygen molecule if you
remove an electron the bundt order
increases what does that means the
overall stability of o2 plus is greater
than Oh - now if you add an electron
bond order also decreases that means it
becomes actually less stable than
neutral oxygen molecule the the anion
here is less stable oxygen molecule and
the cation is less stable but in this
case it is just the other way around it
as a matter of fact clear so you see how
we use the bond order to compare the
stabilities and also to find out how
edit electron or remove electron changes
the overall stability by
the bond order of the molecule
okay let's yes this is again another
linear combination of an S orbital and
the p orbital to form a bonding and
anti-bonding molecular orbital as a
matter of fact and it has a sigma
symmetry like this you can simply
combine that and bonding and bond
antibonding molecular orbitals
now this explained something but I will
just show you later now uh I should
actually what really happens in diatomic
molecules particularly homonuclear
diatomic molecules such as n 2o 2 c2
carbon again as fluorine to chlorine -
those are a diatomic now the energy
order of the orbitals are like this but
when the energy difference between the
2s and 2p orbitals is small here what
happens that this orbital Sigma 2 s and
Sigma 2p which has the same symmetry but
now when I say same symmetry it's
different than the Sigma and PI type
symmetry actually the real symmetry wise
they're both G type orbitals G symmetry
orbitals they interact with each other
and when they interact with each other
the energy of this becomes higher up
here pushed up and the energy of that is
also lowered like this right and this
this figure actually shows that you see
again those two orbitals they have this
as I show you that becomes more stable
and that become the that becomes more
unstable as a result what happens Sigma
2p zet orbitals energy becomes above the
PI 2p orbitals again let me show it over
here like this that simply means that
the energy of this orbital becomes over
here and that above this as a matter of
fact and then I will have simply the
order of the orbitals molecular orbitals
Sigma 2 s Sigma 2 s star this will be
lower than the PI 2p then the Sigma 2p
again this this part stays the same 2p +
2 PZ so when we put the electrons into
those Phil the electrons into those
orbitals in diatomic molecule actually
the number of unpaired electron changes
due to the change of the energy order of
that Sigma 2 P and Sigma 2 pi orbitals
here are the examples you see again here
this is the boron diatomic boron boron
molecule carbon molecule diatomic carbon
molecule and the diatomic nitrogen
molecule the order energy order of the
Sigma and PI 2 P PI and 2 P Sigma
changed right there above these are
regular as in the figure here again
right this is the 2p no I should say
over here that Sigma goes over or stays
there in these two K these three cases
OH - f - n neon molecule we have simply
Sigma 2 P lower energy than the Sigma 2
pi but here Sigma 2 pi is lower energy
than the Sigma Pi 2 pi Xu lower energy
than the Sigma 2 pi PI becomes more
stable PI P orbitals
Sigma P orbitals becomes about so when
you put the electrons into these
molecules diatomic molecules you see a
game boron molecule will be paramagnetic
right and carbon molecule will be
diamagnetic nitrogen molecule will be
diamagnetic oxygen molecule is
paramagnetic as we have simply shown
here two unpaired electron fluorine
diamagnetic neon actually does not exist
why because the bond order of neon
molecule is zero just like the same as
helium remember all of the noble gases
do not form molecules they are in mono
atomic form in the gas phase again
because the bond order is 0 that means
energetically there is no gain advantage
in fact forming a molecule over the
separate atoms ok this simply explains
the diamagnetic and paramagnetic
properties of the oxygen molecule this
is again shows how the oxygen molecular
oxygen is paramagnetic if you simply
take a liquid oxygen liquid oxygen and
pour it on a magnet and you will see
that it will be attracted by the
magnetic field simply saying that the
oxygen molecule is paramagnetic oxygen
molecule is paramagnetic as the
molecular orbital theory predicts it now
when you have heteronuclear diatomic
molecules that means instead of two
nitrogen to oxygen what happens if you
have one nitrogen and one oxygen for
example or one carbon and one oxygen now
what will change
remember when you change the atoms this
is nitrogen this is the oxygen this is
the enno molecule for example and the
only thing will change is the energies
of the 2s and 2p orbitals like this
you see again in this case the oxygen
atom the 2's orbital energy wise is
lower than the 2's orbital in the
nitrogen same is true for the P but
there not much difference again you know
just a little bit more stable and they
will combine exactly the same phase
fashion as homonuclear diatomic molecule
to form bonding and anti-bonding
molecular orbitals and the electrons in
the atomic orbitals will simply fill in
to these molecular orbitals like in this
case for the nitrogen and O molecule you
see there is one unpaired electron there
now if you calculate the bond order for
the N o molecule for the nitrogen
remember it was 3 and for the N o
molecule the number of bonding electrons
anti bonding electron becomes 5 and 10 -
5 - there will be 2 point 5 so you can
calculate the bond order for n o it will
be 2 point 5 again right 2 point 5 okay
this is the end of this chapter any
questions yes
if they have one yes at the same spin oh
that doesn't doesn't matter because the
electrons in the hydrogen anthem may be
in different spins like this but when
they form a molecule when those two goes
into the bonding molecular orbitals they
have to pair up they have to pair up
again just like the same as the atomic
orbital if you put three electrons in
one orbital it has to be paired up you
can't put them up like this because
that's again in case of atoms against
the Pauli exclusion principle remember
you can't have two electrons in the same
orbital with the same spin but if there
are different orbitals they will be Hans
rule says that they will have the same
spin directions actually okay oh I guess
this is the end of this hour we will
start the next chapter next hour
let's have 10 minutes break now but
please make it exactly ten minutes