Valence Bond Theory Video Lecture - Class 11

hello friends this video on chemical
morning part 19 is brought to you by
exam feel calm no more fear for exam
before watching this video please make
sure that you have watched part 1 part
18
let's now discuss the theories based on
quantum mechanics we have seen the other
approach we had was the latest theory in
the best burger if it helped in writing
the structure of the molecules but it
failed to explain the formation of
chemical bond why the bond form doesn't
explain that it doesn't give any reason
why H 2 and F 2 have different born in
honkies it also doesn't talk much about
the sheep or the polyatomic molecules
Vespa theory wish we had this criticized
act because it is not pointed it thus
based on observation they found it but
is not pointed if there is a crude
reason why it should be or the
particular shape just being based on
observation they have given this theory
but it is not point it is not
structurally accurate and to overcome
these limitations from this experimental
ways they came out with the theories
based on quantum principles and these
theories where valence band theory and
the molecular orbital theory
valence bond theory is jelly gives the
shape and the molecular orbital theory
is also for the shape but jelly gives
the properties properties of molecule
whether it is paramagnetic or what is
the bond length stable or not since more
about the clinical properties the
molecule is given by the molecular
orbital theory and the shape is more
thing into the valence bond Theory so we
learn valence bond Theory now this
theory focuses on how atomic orbitals
combine to form chemical bonds in the
model and this based on the energy
reduction the whole quantum theory is
based on the energy
they told energy is like stress anyone
do you want to reduce the stress really
autumns want to reduce the energy and
this theory is all based on energy
reduction and according to this theory
the covalent bond is formed between the
two atoms by the overlap of half filled
valence atomic orbitals of each hatton
having unpaired electron please note a
liquid the shape is a very critical
state line yet so it says the covalent
bond is formed between two atoms by
overlap of half filled balanced atomic
orbitals of each atoms containing one
unpaired electrons I'll explain more on
this I think the next few slides we
don't understand this say in same
example I have one s orbital let's
understand the concepts of had in the
hydrogen atom it's understand the
valence bond concept will take two
hydrogen atoms and if he assumed these
hydrogen atoms are coming towards each
other she's not pcqc now for force
appears right this is the attractive
force appears so if you see this is my
nucleus one this is nucleus - this is my
lectronimo is my neck trauma so there is
a attraction between nucleus one
electron two right there is again a
attraction between niklas to an electron
one right there is already attraction
between this the dark blue line is
already existing attraction between
nucleus - an electron - and if this one
electron on glass more attraction came
the blue one is attraction and the green
one is the repulsion actually new
attraction develop so I say my whole
attraction was old attraction averse
nucleus one left on one if you use to
network
my new attraction is nucleus one
electron 2 and nucleus two electrons
my new repulsion also game that is
nucleus one nucleus two electron one
electron so these new forces came into
play the one cannot have the new forces
the blue one is the attraction force and
the green one is the replacement portion
right but oh it hasn't seen that the the
way it comes to each other if it is an
infinite distance there is no energy
actually the zero energy if it comes to
each other the energy decreases the
energy decreases and comes to very
minimal level and then it goes up again
why because if you try to bring them
closer more it will repel more right
there is a one place where it achieves a
stable configuration and that is at this
stage where both are almost combined and
this is found experimentally for Hydra
this is what hydrogen so what they have
done is they have taken two hundred
atoms separated there is no energy
between them and it was loaded energy
because it's in a separate plane finite
distance when the camp together the
energy goes down goes down it went to a
minimum level of minus four thirty five
point eight kilo Joule per mole and
again it went up because all of this if
you try to squeeze these two atoms will
be more repulsion by these two nucleus
and these two life will become unstable
but at this position it is 20 Steven
this is a stable state and this is
derived experimentally experimentally
they found that if you take a hydrogen
atoms at this state it is more stable
because the energy is lower here hey
when to them and this is the part there
is no energy is all zero energy but when
it comes near it it loses energy it
becomes very stable in the game it
becomes unstable so at this point it
becomes very stable and
is the cruise for the valence bond
theory please understand this is the
main concept behind hi valence bond
Theory experimental they are formed for
hydrogen atom as a distance between the
atoms is decreasing the energy also
decreases because no stable and if you
further decrease the energy the distance
if you further leak the distance the
energy increase and the atoms becomes
also so as I have told experimentally we
have found this that the new force is
there the the magnitude of new force
that comes is more than the new universe
support the new attractive force is more
stronger than the new repulsive force
and that's why when they approach each
other the energy of the system decreases
ultimately I saw surely the stage where
the net force of attraction balances the
represent force and this attains a
minimum energy and at this stage the
distance between two hydrogen atom is 74
Pico meter and that's why the bond is of
hydrogen is 72 Pico meter and since the
energy is released in this the h2
molecule is more stable than in a
certain hydrogen atom so my h2 hydrogen
molecules becomes had two atoms becomes
molecule and in this energy is released
so this is one stable and this energy
releases for 35.8 for one mole of
hydrogen atoms and so this is called
bond enthalpy correct conversely if you
have a hydrogen molecule if you want to
convert into hydrogen atoms you need
this for thirty five point eight kilo
Joule of energy for you convert one mole
of hydrogen molecule to hydrogen so
hunter she did the picture we had the
distance when the distance became 74 be
committed between the nuclei of two
hydrogen like this
seventy-four picometer this was Steven
and in released four thirty five point
eight kilo Joule of energy but one
mullah hydrogen right okay so now let's
discuss the orbital overlap concerned so
what we have seen till now is
experimented things right experimentally
they have two hydrogen atoms near to
each other and then they found some
energy was released in h2 us
molecules form which was stable now this
was an experiment phenomena you explain
this phenomena orbital overlap concept
came this is a Curie action is earlier
life what we saw earlier was
experimental you see the theory which
came to explain the experiment right so
it says that when we have seen the final
the last experiment where the hydrogen
molecule is formed there is a minimum
energy state at 74 peak amid resistance
where it sees as hydrogen atoms undergo
partial interpenetration that means the
orbitals overlapped for the two hydrogen
atoms example if you see here SS orbital
they overlap to form SS overlapping
where if you see there is a place where
there's overlap it is there is s4
orbital overlap with p orbital then also
we have some overlapping so say that
overlapping nothing but inter
penetration of orbitals partial in
independent Islamic orbital a partial
merging of orbitals right this is called
overlapping of a number and they are
saying that this is the result of
electron feeling so there is one
electron here my electron here and they
pair up right and the extent of overlap
designed the strength or the covalent
model if the overlap is more strength is
more overlap is less strength is less
right so I got
the overlap concept it's a understand
once again the formation of covalent
covalent bond is the result of pairing
of electrons never present in the
valence shell of two different atoms and
they have opposite spin so there is one
spin here one spin here I'm talking
about the valence cell and they overlap
and they get something like this all
right thank you
visit exam viacom to watch free
education videos try free online tests
get the best quality study materials
study from the best tutors and mentors
and much more thanks again