L5 - Writing Molecular Formula - Atoms and Molecules, Class 9, Science Video Lecture - Class 9 | Best Video for Class 9

writing formulas of a molecular compound
now this is as simple as it can get is
just that we have the tendency to
complicate now what will happen in
compound and compound obviously there
are two elements let me call them as e1
and e2 okay both these elements would
have their own valence ease right so
let's say for example the valency of e1
is X + e2 is y right once you know these
valence ease it actually becomes pretty
simple the most simplest thing you need
to do is you need to do something which
is called crossovers right so what does
that mean that basically means that
whatever is the valency of e 2 goes and
sits as a subscript of e 1 and whatever
is the valency of e 1 goes and sits as a
subscript of e 2 right and the formula
is basically written as e1 by e2 X right
now this might be something a little
complicated let's take an example and I
will clear it for you so let's talk
about carbon monoxide ok this might
sound a little complicated so let's take
an example so let's work out for carbon
dioxide well know what's the formula of
carbon dioxide is co2 right but how does
it comes in so there are two elements
here one is obviously carbon and the
other one is oxygen right carbon and
oxygen do you know the valency of carbon
the valency of carbon is 4 and the
valency of oxygen is
- right then we do the crossovers so
what's the formula of carbon it's C and
a here it is o so C and then the
crossover of the valency of oxygen so C
- right and oxygen is o and four right
but in this case is if we see the
subscript have a common divisor of two
so we just divide this entire thing by
two and what do we get we get C 2
divided by 2 gives you 104 or divide 4
divided by 2 gives you 2 C 1 can again
be written as co2 that is what the
formula of carbon dioxide is right so
let's take another example and this time
maybe
let's take hydrogen sulfide right so
this is a molecular compound which are
the two elements which are present here
one is obviously hydrogen and the other
one is sulphur so H and s what is the
valency of hydrogen valency of hydrogen
is 1 and sell 4 is 2
this you can find out from any of the
books that you have I mean these
balances are available then what do we
do we do the crossovers so we write
hydrogen and cross over the valency of
sulfur - this - then we write sulfur and
we cross over the valency of hydrogen to
this H 2 s 1 or simply H 2 s
right you want to do one more okay let's
take hydrogen chloride so which are the
two elements which are there right come
on quickly right on the first one right
on no it's hydrogen and what's the
formula of hydrogen H flowing what's the
formula of chlorine now come on don't
tell me that you don't know this quickly
CL right the valency is obviously you
might not have the table before you but
incidentally the valence ease of both
ways is 1 in each case so when we write
down the formula we write HCl we cross
over the valency of this to this and
this to this so one comes with hydrogen
and one comes with chlorine since you
don't need to mention the subscripts
specifically because h1 in any case
means H we can just write as x CL right
that's how basically you from the
formula for a molecular compound I hope
you would have had a good time
understanding this thank you for being
with us today