Basic Chemistry Concepts - Chemistry, Class 11 Video Lecture - Class 11 | Best Video for Class 11

hello welcome to the penguin Prof
channel today I want to get started on a
chemistry concept series by popular
demand we're going to be talking about
elements atoms really why it's all about
the electrons and some Lewis dot
structures which in my opinion may be
the key to happiness when we get started
here we're really talking about what
stuff is made of and stuff is made of
matter from here you can really kind of
go off a cliff in terms of how far you
want to go into this this is really the
realm of physics so I just want to say
from the outset that this is a highly
abbreviated version of an understanding
of what the stuff is in the universe the
matter is made of elements elements are
pure chemical substances that you can't
break down into anything else
now we organize elements in this amazing
table called the periodic table of the
elements it's really amazingly organized
and I mean I'm just gonna say it it's
beautiful
ok how much of a geek am I to say that
but it really is because if you
understand how this table is organized
this is tremendously powerful notice I
didn't say if you memorize it I do not
have my students memorize any part of
this table that's what it's there for
that's why you can look it up but you
have to understand how to use it and
what it means in terms of elements in
the universe the vast majority of them
are the two smallest elements hydrogen
and helium on our planet in the Earth's
crust by weight we've got oxygen silicon
and aluminum if you take the earth as a
whole not just the crust by weight the
most abundant element is iron in terms
of biology only 25 elements are
essential for life and of those 25 only
for makeup the vast majority of most
living things including penguins so if
you're wondering well what are we made
of you'll be thrilled to know that you
can actually buy a shirt with human
ingredients on it oh my gosh I love
these geeky gifts I don't know what that
barcode is at the bottom but anyway so
here's a nicer view table of the
elements of the human body and again you
can see that 9 over 96% of humans are
made of
oxygen carbon hydrogen and nitrogen so
if you go back to the periodic table and
it is kind of overwhelming I admit but
you remove all of the elements that are
not found in humans it's really not too
bad at all this this looks somewhat
manageable right understanding the
periodic table also allows you to
understand all these stupid geeky gifts
like this bacon shirt which I just I
don't know I was just in a weird mood
okay so the smallest unit of an element
which retains all of the properties of
that element is called an atom that word
comes from the Greek a term which means
not to be divisible meaning you can't
cut it up into smaller pieces and still
retain the properties of the element we
can actually see atoms today this is a
really cool transmission electron
micrograph done by the physicists at UC
Berkeley the red arrows are hydrogen
that black arrow is a carbon atom shown
on the surface of graphene now in your
textbook you will see atoms drawn like
this electron clouds the shell or Bohr
model and I gotta be honest with you
this is just not true
atoms do not look like this we use these
drawings because they represent it sort
of an easier way for us to talk about
them but this is not what atoms really
look like atoms are really hard for us
to wrap our brains around because they
are unbelievably small that black bar
right here at the bottom represents one
angstrom or 100,000 of femtometers so an
angstrom is 10 to the minus 10 meter and
the vast majority of the atom is
actually empty space so over 99% of the
mass of the atom is concentrated at the
center in the nucleus and the rest of it
where the electrons are the orbitals are
just hugely spread out and it is really
difficult for us to really draw and
think about what atoms really look like
so I have a couple of analogies for you
of course one is an analogy of scale and
this is a soccer field and if
you take the nucleus to be the size of
the soccer ball then an electron could
be a fly buzzing around the spectators
and most of the stadium represents the
space between the atomic nucleus and the
electron so most of the atom is empty
space
now this analogy is not really accurate
either because it shows the electron as
a particle as a fly and actually that's
not true one of the best analogies that
a chemist gave me for this is with
cotton candy and he said okay where's
the sugar well
the sugar is kind of everywhere I mean
the whole cotton candy is made of sugar
that's kind of the same idea with these
electrons so the orbital itself really
is the electron but that's really hard
to draw and describe so what I always
say is you know strive for understanding
strive for the real truth but we like
all of your textbooks and basically
everybody else we're going to use these
models and drawings we're gonna draw the
little electron as like a ball or a bead
so that we can keep track of everything
okay but realize that that's not really
the truth so getting back to the atoms
in the periodic table there's a couple
of numbers that we need to concern
ourselves with one is the atomic number
and that's just the number of protons
that an element has and I'm picking
carbon here as an example the atomic
number for carbon is 6 what that means
is that carbon has 6 protons that number
does not change it's kind of like your
calling card so if you add a proton to
carbon it's not carbon anymore so now
your atomic number would be 7 and now
your nitrogen so the atomic number does
not change the mass number or what we
call the atomic weight that's the number
of protons plus the number of neutrons
the first thing that you notice is that
these numbers are decimals so maybe
you're wondering well are there
fractional protons and fractional
neutrons and you know what's up with
that well what we're really getting at
here is the mass number is actually an
average so the number of protons does
not change but the number
of neutrons can change so if we look at
carbon carbon in the universe there are
different numbers of neutrons in various
carbon atoms that you could find we call
these isotopes most carbons in the
universe have six neutrons and of course
the number of protons doesn't change so
6 plus 6 is 12 so the vast majority of
carbons that you would find have the
mass number of 12 you will also find
those some carbon atoms with an extra
Neutron and if you add 6 plus 7 that
gives you the mass number of 13 some
atoms have two extra neutrons 2 more
than the normal number and the mass
number here is 14 this is probably the
one that you're most familiar with
carbon-14 is what we use for dating not
not dating will you go out with me but
dating how old is that tree we use
isotopes for all kinds of purposes
actually in medicine as well as
radioactive dating because they decay at
a constant rate so if you look at the
decay rate of carbon-14 this is what it
means so we have x 0 some amount of
carbon-14 that's present after one
half-life which for carbon-14 is about
five thousand years after one half-life
you have half that amount still
remaining after two half-lives you have
half of that half and after three
half-lives you got another half so it's
not like after two half-lives it's all
gone that's a big misconception so what
you should notice is this exponential
rate of decay and this rate is constant
so that's why we can use isotopes to
date living tissue to date rocks and
things like that but enough about the
nucleus chemistry is really all about
electrons so we're going to spend the
rest of this video talking about them
now atoms like to be stable and in order
to become stable sometimes they have to
play with other atoms in this way
they're kind of like people but you
might ask the question does everybody
need to play with others in order to be
happy humans are social creatures so
most of us do but there are actually
individuals that don't and the same
thing is true in the periodic table we
have
to the far right the noble gases noble
gases so named because like nobility
they do not mix with the commoners and
in this video we're going to explore why
why don't they form bonds with other
atoms everybody else on the table they
form bonds but not the noble gases
you've got to understand the electrons
in order to understand why so electrons
fill around the nucleus from the inside
out and what I mean by that is as you
look at different atoms and different
atoms have different numbers of
electrons they will fill around those
nuclei from the inside out just like
water fills in a glass when you fill a
glass with water it fills from the
bottom up ok you can't do it any other
way if you can I'd like to see that
shell number one is gonna hold two
electrons shells 2 & 3 are gonna hold 8
yes for you chemists I am
oversimplifying this when you go on and
take more general chemistry you will see
that e electron configurations are a
little more complicated than that
but for the purposes of general biology
what I'm giving you should get you
through most of the problems that you're
going to come in contact with I've just
got to say don't don't get your heart
rates up when you see stuff like this
even at first glance you notice that
there is a pattern so there really this
is manageable but we're not going to go
into that in this video we're gonna
focus on happiness what you got to
remember is that atoms are happy when
their outer shell is full of electrons
the outer shell turns out to be so
important we actually give it a name we
call it the valence shell so we're going
to talk about how they get to be happy
right now as we go through this we're
going to see that carbon is not happy by
itself whereas neon is we're gonna see
why my analogy for this is a stadium in
the round so a stadium with a stage
inside and spectators all the way around
and of course we're gonna have penguins
in there too
so we're gonna have a rule and a goal
the rule is we're gonna sell tickets
closest to the stage first the goal is
we want to make each shell around the
stage full of spectators now in our
little theater it's
to be a very little theater the first
shell is only going to have two seats in
it the second shell is going to have
eight seats and the third shell is also
going to have eight seats so just go
with me on this okay it's a small
intimate theater so here are our seats
if we're gonna look at different
scenarios and see what it looks like so
here's the first scenario we sell one
ticket so we're gonna satisfy the rule
the the tickets that we sell are going
to be closest to the stage the goal is
not met though so that's not so happy we
have one empty seat in the first ring
and this is actually hydrogen another
scenario we sell two tickets so the rule
is they're going to be the ones closest
to the stage so that's all good and we
actually met the goal the shell is full
so that's a really happy stable scenario
and this is actually the element helium
what if we sell seven tickets so notice
the rule we're selling the tickets
closest to the stage first but you
notice now that we have three empty
seats in that second shell so this is
nitrogen this explains why nitrogen
would want to form bonds to try and fill
those empty seats what if we sell ten
tickets though now you notice that the
first two shells are full this is a very
stable configuration and this is
actually the noble gas neon what about
13 tickets so now we're moving into that
third shell we're always filling from
the inside out but now that third shell
has only three occupied seats and five
empty ones so that's not so happy and
that is the element aluminum in our last
example we have 18 seats and you notice
we have all three shells full this is a
very stable configuration this is the
noble gas argon so maybe you've noticed
that it seems to be about the valence
shell
if the valence shell is so important why
don't we just focus on that I mean who
cares what the core of the beast looks
like let's just look at the outer shell
since that's what's going to determine
how happy an atom is if you're thinking
that
that's awesome you're not alone because
some other really smart people have
thought exactly the same thing like dr.
Gilbert Lewis who is a physical chemist
he spent most of his career at UC
Berkeley and he did exactly that
he said we should draw elemental symbol
and then we're gonna draw the valence
shell electrons around it and I always
tell my students if you keep track of
the valence electrons you're gonna be
happy so let's see how this works we're
gonna look at carbon the most important
part in terms of carbons reactivity is
the valence shell so let's just get rid
of everything else and we're gonna draw
only the electrons in the valence shell
around carbon that's the most important
part so carbon has the atomic number six
and now you know that two of those
electrons are in the first shell leaving
four for the valence shell now I just
want to remind you those penguins
represented electrons but for heaven's
sakes don't draw penguins and your Lewis
and dot structures you're gonna get
kicked out of class so what we do is we
draw literally dots around the element
to represent the valence electrons and
that is the lewis dot structure for
carbon how easy is that so let's look at
nitrogen so to draw the Lewis dot
structure for nitrogen all you do is you
draw the symbol for nitrogen and then
you go around it and draw the valence
electrons starting at the top now this
how you draw these varies this is the
way that I was taught starting at the
top and going around nitrogen looks like
that so let's look at aluminum so here's
the symbol for aluminum aluminum has
three electrons in its valence shell so
you draw them around going one two three
that's aluminum so finally let's do
argon argon has a full valence shell so
we're gonna draw one two three four five
six seven eight a full valence shell for
argon and it's as simple as that
so check this out if you look down the
column of all the noble gases and you
draw the Lewis dot structures for each
one you will find that they all look the
same because all of them have full
valence shells and this explains why all
the noble gases are happy as they are
and they don't form bonds with other
atoms because if the goal is to have a
full valence shell and you already have
one you don't need to play with others
the only thing that might confuse you is
helium and what you have to remember is
that helium is very small atomic number
two so it has only two electrons and so
you have to remember that that first
shell is in fact full with two where as
everybody else going down this group
they will all have an outer shell full
with eight so let's practice when you
practice drawing the Lewis structures
all you need without the Penguins is a
periodic table so lithium atomic number
three it's gonna have two electrons in
the first shell leaving you one in the
second shell so when you draw the Lewis
dot structure that's it one electron in
the valence shell for oxygen you've got
two electrons in the first shell six in
the second for a total of eight so you
draw your oxygen with one two three four
five six electrons in the valence shell
that's your Lewis dot structure for
oxygen sodium has two in the first shell
eight in the second one in the third for
a total of eleven sodium is drawn just
like that so it's really not too bad and
even though most biology instructors
don't really show you Lewis dot
structures you're gonna have to do it in
chemistry anyway and truthfully if you
understand Lewis dot structures
everything else is going to make a lot
more sense so here are the first twenty
elements and the Lewis dot structures
for each and what you notice is that as
you go down the vertical columns which
we call groups they all have the same
Lewis dot structures and that explains
why they have the same chemical
reactivity and why they'll form the same
kind of bond isn't that cool how
powerful is that
so understanding Lewis dot structures is
gonna make predicting atomic
interactions so much easier and you're
gonna be able to predict what's gonna
happen when for example two hydrogen
atoms get together because unless you're
a noble gas
you're gonna have to play with others in
order to become happy and that's what
atomic bonding is and that's why two
hydrogen's get together to form h2
hydrogen gas so they're both going to
share their lone electron so that each
one can have a stable full shell as
always I hope this was helpful thank you
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