Periodic Properties - Classification of Elements Video Lecture - Class 10

okay welcome back we're going to
continue with our periodic property
discussion and we'll continue talking
about the periodic table we just
finished up talking about Dmitri
Mendeleev in this first periodic table
and how the periodic table is arranged
in order of increasing atomic number
once again Mendeleev used atomic mass
we've changed that to increasing atomic
number and when we do that we see a
recurring pattern a cycle so to speak in
both the physical and chemical
properties of the elements of course
that cycle means that these physical and
chemical properties repeat themselves
over and over and over again and we
talked about melting point we talked
about how elements react with both
chlorine and oxygen and the ratio in
which they react to illustrate those
cyclical properties of the elements now
a couple more vocabulary terms and I
want to talk about some periodic
properties the first pit the first
vocabulary term is called the periodic
law now the periodic law states that
many of the physical and this is
basically what I've just talked about
and chemical properties of the elements
now tend to recur in a systematic manner
with running other room here increasing
atomic number so we arranged the
elements in order of increasing atomic
number many of the physical and chemical
properties of the elements tend to recur
in a systematic manner now i also want
to define the octet rule for us we've
talked about this in class several times
but we'll go ahead and put it down on
paper right now the octet rule simply
states that atoms tend to react to
achieve a full outer energy level and we
call that the valence level level see if
I can spell here a full outer energy
level or have eight electrons in the
outer level now another way of saying
eight electrons we like to say four
pairs of electrons so atoms tend to gain
electrons or lose electrons so they can
have four pairs of electrons in their
valence level we call this the octet
rule of course it refers to the number
eight we know that the noble gases are
all considered stable because they have
full octet of course with the exception
of helium that can't have an octet
because it only has two electrons but it
has a full outer energy level neon argon
Krypton xenon and radon are
considered noble because they rarely
react with anything at all just the
larger noble gases can react at high
temperatures with the element fluorine
now the halogens fluorine chlorine
bromine iodine and astatine tend to
react to gain an electron so that they
can have an octet remember fluorines
electron configuration is to s2 2p 5 so
that's seven valence all it needs to do
is gain one more somehow and it can have
a noble gas configuration and octet just
like me on the same is true with
chlorine bromine iodine and astatine of
course the oxygen family these guys need
to gain two electrons to gain their
octet so they're probably not as
reactive as the halogens because they
need to gain two instead of just one to
take a look at the alkali metals lithium
sodium potassium rubidium cesium and
francium they have one valence electron
so you might think well boy how are they
going to get an octet well in the case
of lithium it has three electrons if it
can get rid of one of those it can
become like helium which has two
electrons which is a noble gas
configuration sodium has 11 electrons
hmmm well neon has ten which is stable
so what do you think sodium can do well
of course it can lose one electron
potassium rubidium cesium and francium
will do the same thing to achieve our
noble gas configurations what about
group to the alkaline earth metals
really a must for electrons to attain a
noble gas configuration like helium it
would need to lose to magnesium would
need to lose too so it could become like
me on and so on down the road which of
these elements the alkali metals or the
alkaline earth metals which you think
would be more chemically reactive yeah
probably the alkali metals because they
only need to lose one electron to become
like a noble gas whereas the alkaline
earth metals have to lose two which is a
bit more difficult so they would not be
suspect is chemically reactive that's
high let's talk about a couple of
chemical properties let's define the
chemical property we're going to keep it
sort of simple here this is a property
of an element that is predictable based
on the elements position on the periodic
table so by looking at the periodic
table and identifying the elements
position we can identify certain
properties of that element and there are
many of them when you get a chance I
want you to take a look at a YouTube
video which is entitled um let's see
it's entitled the alkali metals reacting
with water now that's not one that I've
produced but go ahead and search for
that and you'll see a neat little video
that's done where the alkali metals
lithium sodium potassium rubidium and
cesium I placed in water and they react
with water as you move down the group I
mean by down the group down the group
they react more violently with the water
now the reason that they react more
violently think about this is that when
we go from lithium to sodium we gain an
energy level so the outer electron that
lithium needs to lose is in the second
energy level pretty close to the nucleus
sodium's electron that it would like to
lose is in the third energy level it's
easier to lose one that's farther away
nucleus so we would suspect that sodium
would react more violently or rapidly
with water potassium reacts you even
faster in rubidium and wait till you see
what happens with cesium now I'll show
this in class but you guys go ahead and
take a look if you'd like it's really an
interesting video I think you'll enjoy
it now periodic property that we really
need to talk about as atomic radius this
really helps us understand all of the
other periodic properties now this graph
here shows the atomic radius of an
element versus it's atomic number and
you can see when i go from lithium to
sodium to potassium to rubidium to
cesium group 1 the alkali metals the
atomic radius increases when i look at
the halogens fluorine chlorine bromine
iodine and astatine once again when they
go down Mac group the atomic radius
increases so I want to answer the
question on the bottom of this page
first what is the general trend ins and
the size of an atom when you go from top
to bottom and a group of the elements in
the periodic table or just saw it
increases the elements radius gets
bigger why is that so well there are two
reasons number one when you go down a
group they gain an energy level and when
you gain an energy level you get those
electrons become farther away from the
nucleus and number two is something
called an increase in shielding now what
the heck does that mean let's talk about
voltages okay and that's turn this place
is the paper over here we're going to
draw on this let's take the element lift
let's go with hydrogen k the first
member of group 1 the nucleus of a
hydrogen atom and it has an energy level
with one electron buzzing around it the
next member of that group is lithium
here's the nucleus its configuration is
1 s 1 so we have one electron to s 1 so
it's 1 s 2 so there's two electrons in
the four
energy level in one in the second see
how that's farther away from the nucleus
than hydrogen's then after lithium is
sodium the sodium's configuration is 1s2
so two electrons in the first energy
level 2 s 2 so 2 p-6 so there's a total
of eight electrons in the second energy
level and then it ends with 3 s 1 so it
does another energy level so that
valence electron is farther away from
the nucleus than both lithium's and
hydrogen's and then potassium of course
a draw it over here we have one energy
level with two electrons the second
energy level has eight right the third
energy level for potassium also has
eight electrons in it 3s2 3p6 and it
ends with 4s one so it's even further
away from the nucleus and that happens
as I go down the group we increase
energy levels of course the atomic
radius increases well what is that
shielding effect that I mentioned well
what do electrons do to each other do
they attract ordered a repeal now being
evening absolutely they repel how many
energy levels is the valence electron
and hydrogen repelling a dumb question
right and only as one energy level it's
attracted to the nucleus there's nothing
between it into the nucleus there's
nothing repelling this electron farther
away but look at lithium this valence
electron is being repelled by and energy
level of electrons pushing this one
farther away that's called shielding
look at sodium look at this valence
electron isn't it being repelled not
only by this energy level but this one
down below as well it's being repelled
by two energy levels two layers of
shielding and potassium has three layers
layers of shielding on that valence
electron and of course rubidium cesium
entrants maybe even more so that pushing
that valence electron farther away
increasing their
this so in the atomic group the radius
increases because you're gaining energy
levels and there's an increase in
shielding so I would expect France team
they have the largest atomic radius of
any member of group 1 Radian to have the
largest radius of any element of group
two I would expect polonium to have the
largest radius of any member of group 16
astatine the largest and any number of
group 17 and so on and of course at the
top of the group's we would have the
smallest atomic radius now the one
that's harder to understand that we need
to spend a little bit of time left when
I go across the period notice what
happens to the atomic radius when I go
from sodium to argon the atomic radius
starts out big but then it decreases
which is counterintuitive sodium has an
atomic mass of about twenty-three atomic
mass units are gone has an atomic mass
of about 39 or 40 atomic mass units it
is heavier so once you expect as I want
to cross period three the radius to get
bigger is that what happens does it get
bigger when I go across period number
three nope it gets smaller so I'm going
to let that down when you go across a
period the atomic radius gets smaller
now that is strange indeed why does it
get smaller let's write the answer down
then we'll talk about what it means when
you go across the period you increase
the number of protons and the nucleus
now here's the kicker but you don't
increase
the energy level when you go across a
period you get more and more protons in
the nucleus that the energy level stays
the same what does that mean let's take
a look at sodium it's in the third
period it is electrons in the third
energy level it ends with 3s one I
museum in the third period it has
electrons in the third energy level it
ends with 3s too we go all the way
across the argon it has electrons up
into the third energy level that ends
with 3s2 3p6 all of these elements in
period three have electrons in the third
energy level but notice the number of
protons goes from 11-4 sodium up to 18
for argon so while the energy level
remains the same we are socking away
more and more positive charges in the
nucleus what do protons positively
charged particles do with electrons well
they attract them so when I go across a
specific period the atomic radius gets
smaller okay we increase the number of
protons in the nucleus but we don't
increase the energy level and that
causes a decrease in the atomic radius
and students have a hard time
understanding now that happens across
any period of the periodic table let's
take a look at this graph right over
here let's see if I can find it again
when I go across period number two when
i go from lithium to fluorine sure the
atomic mass increases but the atomic
radius gets smaller sodium to chlorine
potassium to bromine rubidium two on 18
cesium all the way across to astatine
now of course there are a few things
that change that trend as we get farther
down the periodic table but very much
speaking for the most part i should say
the atomic radius gets smaller when you
go across
a period so the that atomic proxima
periodic property of atomic radius is
very important once again remember that
when you go down on the group the atomic
radius increases make sure you
understand the reasons why and then you
go across a period the atomic radius
decreases and make sure you understand
why this is the basis behind many of the
other periodic properties which we will
talk about later thanks