so this video will go through section
2.7 on valence electrons so the chemical
properties of an element depend on the
number of electrons in the valence shell
or the other electrons in the rest of
the atom are not influential on the
properties as much as the valence
electrons the valence shell is the
outermost shell develop the shell with
the highest value of n the quantum the
principle quantum number and so the
electrons in the valence shell are
called valence electrons all the others
are called core electrons but really the
chemical properties depend on the
valence electrons and so if we look at
barium for example it's electronic
configuration is 1s2 2s2 we are not
asking you to write down those electron
configuration but how you need to
interpret this is that in the first
shell and s orbital there is two
electrons and then in the second shell
the s orbit the other s orbital
there is also two electrons for the
beryllium atom beryllium is on the left
side of the periodic table it's atomic
number four and so those two electrons
in the 1s plus two electrons in the 2s
corresponds to the total number of
electrons for beryllium and so the
valence shell for beryllium is the
second shell because the second shell is
the highest n value 4 beryllium so the
valence shell is for n equal 2 and the
number of valence electrons in that
shell is too so beryllium has two
valence electrons
another example with chlorine-chlorine
electron configuration is 1s2 2s2 2p6
3s2 3p5 so there's two electrons in the
1s orbital two electrons in the 2's
orbital six electrons in the 2p orbital
and then two electrons in the 3s orbital
and five electrons in the 3p orbitals
and so you can see that for chlorine and
the valence shell is the 3rd shell
because the highest n is N equals 3 for
chlorine and you can see that chlorine
is in the third period on the periodic
table and and so if we count the number
of valence electrons in that valence
shell there is two in the 3 s orbital
and five in the 3 P of orbitals so five
plus two is seven and so the number of
valence electrons for chlorine is seven
and so there is an easier way than
writing the electron configuration to
figure out the number of valence
electrons for an atom and it's related
to the group number of the element in
which group it is located and we have
elements are not ordered randomly in the
periodic table elements in the same
group
have the same number of valence
electrons which is why they have this
similar chemical properties we said that
elements in the same group have the same
properties and the reason why they have
the same properties it's because they
have the same number of valence
electrons so then by looking at the
group number and I'm talking about the
group number with the letter A this this
group number is the number of valence
electron for the elements in that group
so
for the main group elements those that
are numbered one a to eight a the group
number is equal to the number of valence
electrons so four elements in group 1a
hydrogen lithium sodium potassium
rubidium they all have one valence
electron group 2a like beryllium
magnesium calcium strontium barium they
all have two valence electrons
so it doesn't work for the big group
it's only for the a group so you move to
the three a group well you have boron
aluminum gallium indium they have three
valence electrons in for a four valence
electrons 5A five valence electrons
6A six valence electrons 7A
have seven valence electrons
and the noble gases in group 8 a of
eight valence electrons there is an
exception there do
the exception is areum areum has only
two valence electrons even though it's
in group 8 a if you look at areum in the
periodic table its atomic number is 2
which means areum has only two electrons
in total two protons two electrons wins
neutral so it can only have two valence
electrons so is two electrons around is
valence electrons
so this is something to remember areum
is an exception so select correct number
of valence electrons for each of these
elements again pause the video to do
this on your own before you watch the
answer so oxygen how many valence
electrons so for this you need the
periodic table so if you look at your
periodic table oxygen is in group six a
so oxygen has six valence electrons
aluminum is 3a so aluminum has three
valence electrons chlorine group 7a it's
an halogen
7A means 7 valence electrons elements of
the group 6A they will have 6 valence
electrons tin symbol Sn is in group 4A
so it has 4 valence electrons so we will
now represent the valence electrons
using electron dot symbol for the atoms
the dot will represent valence electrons
and they will be placed around the
symbol of the atom around the symbol of
the element and we will use the four
sides the top the bottom the left side
and the right side of the element and
each dot will represent one valence
electron and you will place one valence
electron around each side if you have
one to four valence electrons and you
have more than four you will start to
pair them and you can have a maximum of
two electrons per side so for example 
hydrogen in group 1a as one valence
electron and so its electron that symbol
would be H with a dot the dot can be on
the right side like here on the left
side above or below it doesn't matter
where it is it only needs to be wondered
carbon in group 4a as four valence
electrons and because we have four sides
we will place one dot on each side and
you do not pair them if you have the
room to put a dot on each side so carbon
electron dot symbol can only be the one
you see there with the dots one dot on
each side oxygen group 6a as six valence
electrons so you give one dot on each
side and then because you have two more
you have two pair dots on two sides so
here there is two dots on the left side
and on top of oxygen and one dot below
and on the right but the paired
electrons could be to the right or below
the symbol it doesn't matter as long as
you have six valence electrons and a
maximum of two dots per side chlorine an
halogen from group 7A has seven valence
electrons and here you will have to put
four dots around each side and then you
will have to pair three sides so there
is only one side where there is one dot
and again the side where you have one
dot can be the right side like here or
the left side or it could be above or it
could be below it doesn't matter where
it is as long as you have no more than
seven dot and no less it should be seven
so let's practice this by doing the
lowest symbol yes
electron dot symbols also called li with
symbols so of strontium Tim and iodine
please do this on your own before you
continue with the video so strontium
symbol Sr atomic number 38 is on the
left side of the periodic table
it's in
group 2A so it has two valence electrons
so you would place two dots around
strontium and here the two dots are one
on the left one on the right but you
could have one on the left and one on
top or one on the left one on the bottom
or one on the right and one on top you
know all combinations are possible as
long as you don't pair the dots when
there is room you do not pair on the
dots tin symbol s n atomic number 50 is
on the right side of the periodic table
in group 4A so it has four
valence electrons and because it has
four valence electrons each side will be
occupied by one dot it's like carbon
over here you have to have one dot on
each side a single dot you cannot pair
them this would not be correct if you
pair the dots of course the dots should
be the same size not like on that slide
but otherwise there is no choice you
have to have one dot on each side and
finally iodine group 7A iodine is an
halogen with seven valence electrons
and so we will need to pair the
electrons on three sides to have six
dots and then a single dot on the fourth
side and here we placed a single dot on
the right but the single dot could be on
the left or above or below the symbol of
iodine
so we will not look now at some physical
properties of atoms that very that that
have trends that you can follow with the
periodic table and one of these
properties is the atomic size so the
size of an atom increases as you go down
the colon because as you go down the
periodic table when you in go down in
the periods from 1 2 3 4
it means you are adding shells right
period one has only one shell second
period two shells sub-period three
shells force periods four shells and as
you add shares the size of the atoll
increases because each shell gets larger
and larger
random Clips so valence electrons get
further and further from the nucleus and
because the valence shell in you know
gives the size to the atom then you get
bigger and bigger atoms as you go down a
column the size of the atom decreases
across a row and this is not intuitive
and so the reason why is that the number
of protons in the nucleus increases as
you go from left to right with in a row
within a row you're staying in the same
shell so the shell does not increase
it's the same shell however you increase
the number of protons in the nucleus and
you increase the number of electrons in
the valence shell and the more positive
and negative charges there is and the
stronger the attraction between the two
so the more protons and electrons there
is in a shell protons are in the nucleus
but the more electrons you have in the
shell do more protons you have in the
nucleus and
stronger or the attraction and so that
will pull the electrons closer to the
center when you have more of them and so
that's why within a row and within the
same shell the size of the atom will
decrease as you go from left to right
because the electrons are pulled more
strongly closer to the nucleus so let's
practice this knowledge by selecting in
select the element in each pair with the
Roger atomic radius so between lithium
and potassium which one is the larger
atom determine potassium being in the
same group and lithium is above
potassium so potassium has more shells
and it will be the bigger atom between
potassium and bromine potassium and
bromine are in the same row the force
period which means they have for their
valence shell is the fourth shell for
both of them except potassium is on the
left and bromine is on the right which
means bromine has a lot more protons and
electrons in its shell and they will be
held more strongly together the
attraction will be stronger and so that
will pull the outermost shell closer to
the nucleus and so bromine will be the
smaller tone which means potassium is
again the larger atom between the two
between phosphorus and chlorine
phosphorus and chlorine are in the same
row again it's the third shell so they
have the same shell four valence shell
except chlorine is to the right of
phosphorus which means it will have more
protons and more electrons so the
attraction will be stronger and this
will pull the shell closer to the
nucleus and chlorine will be smaller so
phosphorus here is the bigger at home
between the two between nitrogen and
arsenic nitrogen arsenic are both in
group 5A nitrogen is above arsenic a
nitrogen is smaller at home because it
has only two shells arsenic is in the
fourth period so that means it has full
than four shells and so it's balanced
share is the fourth which is a bigger
rounder shell so that will be the larger
atom between the two the second
physical property we will look at that
has a trend in the periodic table is the
ionization energy so what is the
ionization energy it's the energy needed
to remove an electron from a neutral
atom ionization here means to make an ion
and you start from a neutral atom
and the ionization energy is the energy
needed to pluck an electron from the
valence shell and the atom will become
positively charged from losing an
electron so ionization energy decreases
down the column as the valence electrons get
further away from the positively charged
nucleus right as you go down
a column the size of the atom increases the
valence shell gets further and further
from the nucleus and
and so the electron is further from the
nucleus and held less
strongly because it's further the
attraction decreases as you put some
distance between the electron and the
nucleus so it is easier to pluck
electron from larger atom and from a
smaller atom so the energy needed will
decrease as you go down the column and the
ionization energy increases across a row
as the number of protons in the nucleus
increases so remember as we go from left
to right within a row it's the same
shell same valence shell what changes is
the number of protons and the number of
electrons and so the attraction gets
stronger and stronger as you add
positive and negative charges and so it
is harder to pluck an electron from an
atom on the right side than on the left
side where there is less protons and
electrons another way to remember this
is that the ionization energy varies in
opposite direction than the size of the
atom so as your atom gets bigger it
requires less energy to remove an
electron from the valence shell another
way to see it is ionization
energy varies in opposite direction to
the size but also the metals
the metallic character also influences
that metals have a few valence electrons
right the group number for the main
group elements are 1A and 2A and so they
only have 1 to 3 valence electrons 1 a 2
3 a actually
and so they have only one two three
valence electrons and so it is easier to
remove electrons when there's few of
them in the shell and so they will have
a lower ionization energy nonmetals
which are on the right of the periodic
table they have a lot more valence
electrons because we have filled the
shells the valence shell is getting full
as you move to the right and so you have
more protons - and it is harder to
remove electrons from the nonmetals than
the meters so nonmetals will always have
a higher ionization energy than meters
and the noble gases are the ultimate
atoms with the highest I uni sation
energy possible because they have
complete shells right they have eight
valence electrons in their shells which
is in their valence shell I should say
so it's the maximum you can have in a
valence shell and so their shells are
complete and so there are these
electrons are held very strongly and
noble gases will then have the highest
ionization energy in a period remember
that helium has only two valence electrons
right it's an exception but still it has
a high ionization energy
so trying to remember this select
element in each pair with the higher
ionization energy
so between lithium and potassium lithium
and in potassium are in the same group
Group 1A lithium is a smaller atom
than potassium because it's higher up in
the periodic table valence shell is the
second for region and valence shell is
the fourth for potassium and so it will
be
year to pluck an electron from the
potassium valance shell than from the
lithium
with the higher ionization energy will
be lithium between potassium and bromine
they are in the same period the fourth
period so they have valence electrons in
the fourth shell valence there is a lot
more valence electrons in the bromine
fourth shell in the potassium fourth shell
potassium has only one valence electrons
when bromine has seven and so those
seven electrons are held held more
strongly closer to the nucleus because
there is seven more protons to attract
them so it will be harder to remove an
electron from bromine and its ionization
energy then will be the highest between
the two between phosphorus and chlorine
again they are in the same row it's the
third period so they have have electrons
valence electrons in the third shell
chlorine has two more protons and two
more electrons which means they will be
held more slightly more strongly by
those two more protons and then it will
be harder to remove an electron from
chlorine than from phosphorus and its
ionization energy will be higher between
nitrogen and arsenic nitrogen and
arsenic are in the same group nitrogen
being above arsenic is a smaller item
arsenic has four shells and so is its
fourth shell will be further from the
nucleus and it will be easier to pluck an
electron from arsenic than nitrogen who
has
only two shares and it's balanced share
is just the second so
nitrogen is the one with the higher
ionization energy because it's the
smaller atone and it's the electrons are
closer to the nucleus and it will be
harder to remove an electron from
nitrogen than arsenic the third physical
property we will look at is the metallic
character we already know that we have
metals on the left side of the periodic
table and nonmetals on the right side
the metallic character then increases
going from right to left it's increasing
and going down the column it also
increases and you can see that very
easily with group 3A to 6A you start at
the top of the periodic table with
nonmetals then you have metal weights
and then on the bottom you have meters
so it definitely can see how the
metallic character is increasing as you
go down the column
so select the elements in each pair with
the higher metallic character
so between beryllium and nitrogen barium
and nitrogen are in the second period
and barium is on the left side
nitrogen is on the right side and so
barium is a metal and nitrogen is a
nonmetal so the most metallic of the two
is of course be on between sodium and
bromine sodium is on the left side and
bromine is on the right side again
sodium is the metal bromine is a
nonmetal so the more metallic of the two
is sodium between silicon and tin they
both belong to group 4A silicon is
above 18 as you go down group 4a you go
from carbon to silicon to germanium to
tin and so the metallic character
increases as you go down the group
silicon is a metalloid and tin is a
metal so tin is more metallic and
between chlorine and cobalt cobalt is
metal in the big groups it's in really
in the middle of the periodic table
their atomic number 27 and chlorine is
to the right of the staircase so it's a
nonmetal so the metallic of the two is
obviously cobalt and that's the end
of section 2.7
