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in this video I want to tell you how you
can calculate valence electrons but also
to understand it as well so let's look
at an atom of nitrogen so nitrogen has
an atomic number of seven and an average
atomic mass of fourteen point zero one
so an atom of nitrogen has seven protons
and seven electrons atoms are
electrically neutral now let's draw a
simple Bohr model of the nitrogen atom
in the first shell or the first energy
level nitrogen carries two electrons
that's the maximum number of electrons
you could have in a first energy level
in the second energy level you could
have up to eight electrons but what we
have is a total of seven so we need to
place five and the second energy level
so we could have a total of seven the
valence electrons are the electrons in
the outermost energy level so therefore
a nitrogen has five valence electrons so
here are the five valence electrons of
the nitrogen atom now the inner shell
electrons the ones on the inside the
ones that are not valence electrons
these are known as core electrons so
nitrogen contains two core electrons for
a total of seven electrons which is
which goes with the atomic number of
nitrogen now keep in mind for atoms the
number of protons and electrons are the
same but for ions they differ so you
have to adjust the base on a charge now
let's look at another example let's
consider the aluminum atom which has an
atomic number of 13 and an average
atomic mass of 26 point nine eight how
many valence electrons and core
electrons can be found in the aluminum
atom so let's draw a picture
so this is gonna be the first shout and
this is the second energy level and this
one is gonna be the third so in the
first energy level there's gonna be two
electrons now we said the second can
hold a maximum of eight electrons so
right now we have a total of ten but we
need to get up to 13 so that's 13 so
aluminum has three valence electrons
those are the electrons in the highest
energy level now how many core electrons
does have it has two in the first shell
and then eight in the second shell so it
has a total of ten core electrons if you
add ten plus three you're gonna get the
atomic number for the atom thirteen
electrons in total so just make sure you
understand is the valence electrons are
the electrons in the outermost energy
level these are the electrons that
participate in a chemical reaction the
inner core electrons for the most part
really participate in a chemical
reaction
it's the valence electrons that are
involved in reactions that we see in a
day-to-day basis now another way in
which you can identify the number of
valence electrons it is by writing the
electron configuration so let's write it
for nitrogen and for aluminum the
electron configuration for nitrogen I'm
assuming that you already know how to do
this
if not you can check out another video
that have on YouTube it's 1s2 2s2 2p3
so notice that the exponents add up to
seven because nitrogen has a total of
seven electrons now in the highest
energy level that's in the second energy
level we have two sub levels 2s and 2p
but in the second energy level notice
that we have a total of five electrons
so those five are the five valence
electrons
those are the electrons in the outermost
or the highest energy level all the
other electrons are known as core
electrons so you can see that nitrogen
has two core electrons now let's
consider aluminum the electron the
figuration is 1s2 2s2 2p6 3s2 3p2 on the
s sublevel can only hold two electrons P
can hold up to six electrons and D can
hold up to ten but we look at the
highest energy level of aluminum which
is the third energy level we could see
that there are three electrons there two
plus one extreme so aluminum contains
three valence electrons all the others
represent core electrons so we got two
plus two plus six so aluminum contains
ten core electrons so that's how you can
identify the number of valence electrons
in core electrons using the electron
configuration now let me give you
another example let's use chlorine which
has an atomic number of 17 so in an atom
of chlorine write the electron
configuration and then identify the
number of core and valence electrons so
the electrical figuration of chlorine is
1s2 2s2 2p6 3s2 3p5 all the exponents
out of 17 so the valence electrons are
the electrons in the highest energy
level so in the case of chlorine we have
two plus five so seven valence electrons
and the rest our core electrons so we've
got a total of ten core electrons
another way in which you can identify
the number of valence electrons is using
the periodic table you can identify it
based on a group number of
Representative elements so in group
number one we have elements like
hydrogen lithium sodium potassium these
elements all have a valence electron of
one they have one valence electron now
in the second group you have the
alkaline earth metals like beryllium
magnesium calcium so these metals have
two valence electrons they're in group 2
of the periodic now we're gonna skip the
transition metals and move on to group
13 also known as group 3a so these
include aluminum and gallium and I
believe indium is the next one so these
elements have three valence electrons
and I'm forgetting something
can't forget about boron after boron is
aluminum then we have helium so these
elements contain three valence electrons
and then after born you have carbon
silicon germanium and those elements
contain four valence electrons and then
you have nitrogen phosphorus and I
believe arsenic so these they're found
in group five a so they contain five
valence electrons then you have the
chalcogen like oxygen sulfur selenium
these guys they hold up to six valence
electrons and then you have the halogens
like fluorine chlorine bromine iodine
those guys they have seven now be
careful of this one helium is a noble
gas but it only has two valence
electrons because it has a total of two
electrons so here having but the other
ones after it like neon and I think it's
a argon krypton all of these they have
eight valence electrons now I have
another question for you let's focus on
iodine which has an atomic number of 53
and an average atomic mass of 120 6.9
now how many core electrons are found in
an atom of iodine think about it now you
can use any one of the techniques that
we've illustrated in his video you can
draw the Bohr model of the atom which is
gonna take time to draw 53 electrons you
can also write out the electron
configuration which works as well but
because iodine is a relatively large
atom it's going to take some time to
draw the electrical figuration so is
there an efficient way to determine the
number of core electrons what's the
easiest way to
quickly give the answer now we can start
with an equation the total number of
electrons is equal to the sum of the
core electrons plus the valence
electrons keep that in mind
now for an atom of iodine we have 53
electrons in total now in order to find
the core electrons we need to find the
valence electrons first now we could do
so by using the group number iodine is
in group 7a it's a hill agem so it
contains seven valence electrons so the
number of core electrons is simply the
total number of electrons which you can
find a base on the atomic number of an
atom minus the valence electrons found
by the group number so it's gonna be 53
minus seven which is 46 core electrons
because 46 plus seven is 53 and so
that's an efficient way to find the
number of core electrons when you're
dealing with an element that's pretty
large you don't want to draw it out over
at out the electron configuration that's
gonna take time
so just identify the atomic number and
the group number subtract those two and
that will give you the number of core
electrons
