Need up to 30 seconds to load.
in this lesson we're going to talk about
how to calculate the number of protons
neutrons and electrons in an atom and in
an ion so let's use sodium as an example
and on a periodic table you might see
something that looks like this the
smaller of the two numbers is the atomic
number which is also equal to the number
of protons the larger of the two numbers
is known as the average atomic mass now
sometimes you might see a problem that's
written this way they may tell you to
find the number of protons neutrons and
electrons for this particular element
now in this format you can see that the
atomic number is on the bottom the
number on the top noticed that it's not
the same as twenty two point nine nine
in fact it's rounded this is not the
average atomic mass but rather it is the
mass number so this is a specific
isotope of sodium if you average all of
the isotopes of sodium on earth you're
gonna get this number so because that
number is close to 23 this is the most
abundant isotope of sodium on the earth
so in addition to sodium 23 you also
have sodium 24 and is also sodium 22 but
as we said before because the average
atomic mass was twenty-two point nine
nine this is the most abundant isotope
which is also the most stable one on the
earth the other forms of sodium they're
not stable and they're not very common
but these are other isotopes of sodium
now what you need to know is that the
atomic number is associated with
the identity of the element so sodium
always has an atomic number of 11 now
the mass number can change and it will
change for different isotopes so that
number can vary but the identity of the
element is dependent on an atomic number
now let's talk about the formulas that
we need in order to calculate the number
of protons and neutrons and electrons so
let's start with the number of protons
this is always equal to the atomic
number that's it so sodium always has 11
protons now the number of neutrons is
the difference between the mass number
and the atomic number and then the
number of electrons is equal to the
atomic number minus the charge of the
species so for an atom let's say like
aluminum which doesn't have a charge the
charge is zero the electrons will thus
be equal to the atomic number so for an
atom the number of electrons and protons
are the same because the atom is neutral
but if we have in an ion let's say a
positively charged cation then the
number of electrons will change in this
case there's gonna be three more protons
than electrons or if we have an anion
like the phosphide ra9 there's gonna be
three more electrons than protons so for
ions the number of electrons and protons
are different but for atoms which are
electrically neutral the number of
protons and electrons are the same so
let's start with this example go ahead
and calculate the number of protons
electrons and neutrons for this element
so the number of protons is equal to the
atomic number which is
the smaller of the two values so that's
gonna be 11 the number of neutrons is
the difference between the mass number
which is 23 and the atomic number which
is 11 so it's 23 minus 11 and that gives
us 12 so we have 11 protons 12 neutrons
now for the number of electrons it's
equal to the atomic number which is 11
minus the charge now there's no charge
for this particular element it's an atom
not an ion so the charge is 0 so we have
11 electrons so as we can see for
electrically neutral atoms the number of
protons and electrons are the same now
let's consider another example aluminum
27 with a 3 plus charge so here we have
an ion and go ahead and calculate the
number of protons and neutrons and
electrons in this ion so we can see that
the atomic number is 13 and the mass
number is 27 so the number of protons is
always equal to the atomic number which
is 13 in this example now the number of
neutrons is the difference between the
mass number and the atomic number so
it's 27 minus 13 which gives us 14 now
for the number of electrons it's going
to be the atomic number which is 13
minus the charge on the ion which is
positive 3 so 13 minus 3 is 10 so 4
positively charged cations there's going
to be more protons and then electrons so
because it has a positive 3 charge
there's 3 more protons and then
electrons now let's try a negatively
charged anion so this is the sulfide ion
and let's calculate the number of
Tod's neutrons and electrons so we have
an atomic number of 16 and a mass number
of 33 so the number of protons just like
before
is equal to the atomic number so that's
16 the number of neutrons is a
difference between the mass number and
atomic number so 33 minus 16 so we have
17 for this example and finally the
number of electrons is the atomic number
which is 16 minus the charge which is
negative 2 so 16 minus negative 2 is the
same as 16 plus 2 so we get 8 and
electrons thus 4 negatively charged ions
or anions you're going to have more
electrons than protons for an ion with a
negative 2 charge there's two more
electrons than protons now let's try a
different example or at least one that's
presented in a different way find the
number of protons neutrons and electrons
in silicon 29 so in this example we need
to write this in a format that we're
familiar with so silicon 29 that tells
us that the mass number is 29 but what
about the atomic number what we need to
use the periodic table so if you go to
it if you don't have one you can go to
Google Images you'll find that silicon
is the 14th element in the table so it
has an atomic number of 14 now there was
no charge indicated in this problem so
we're gonna assume that we're dealing
with an atom and not an ion thus we have
14 protons because that's the atomic
number and the number of neutrons is
going to be the mass number
- atomic number so that's 29 - 14 which
gives us 15 finally the number of
electrons is going to be the atomic
number minus the charge so it's 14 so
for all atoms the number of protons and
electrons are the same now let's work on
a challenge problem so let's see if you
truly understand the lesson in this
video so we have a cation with a 4 plus
charge and it has 26 neutrons and 18
electrons what is the identity of the
element feel free to pause the video if
you want to try it by the way for those
of you who haven't done so already feel
free to subscribe to this channel and
don't forget to click on that
notification bell and now let's begin
working on this problem so right now
we're going to say that the unknown
element is element X so we know it has a
4 plus charge and the number of neutrons
is 26 the number of electrons is 18 in
order to determine the identity of the
element what do we need to know remember
the atomic number identifies the element
so if we can determine the number of
protons we can identify the unknown
element and so that's what we need to do
in this example now which of the three
formulas that we wrote in a beginning
can help us here is it this one the
number of protons is equal to the atomic
number now this is not going to help us
because we know neither of these could
we use this the number of neutrons is
equal to the mass number minus the
atomic number now we have the number of
neutrons but we don't know the mass
number or the atomic number
so the formula that is going to be
helpful is the third one the number of
electrons is equal to the atomic number
minus the charge because we know the
charge and the number of electrons with
that we can calculate the atomic number
so there's 18 electrons and I'm just
gonna write a four atomic number and the
charge is positive four so this becomes
a minus four so we need to add four to
both sides and so the atomic number is
22 and the number of protons is equal to
the atomic number which is 22 now if we
wish to calculate the mass number we can
the number of neutrons was the
difference between the mass number and
the atomic number if you rearrange that
equation you'll find that the mass
number is the neutrons plus the atomic
number or you could say is the neutrons
plus the protons because the protons is
the same as the atomic number so it's 26
plus 22 which is 48
now granted that last step wasn't really
necessary but I decided to do it anyway
for those of you who were curious on how
to get it but let's focus on this number
so if you go to the periodic table and
look for element 22
let me get mines real quick you'll find
that this corresponds to titanium so
this is the titanium 4 plus ion and this
is the answer
so titanium is the unknown element and
that's how you can identify any unknown
element for these types of problems once
you identify the number of protons you
could use a product able to identify
them as an element
