Atomic Number and Mass number Video Lecture | AP Physics 2 - Grade 9

here's a diagram of an atom it's not
perfectly accurate but for what we need
right now it's going to do just fine
so in this lesson we're going to look at
characteristics that we can use to
describe atoms like this one
you know if somebody asks you to
describe a person chances are you use
characteristics like the color of their
hair color their eyes what their height
is and so forth just like that there are
characteristics we can use to describe
atoms the first of these characteristics
and maybe the most important is the
atomic number atomic number is often
abbreviated by a capital letter Z and
what the atomic number is is it's the
number of protons in the nucleus of an
atom in my diagram here I'm using these
red circles to represent protons so I
have 1 2 3 protons in this atom so the
atomic number is going to be 3 for this
atom here atomic number is particularly
important because the number of protons
in an atom tell us what kind of an atom
is and by that I mean is it a carbon
atom is it an oxygen atom is it a sodium
atom we're going to talk more about that
in the video on isotopes so you don't
have to worry about it for right now the
next important characteristic that we
can use when describing atoms is the
mass number often abbreviated by an
uppercase a here mass number is the
number of protons plus the number of
neutrons in the nucleus of an atom
people always get confused by mass
number here's why atomic number is the
number of protons so everybody always
wants mass number to be the number of
neutrons but it's not it's a number of
protons and neutrons in fact there's not
really a name for the number of neutrons
just the number of neutrons an atom we
just don't use it that often so anyway
for mass number we want to count all of
these together 1 2 3 4 5 6 7 the mass
number of our atom here is going to be 7
now finally the third characteristic
we're going to talk about here is
probably the
complex and that's called net charge I
don't have a good way to define that
charge I mean I don't have a good
definition so what I'm going to say here
is it's how protons and electrons
balance each other out
let's talk a little bit more about this
so net charge is about how protons and
electrons balance each other out so
protons as you know have a positive
charge and electrons have a negative
charge so that means that if we have the
same number of protons and electrons in
an atom like I have right here three
protons and three electrons I'm not
drawing the neutrons here because we're
not worried about here we have the same
number of protons and electrons they're
going to balance each other out this
three minus is going to balance out the
three positive so we're going to end up
with an atom that has a net charge of
zero because these two things balance
out and we can call this atom neutral
which means that it has no net charge
now what would happen in an atom where
we have four protons and three electrons
we don't have the same number of
electrons and protons anymore so they
can't totally balance each other out
instead we have more protons than
electrons so that we're going to have a
net charge that's positive because
there's going to be some of this
positive charge in the protons it can't
be balanced out by the electrons in this
case we have one more proton than
electron so this is going to give us a
net charge of plus one since our atom
has a net charge we give it a special
name we call it an ion an ion is any
atom that has a net charge neutral is
what you call it if you don't have a
charged ion is what you call it if you
have any charge okay this is where we
have one more proton than electron what
about this case here what if we had five
protons and three electrons in this case
the charges don't balance out either but
now I have two more protons than
electrons I've got two more protons that
aren't getting balanced out by these
electrons so that means that I'm going
to have a net charge of two
plus for these two protons and again
this is going to be an ion because it's
got a charge now the opposite can happen
as well here is an example where an atom
has four electrons and three protons so
now it's got more electrons and protons
these three protons can only balance out
three of the electrons so have one more
electron that's not getting balanced
down that means that this total charge
is going to be minus one that's a charge
of this one more electron that I have in
the proton and if I had a case in which
I had five electrons oh I should have
said we call this an ion because it has
a charge just like this negative charge
positive charge it doesn't matter if
it's got a charge we call it an ion okay
so anyway we have a case where we have
five electrons and three protons now
there are two more electrons and protons
so I have two negative charges that
aren't getting balanced out by the
proton which means that I have a net
charge of minus two and this also is an
ion so that's how you can figure out net
charge so what's the net charge for this
end well we have one two three four
electrons and we have one two three
protons since there are different
numbers the protons and electrons aren't
going to be balancing each other out we
have one more electron than proton so
that means that we have a net negative
charge and since there is one more
electron than proton this atom here is
going to have a net charge of minus one
so now that we've talked about these
three characteristics you can use to
describe an atom let's look at the
practice problem video so you can go
over this and a little bit more depth
and try a bunch of examples