This content explains the fundamental characteristics used to describe atoms: atomic number, mass number, and net charge, emphasizing how these properties define an atom's identity and state.
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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 asked you to describe a person
chances are you'd use characteristics
like the color of their hair the color
of 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 three protons in this atom so
the atomic number is going to be three
for this atom here atomic number is
particularly important because the
number of protons in an atom tell us
what kind of an atom it 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 in 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 seven now finally
the third characteristic we're going to
talk about here is probably the most
complex and that's called net
charge I don't have a good way to Define
net 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 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
them here we have the same number of
protons and electrons they're going to
balance each other out this 3 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 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 that 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 charge 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 Balan 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 I have one
more electron that's not getting
balanced out 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 the proton and if I had a
case in which I had five electrons oh
and 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 2 and
this also is an ion so that's how you
can figure out net charge so what's the
net charge for this atom well we have 1
protons since they're 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
minus1 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 that you can
go over this and a little bit more depth
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