here's a very simplified model of an
atom the nucleus at the center of the
atom is where the protons and neutrons
live but they're kind of boring because
for the most part they just sit there
the real star of the show is the
electron the electron gets to do all the
interesting stuff like move around jump
around bind with other atoms these
dashed lines represent the different
energy levels the electron can have
while in the atom we like representing
these energy levels with an energy level
diagram the energy level diagram gives
us a way to show what energy the
electron has without having to draw an
atom with a bunch of circles all the
time let's say our pretend Adam has
electron energy levels of zero Evie for
Evie six Evie and seven Evie note that
moving left or right on an energy level
diagram doesn't actually represent
anything meaningful
so technically there is no x-axis on an
energy level diagram but we draw it
there anyway because it makes it look
nice all that matters is what energy
level or wrong on the ladder the
electron is f note that the electron for
our hypothetical atom here can only
exist with zero evey for six or seven
Evie the electron just cannot exist
between energy levels it's always got to
be right on one of the energy levels
okay so let's say our electron starts
off on the zero evey energy level it's
good to note that the lowest energy
level an electron can have in an atom is
called the ground state so how could our
electron get from the ground state to
any of the higher energy levels well for
the electron to get to a higher energy
level we've got to give the electron
more energy and we know how to give an
electron more energy you just shoot
light at it if a photon of the right
energy can strike an electron the
electron will absorb all the photons
energy and jump to a higher energy level
the electron in this ground state needs
four Evie to jump to the next energy
level that means if a photon that had an
energy of four Evie came in and struck
the electron the electron would absorb
the energy of the photon causing the
photon
to disappear and that electron would
jump up to the next energy level we call
the first energy level after the ground
state the first excited state once the
electrons at the higher energy level it
won't stay there long electrons if given
the chance will fall towards the lowest
energy level they can so our electron
will fall back down to the ground state
and give up for evey of energy the way
an electron can give up energy is by
emitting a photon so after falling back
down to the ground state this electron
would emit a four evey photon electrons
don't have to just jump one energy level
at a time though if the electron in our
ground state were to absorb a six evey
Photon the electron can jump all the way
up to the six evey energy level now that
the electrons at a higher energy level
it's gonna try to fall back down but
there's a couple ways it could fall back
down in this case the electron could
fall down to the ground state all in one
shot giving up a six evey photon in the
process but since the electron started
at the six evey energy level it could
have also fallen first to the four evey
energy level emitting a to evey photon
in the process it's a to evey photon
because the electron dropped two
electron volts in energy and now that
the electrons at the four evey energy
level it'll fall back down to the ground
state emitting a four evey photon in the
process so electrons will sometimes drop
multiple energy levels at a time and
sometimes they'll choose to take
individual steps but regardless the
energy of the photon is always equal to
the difference in electron energy levels
what if our electrons in the ground
state and we send a five evey photon at
it if the electron were to absorb all of
the energy of the five evey photon it
would now have five electron volts but
that's not an allowed energy level so
the electron can't absorb this photon
and the photon will pass straight
through the atom keep in mind the
electron in the atom has to absorb all
of the photons energy or none of it it
can't just absorb part of it all right
so now we could figure out every
possible photon this
Adam could absorb if the electrons in
the ground state it could absorb a for
evey photon or a six evey photon or a
seven evey photon if the electrons at
the second energy level also called the
first excited state the electron could
absorb a to evey photon or a three evey
photon and if the electron were at the
third energy level or the second excited
state the electron could absorb a one
evey Photon those are the only photons
that this atom will be seen to absorb
2.5 evey photons will pass straight
through five evey photons will pass
straight through 6.3 evey photons will
pass straight through what this means is
that if you were to shine light that
consisted of all possible wavelengths
through a gas that was composed of our
pretend atoms all the wavelengths would
not make it through some of the
wavelengths would get absorbed then
scattered away in random directions this
would manifest itself as dark lines in
the spectrum missing wavelengths or
missing energy levels that correspond to
the energies of photons that our
electron can absorb this is like a
fingerprint for an atom and it's called
that atoms absorption spectrum if you
were to ever see this progression of
dark lines in these exact positions you
would know that the gas you were looking
at was composed at least partly of our
hypothetical atom this also allows
astronomers to determine what stuff in
our universe is made out of even though
we can't get close enough to collect a
sample all we have to do is collect
light from a distant star or quasar that
shines through the stuff we're
interested in then just determine which
wavelengths or energies got taken out
the details are a little messier than
that but this provides astronomers with
maybe the most important tool at their
disposal now the absorption spectrum are
all of the wavelengths or energies that
an atom will absorb from light that
passes through it you could also ask
about the emission spectrum the emission
spectrum are all of the wavelengths or
energies that an atom will emit due to
electrons falling down an energy level
you could go through all the
possibilities of an electron falling
down again but you'd realize you're
going to get the exact same energies for
the emission spectrum that you got for
the absorption spectrum so instead of
letting light pass through a gas
composed of your hypothetical atoms
let's say you made a container that had
the gas of your hypothetical atoms and
you ran an electric current through it
exciting those electrons to higher
energy levels and letting them fall back
down to lower energy levels this is what
happens in neon lights or if you're in
science class is what happens in gas
discharge tubes so for the emission
spectrum instead of seeing the whole
electromagnetic spectrum with a few
lines missing you're going to only see a
handful of lines that correspond to the
energies of those photons that that atom
will emit okay I've got to be honest
about something if any physicists are
watching this video they're cringing
because the energies that electrons will
have in an atom are not positive the
energies an electron can have in an atom
are actually all negative values this is
because the electrons bound to the atom
anything that's bound to something else
will have total energies that are
negative this is analogous to a ball
stuck at the bottom of a ditch if the
ball is not moving it has no kinetic
energy and if we assume that ground
level is the h equals zero position then
this ball is going to have a negative
gravitational potential energy
since this ball has a negative total
energy it's stuck and bound to the ditch
if someone could give this ball enough
energy so that it would have positive
total energy the ball could leave the
ditch it would not be found any more so
to make our hypothetical atom a little
more realistic let's subtract 10 Evie
from each energy level this doesn't
really change anything in order for the
electron to get from the negative 10
Evie ground state to the negative 6 Evie
first excited state it's still going to
take a 4 Evie photon people do get
confused with the negative signs though
so be careful in order to find the
energy of the photon that was absorbed
or emitted you always take the higher
energy
and subtract from it the lower energy
level so in this case we would take
negative 6 evie and subtract from it
negative 10 evie which tells us that it
would take a for evey photon to bump an
electron up to that energy level and the
electron would emit a 4 evey photon if
it dropped it back down from that level
something else that's unrealistic about
our hypothetical atom is that real atoms
wouldn't just stop at negative 3 evey
for the highest energy level real atoms
have energy levels that get closer and
closer together as you approaches zero
evey what happens when an electron gets
more than zero evey energy well if an
electron has more than zero energy that
means it's got positive energy and if
it's got positive energy it's not bound
to the atom anymore it'll be free to
leave it'll be gone and we'll say that
we've ionized the atom by removing an
electron so for example say the
electrons started at the negative 4 evie
energy level and it absorbed a 7 evey
photon that electron would have a total
energy of positive 3 evey and so it
would be gone from the atom