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