Atomic spectra | The Bohr model and atomic spectra | High school chemistry | Khan Academy

- [Narrator] 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 atom
has electron energy levels

of 0 eV, 4 eV, 6 eV, and 7 eV.

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 rung on the ladder the electron is at.

Note that the electron

for our hypothetical
atom here can only exist

with 0 eV, 4, 6 or 7 eV.

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 0 eV energy level,

it's good to note that
the lowest energy level

and 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 photon's energy

and jump to a higher energy level.

The electron in this
ground state needs 4 eV

to jump to the next energy level.

That means if a photon that
had an energy of 4 eV 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 4 eV 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 4 eV 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 6 eV photon,

the electron can jump all the way up

to the 6 eV energy level.

Now that the electron's
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 6 eV photon in the process.

But since the started at
the 6 eV energy level,

it could have also fallen first

to the 4 eV energy level
emitting a 2 eV photon

in the process.

It's a 2 eV photon

because the electron dropped
2 electron volts in energy,

and now that the electron's
at the 4 eV energy level,

it'll fall back down

to the ground state emitting
a 4 eV 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 electron's in the ground state

and we send a 5 eV photon at it?

If the electron were to
absorb all of the energy

of the 5 eV photon,

it would now have 5 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 photon's
energy or none of it,

it can't just absorb part of it.

Alright, so now we could figure
out every possible photon

this atom could absorb.

If the electron's in the ground state,

it could absorb a 4 eV
photon or a 6 eV photon

or a 7 eV photon.

If the electron's at
the second energy level,

also called the first excited state,

the electron could absorb a 2 eV photon

or a three eV photon,

and if the electron were
at the third energy level

or the second excited state,

the electron could absorb a 1 eV photon.

Those are the only photons

that this atom will be seen to absorb.

2.5. eV photons will
pass straight through,

5 eV photons will pass straight through,

6.3. eV 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
atom's 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 distance 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 in energy levels.

You could go through all the possibilities

of an electron falling down again,

but you'd realize you're gonna
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,

it's 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.