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Course: High school chemistry > Unit 1
Lesson 3: The Bohr model and atomic spectraAtomic spectra
Atomic electrons exist at specific energy levels. An electron can be excited to a higher level if it absorbs a photon with energy equal to the difference between the levels. Similarly, an excited electron returns to its ground state, emitting a photon with energy equal to the difference. The specific colors of light absorbed and emitted are called atomic spectra. Created by David SantoPietro.
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- At3:01, it is mentioned that once an electron hits the higher energy levels it will want to fall back down. Why is this the case?(7 votes)
- When electrons jump to higher levels, they get an increase in energy. This extra energy makes the electron unstable. The only way the electron can be stabilized is to lose the energy and fall back down to the ground (or resting) state. This is very similar to, let's say, a person standing on the ground. He is very stable and does not have a very big chance of falling over. But when he gains energy, it's like the person jumping up onto a chair. He has more energy because he is higher off the ground, but the chair can tip over. He is unstable in his more energetic state. Now, the way he can go back to being stable is to get off the chair and return to the ground.
I hope this helps!(36 votes)
- can electrons bump into each other and get their energy??(4 votes)
- Electrons don't usually bump into each other because they both are negatively charged. It's like two negative ends of a magnet. They will deflect or alter paths if you try to slide them together. This is because like charges repulse each other.(12 votes)
- So does each atom have a different combination of eV levels? Do they change slightly? For example would the next element after this hypothetical in the above example correspond to 4.1, 6.1, 7.1, etc? O(6 votes)
- What does eV stand for?(1 vote)
- eV is the symbol for the electronvolt unit. It is a unit of energy for the energy of electrons in different energy levels in an atom.(10 votes)
- So if a photon with 8ev hits an electron at 0, it would just pass right through, right?(3 votes)
- You are correct. If the energy of the photon is not equal to the energy difference between the ground state and any excited state, the photon is not going to be absorbed, and the electron remains in the ground state.(2 votes)
- At3:01, David said there was a couple ways an electron could drop to the ground state. If it's at 6EV, it could lose a 2EV photon and then a 4EV photon to get to 0EV. Or it could just lose a 6EV photon in one shot. What causes it to go step by step or lose it all in one go? Is there some other factor that influences this?(3 votes)
- why does an electron fall back to the lowest energy level(1 vote)
- Electrons fall back to the lower energy levels because that is a more stable configuration. Imagine it's like a rock on a hill. You give the rock more energy the farther you push it up the hill, but once you let go it will roll back down because that is a more stable form than on the edge of the hill. This is how many interactions in the natural world work. Everything wants to get to the lowest energy level that it can.(4 votes)
- Are these levels shown the only possible excited states or energy levels?(2 votes)
- "...give an electron more energy you shoot light at it"
Does the type if light matter?
Does man-made and natural light have the same effect on the electrons?(1 vote)- Yes, the type of light matters. Man-made and natural light can both impart energy to electrons, but different wavelengths of light can have varying effects on electrons depending on factors like intensity and frequency.-hope this helps!(3 votes)
- Wow! So does it mean that you can tell/estimate what things are made of just by observing and classifying the dark lines in the wavelengths?! Amazing!(2 votes)
Video transcript
- [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.