Electron Transitions & Spectral Lines

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  • 0:02 Transitions & Photons
  • 2:25 Photons & Spectra
  • 3:23 The Staircase Example
  • 5:01 Hydrogen Atoms
  • 6:20 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson discusses how electrons transition between energy levels, why they do this, how photons are involved, how absorption and emission spectra come into play, and what the Lyman, Balmer, and Paschen series are.

Transitions and Photons

What does a staircase have to do with atoms and electrons? Well, the obvious answer is it is made of atoms, which contain electrons. But enough of that, smarty-pants. It can also be used as a good metaphor for this lesson's concepts involving atoms, electrons, and transitions. You'll see in a bit what I mean by that.

The transition, or the movement, of an electron between energy levels, in an atom can occur in more than one way. For an electron to transition to a higher energy level, it must absorb energy, just like it takes energy to lift a rocket upwards into the sky or to lift a heavy weight above your head.

If an atom collides with another atom, ion, or electron, the atom can become excited. An excited atom is an atom where an electron has moved from a lower to a higher energy level. You know how when two football players forcefully collide it looks like the helmet jumps up off of their head? Well, a collision can also provide enough energy to get an electron to jump up off of a lower energy level and into a higher energy level.

Another way an atom can become excited is by absorbing a photon, a small bundle of electromagnetic radiation. Electromagnetic radiation, or light, is a form of energy that has wave-like properties. Therefore, the energy of a photon depends on its wavelength; the shorter the wavelength, the higher the energy.

Only photons of specific wavelengths can be absorbed by an atom. Basically, the photon is like an energy drink for a little electron, giving it more buzz to jump up higher into a higher energy level.

But just like people may have a preference for one energy drink to another, atoms have preferences for the kinds of photons they can absorb. Only photons with energies equal to the energy difference between two energy levels in a specific atom can be absorbed. This means electrons can occupy only certain permitted energy levels; there are no energy levels in between.

Consequently, when just the right kind of photon hits an atom, it's absorbed, and the electron jumps to a higher energy level. Since atoms have many energy levels, depending on how energetic this photon is, the electron can jump up one or more energy levels. All of this also means that more than one wavelength of light can be absorbed by a single atom to get an electron to jump into a higher energy level.

Photons and Spectra

If a continuous spectrum of photons (a complete arrangement of colors) shines on a group of identical atoms, these atoms, like sponges, will understandably absorb only certain kinds of photons from the continuous spectrum. When this happens, an absorption-line spectrum will be produced. Because an absorbed wavelength of light removes a color from the original continuous spectrum, the resulting absorption spectrum is also called a dark-line spectrum.

Excited atoms cannot stay excited for long, however, and so the electron must eventually jump down to a lower energy level. As it does so, the electron emits a photon with energy (and thus wavelength) equal to the difference in energy levels between the two levels the electron jumps in between. The photons that are emitted in such a fashion make bright colorful lines against a dark background. These are known as bright-line or emission-line spectra.

The Staircase Example

Let me try and put all of the confusing core concepts of this lesson into a more simple metaphor. Let's pretend you're an electron. A photon of a specific energy (or wavelength) can be like a specific energy drink. The energy levels can be like steps in a staircase in your home.

You are now standing at the bottom step, the lowest possible energy level in the atom. You have a few energy drinks with different strengths next to you. You know that to jump from the bottom step up, you need energy. A little bit of energy to jump to the second step but a lot more energy to jump from the bottom all the way up to the third step in one fell swoop.

Okay, now you take a sip of the first energy drink. You don't move. Why not? It's because that drink didn't provide just the right amount of energy for you to transition between two steps. You can only jump onto a fully-fledged step. You can't jump to a fourth or a half of a step; such a thing doesn't exist on the staircase.

So, if the drink doesn't give you exactly the right amount of energy to jump onto a solid step, you're not going to jump onto anything, are you? Of course not! You'll crash and burn if you do that.

Okay, now you take a sip of a second energy drink. Boom! You jump to the third step. Nicely done. That drink gave you just the right amount of energy to do so. That means a photon of a specific wavelength was absorbed.

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