Before this, knowФотоелектричний ефектPlanck's Quantum Theory — Overview & Planck's Constant
Illustrated · 1 min
Atomic Spectra: How is Atomic Spectra Generated?
Image Source: Leo Dong
Light excites ground-state electrons to an excited state. And excited electrons relax by emitting different wavelengths of light. That is why you see different colors in atomic spectra.
Video
by Ben's Chem Videos
In this video, Ben explains atomic emission spectra. Emission spectra are a result of the Bohr model. Neils Bohr explained that atomic orbitals are quantized. They can only absorb or emit specific energies. Ben explains how electrons transitioning between orbitals create absorption and emission spectra.
Illustrated · 3 min
Atoms contain electrons. That's general chemistry! But did you know that we can excite electrons with energy? What does excited mean? The electron moved to a higher atomic orbital. Do you remember Planck's quantum theory? It taught us that orbitals have specific energy gaps. The fancy word is they are quantized. So, for example, only one specific energy will move hydrogen's electron from the 1s-orbital to the 2s-orbital. This exact energy corresponds to a distinctive wavelength of light. The wavelength of light relates to a type of light found on the electromagnetic spectrum. So what happens if we use the wrong wavelength of light? Absolutely nothing! The light's energy is too high or too low. So, the electron cannot transition to another orbital.
But, something unique happens when electrons move between orbitals. When we add the correct energy, light is also emitted. This occurs when an excited electron relaxes back to a lower orbital. But, to relax, it has to release energy. How? By discharging light. But again, it's moving between orbitals. So there is a specific amount (or quanta) of energy between the excited and resting states. Some orbital transitions release visible light (400-700 nanometers). The light's color is characteristic of individual elements. So we can use it to identify unknown compounds. The light emitted by relaxing electrons produces atomic spectra. Spectra is a fancy term for graphs that display light intensity versus wavelength. You can see hydrogen's atomic spectrum below.
How do we discover atomic spectra? Analytical chemists perform special experiements called spectroscopy. They apply a special instrument called a spectrometer. A spectrometer starts by exciting the sample's electrons. How? With various forms of energy. For example, it might apply a current (electricity), light, or heat. Soon, the excited electrons start to relax. They release light with specific wavelengths. But, the human eye would only see one color. Why? Remember, from the wave-particle duality, we can consider light a wave. The different wavelengths interfere with one another. So, the instrument passes the light through a tiny slit. The slit creates a beam of light. Then it passes through a prism. A prism is a type of optical glass. The different wavelengths travel through it at varying angles. What's the result? The individual wavelengths become separated. The instrument detects the light emitted from the prism. Typically, it will only correspond to a few wavelengths of light. These wavelengths get displayed on a line spectrum. What does it look like? At the detected wavelengths, you'll see a series of lines. The lines get separated by blank spaces. The empty areas indicate nothing got detected.
Image source: Cassie Gates
Chemists have found most of the atomic spectra for the elements of the periodic table. Each one is unique. Earlier, we mentioned that our eyes only see one color. Some elements have emission colors that have become well known. You might recognize some!
Examples of elements and the colors excited atoms emit are:
- Some older street lamps used sodium. Many have been replaced with LED. But, if you see one with a yellowish glow, it's using sodium!
- Hydrogen's emission spectrum mostly has scientific applications. It's used to detect hydrogen in stars. For example, hydrogen fusion is the principal form of energy for our sun. It was also used to confirm the Schrodinger equation. We'll talk more about it when we study the quantum mechanical model of the atom.
- This is the classic neon light.
- Argon is also used as a type of "neon" light.