explain quantum mechanical model of atom
Answers
Matter begins to behave very strangely at the subatomic level. Some of this behavior is so counterintuitive that we can only talk about it with symbols and metaphors—like in poetry. For example, what does it mean to say an electron behaves like a particle and a wave? Or that an electron does not exist in any one particular location, but that it is spread out throughout the entire atom?
If these questions strike you as odd, they should! As it turns out, we are in good company. The physicist Niels Bohr also said, "Anyone who is not shocked by quantum theory has not understood it." So if you feel confused when learning about quantum mechanics, know that the scientists who originally developed it were just as befuddled.
We will start by briefly reviewing Bohr's model of hydrogen, the first non-classical model of the atom.
Review of Bohr's model of hydrogen
As we have seen in a previous article on the Bohr model, the emission spectra of different elements contain discrete lines. The following image shows the visible region of the emission spectra for hydrogen.
Emission spectrum for hydrogen showing purplish blue lines at 410 and 434 nm, a light blue line at 486 nm, and a red line at 656 nm. All lines are against a black background.
Hydrogen emits four wavelengths of light in the visible region. Image credit: Emission spectrum from Wikimedia Commons, CC0 1.0
The quantized emission spectra indicated to Bohr that perhaps electrons could only exist within the atom at certain atomic radii and energies. Recall that quantized refers to the fact that energy can only be absorbed and emitted in a range of allowable values rather than with any possible value. The following diagram of the Bohr model shows the electron existing in a finite number of allowed orbits or shells around the nucleus.
Diagram showing the the first three levels—n=1, 2, and 3—for Bohr's model of hydrogen. An electron is relaxing from n-3 to n=2, as indicated by an arrow starting at n=3 and going to n=2. The loss of a photon is shown for the electronic transition with an energy of hf.
A diagram of the Bohr model of the hydrogen atom. Electrons move in circular orbits that are at fixed distances from the nucleus. Light is emitted when excited electrons, n>1n>1n, is greater than, 1, relax back to a lower energy level. Image credit: from Wikimedia Commons, CC BY-SA 3.0
From this model, Bohr derived an equation that correctly predicted the various energy levels in the hydrogen atom, which corresponded directly to the emission lines in the hydrogen spectrum. Bohr's model was also successful at predicting the energy levels in other one-electron systems, such as \text{He}^+He
+
H, e, start superscript, plus, end superscript. However, it failed to explain the electronic structure in atoms that contained more than one electron.
While some physicists initially tried to adapt Bohr's model to make it useful for more complicated systems, they eventually concluded that a completely different model was needed.
Wave-particle duality and the de Broglie wavelength
Another major development in quantum mechanics was pioneered by French physicist Louis de Broglie. Based on work by Planck and Einstein that showed how light waves could exhibit particle-like properties, de Broglie hypothesized that particles could also have wavelike properties. [What are wavelike properties?]