Wired Chemist

Minimization Of Repulsions

This animation shows three orbitals arranged in pyramidal fashion around a nucleus. This arrangement is not the most stable, however. The repulsions between the electrons in the three orbitals is minimized if the orbitals are directed toward the corners of an equilateral triangle.

Rotational Motion Of A Diatomic Molecule

Up to this point in the course, we have viewed molecules as static entities. That is, we did not concern ourselves about the way in which molecules and the atoms within the molecules move. This animation is the first in a series that attempt to show the various motions of molecules. Here we see the hydrogen chloride molecule rotating or tumbling as well as translating (that is, moving from one point to another). Molecules in the gaseous and liquid states are in constant motion, translating in straight lines until they collide with another molecule or the walls of the container. After such collisions they simply change directions.

Vibrational Motion Of A Heteronuclear Diatomic Molecule

In addition to rotational and translational motion, the atoms within a molecule also undergo an oscillatory motion. One of the fascinating predictions of quantum mechanics is that this vibrational motion occurs even at the absolute zero of temperature.

A Schematic Representation Of A Visible Spectrophotometer

Before you begin the animation examine the pieces of a typical spectrophotometer. From left to right you see the source, in this case a light bulb, a slit that produces a small, thin image of the light source, a prism to disperse the light into different wavelengths, the sample (in this case the sample is in a test tube), and, finally, the detector attached to a voltmeter. As the animation begins you see the source producing light which passes through the slit, and then through the prism. As the light leaves the prism red light is directed to the sample. The wavelength can be changed by rotating the prism so that light of a different wavelength will hit the sample. The detector produces a response to the light that passes through the sample and this response generates a voltage of 350 millivolts. The colorless sample is then replaced with one that contains a blue solution. When the red light again strikes the sample, a considerable amount of the light is absorbed by the sample. The small amount of light that passes through the sample produces a response of 41 millivolts at the detector.

Absorption Of Light By Two Different Triatomic Molecules

Two different molecules along with their electronic energy levels are shown here. A photon is shown striking the first molecule and causing a transition from one energy level to another. The excitation of the second molecule requires a photon of shorter wavelength (and therefore higher energy) because of the wider spacing of the energy levels. Thus, the greater the spacing of the electronic energy levels the greater the energy and the shorter the wavelength of the light required to cause an electronic transition.

The Symmetric Stretching Vibration In A Triatomic Molecule

In molecules containing more than two atoms there are a variety of ways in which the atoms can move during the vibrational process. This animation shows both of the terminal atoms moving in phase; that is, moving together. This is called the symmetric stretching mode.

The Asymmetric Stretching Vibration In A Triatomic Molecule

Here the terminal atoms are shown moving in an asymmetric fashion, with one moving toward the central atom while the other moves away from the central atom. This is the asymmetric stretching mode.

The Bending Vibration In A Triatomic Molecule

In the third vibrational mode for a triatomic molecule the terminal atoms move toward and away from one another in a scissoring motion. In this mode the bond angle changes during the vibration. This is referred to as a bending mode rather than a stretching mode, where the bond angle does not change.