The lesson is organized so that is begins with an interesting fact of molecules and bonding, including a quotation from a quantum theorist. The lesson then moves through a guided instruction, including several practice examples. The lesson finishes with possible extensions and a student practice worksheet. Then I awake with a little shock, for a chemical bond is not a real thing. It does not exist. No one has ever seen one. No one ever can. It is a figment of our own imagination.
Charles Alfred Coulson was a British theoretical chemist who played a central role in the development of quantum theories of chemical bonding. Do chemical bonds really exist? Can we see them? Can we take pictures of them? Can we see them with a microscope? The answer is yes and no.
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Bonds do not exist as a line like we have drawn in the lessons nor do electrons exist as tiny back circles, like they we have illustrated in Lewis dot structures. However, connections do exist between atoms, but they are not always linear.
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Chemical species exist in a 3-D world, but we need a technique to represent them on paper or a computer in a 2-D plane. Hence, scientists have developed models as an illustration technique. It should be noted, however, that in IBM scientists in Switzerland succeeded in imaging a real molecule called pentacene! There are numerous other modeling techniques. We will discuss some of the other most used introductory models, such as ball-and-stick, solid sphere, and computer models. Draw the Lewis dot structure for methane CH 4. Is it a covalent molecule or an ionic compound?
Both carbon and hydrogen are nonmetals and have similar electronegativities; therefore it is a covalent molecule. Perhaps carbon would be white and hydrogen would be black. In a 3-D model, the geometry of the methane model can be shown. The hydrogen atoms are attached to the carbon atom symmetrically in a tetrahedron formation pyramidal.
Use a modeling kit to construct the 3-D model below.
If you have access to modeling kits, use those throughout the remaining examples. You can use or purchase ball-and-stick modeling kits or solid sphere sometime called space-filling kits. If you do not have access to these kits, drawing will work. It does not, however, extend to discussions of molecular geometry as well. Gum drops and toothpicks can also work for 3-D models.
Organic Structure and Bonding Introduction
To make a ball-and-stick model, let carbon be represented by a white circle and chlorine be represented by green circles. Draw a ball-and-stick model for carbon monoxide CO. Carbon monoxide is covalently bonded, with a triple bond between the carbon and oxygen atoms. Let carbon be represented by a white circle and oxygen by a red circle.
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Even though the Lewis dot structure shows a triple bond, geometry dictates a single line be drawn in the ball-and-stick model. Just like the ball-and-stick models, they can be illustrated in a 2-D plane or a 3-D plane. When solid sphere models are represented in a 3-D plane, one or more of the atoms sometimes cannot be seen well. You may draw all of the examples in a 2-D plane.
This lesson does not involve students generating computer-based models, but rather offers pictures of computer based models. Show students copies of the images or display the website listed below each image. Therefore, many models are made using computer modeling programs.
What would a molecule really look like if you could view it through a magical microscope of some kind?
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A possible answer would be this computer-generated view of hemoglobin, a protein molecule found in red blood cells. Have students work individually or in pairs to complete the activity. A discussion of forced atomic microscopy is interesting for students who want more information regarding the techniques used to image the pentacene molecule. Scientists want to show a 3-D representation of a 2-D molecule, but on paper.
Dotted lines and shaded wedges aid in this task.
An example of ascorbic acid is shown below:. You can put a copy of the above structure on the board and have students use their modeling kits to construct this molecule, commonly known as vitamin C. Students who finish their modeling worksheet early can make a ball-and-stick model of ascorbic acid:. You are impersonating. Stop Impersonating. These rules are the basis of a number of structure prediction and modelling techniques which make use of the correlation between bond valence and bond length. A three parameter equation is proposed to describe this relation for those bonds e.
H-O, Na-O, Tl-O which occur with a wide range of lengths and for which, therefore, the usual two parameter equations are not adequate. Unable to display preview. Download preview PDF.
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Structure and Bonding in Metals - Chemistry LibreTexts
Recent developments in the bond valence model of inorganic bonding. Authors Authors and affiliations I. This process is experimental and the keywords may be updated as the learning algorithm improves. This is a preview of subscription content, log in to check access. Acta Crystallogr A — Google Scholar. J Chem Info —69 Google Scholar. Brown ID Predicting bond lengths in inorganic crystals.
The Bonding Model
Acta Crystallogr B — Google Scholar. Chem Soc Rev — Google Scholar. Brown ID, Altermatt D Bond valence parameters obtained from a systematic analysis of the inorganic crystal structure database. J Chem Phys —4 Google Scholar. Am Mineral — Google Scholar. Can J Chem — Google Scholar.