Core Concepts
In this article, you will learn about the different types of visual molecular models used in chemistry.
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What are Molecular Models?
Molecular models are simply representations of the connectivity or structure of a molecule. They are used to help visualize how molecules are structured, predict the physical characteristics of the molecules, and predict the chemical properties of the molecules.
The main types of molecular models covered in this article include the ball-and-stick model, the space-filling model, and the skeletal structure.
Ball-and-Stick Model
The Ball-and-Stick model is one of the most common 3D molecular models. In the ball-and-stick model, balls of different colors representing different elements are connected by sticks representing covalent bonds. This model allows us to see the ideal bond angles of the molecules as well as the possible different orientations of a molecule. It is an accurate representation of the connectivity of the atoms in a molecule. However, the ball-and-stick model isn’t usually used to predict bond lengths since the bond lengths shown are fixed or approximate.
The person credited for popularizing the ball-and-stick model is August Wilhelm von Hoffmann. He was also the one who color-coded the elements that we still use today in ball-and-stick models: carbon as black, hydrogen as white, oxygen as red, nitrogen as blue, chlorine as green, and sulfur as yellow.
In the picture above, a double bond exists between the carbon atom on the left (black) and an oxygen atom (red). The ball-and-stick model shows this double bond using two sticks that connect the two atoms.
Space-Filling Model
The Space-Filling model is also one of the most common 3D molecular models. In the space-filling model, the atoms’ sizes are proportional to their atomic radii. This model is helpful in visualizing the sizes of the atoms in a molecule, how the atoms within a molecule are arranged, and how the atoms take up space. It provides a realistic representation of the molecule. However, it is hard to determine bond lengths and angles; without a solid foundation on certain molecules, it may be hard to distinguish which atoms bond together. Also, in larger molecules, the space-filling model may become very convoluted and confusing.
Skeletal Structure
3D Skeletal structures are similar to the ball-and-stick model. Instead of having balls representing the atoms, we assume that the atoms are at the intersection of the sticks.
2D skeletal structures are more common than those in 3D. In 2D skeletal structures, like Lewis dot structures, lines represent bonds, with each multiple bond represented as an extra line. Unless other information is provided, we assume that the end of each line is a carbon atom with carbon-hydrogen bonds. Skeletal structures simplify larger molecules, because the position of the hydrogen atoms and the identity of the carbon atoms are implied.
Practice Visualizing
Draw or visualize the ball and stick model, the space-filling model, and the skeletal structure for each of the following molecules. Then, search it up to see if you did it right.
- Water (H2O)
- Carbon dioxide (CO2)
- Methane (CH4)
- Glucose (C6H12O6)