Photo provided by Ann Arbor Hands-On Museum in Ann Arbor, MI
We have received questions asking why there aren't four bonds for each carbon atom in the "Build a Giant Carbon Nanotube" component of the Nano mini-exhibition. Catherine McCarthy, Project Leader for the NISE Network, and Keith Ostfeld, Director of Educational Technology and Exhibit Development at the Children’s Museum of Houston, offer some clarity and insights into these inquiries…
It’s true that carbon atoms have four bonding sites, but carbon is very flexible in how it bonds. Carbon has the ability to form several different types of bonds: single, double, and triple. Using hybrid bonds, carbon also has the ability to form different shapes including rings, sheets, and chains.
- In diamond, each carbon atom has a single bond with four other neighboring carbon atoms forming a tetrahedral crystalline structure. This combination of four bonds is known as an sp3 bonded carbon atom. Examples of carbon atom bonding to four other atoms include methane (CH4), propane (C3H8), and many other molecules.
- In graphene, each carbon atom bonds with three other carbon atoms in sheets forming a hexagonal or honeycomb pattern; the sheets then appear to stack upon each other. This combination is known as an sp2 bonded carbon atom. There are weak Van der Waal's forces between the sheets --which is why graphite is soft as the sheets "slip" easily.
- In carbon nanotubes two of the bonds are single, and one is double. The carbon atoms, in this case, bond to only three other carbon atoms, and are arranged in a hexagonal pattern. This combination is known as an sp2 bonded carbon atom. Carbon nanotubes are similar to individual graphene sheets in that each carbon atom bonds with three other carbon atoms but the sheet is rolled up onto itself to form a tube. In the "Build a Giant Carbon Nanotube" model we focused on the structure and angle of the bonds (and did not introduce the complexity of double vs. single bonds).
Types of molecular models
All physical molecular models are a simplification of the very complex real world, and each has their own limitations.
The "Build a Giant Carbon Nanotube" model is a simple ball-and-stick type of model, representing how carbon atoms can form into a carbon nanotube. This type of ball-and-stick model design requires a specific number of holes at specified angles. Given the constraints of this model, visitors are not able to create a diamond structure, which would need four holes at different angles. In the "Build a Giant Carbon Nanotube" model we focused on the structure and angle of the bonds (and did not introduce the complexity of double vs. single bonds).
Commercially available chemistry molecular model sets have several different approaches to this issue, including providing model atoms with different numbers of holes and angles, or flexible materials for bonds such as springs and tubing rather than stiff materials representing bonds.
More on different forms of carbon
We have several NISE Network programs that go into more details regarding the different forms and structures of carbon:
Forms of Carbon Cart Demo (including Forms of Carbon poster): http://www.nisenet.org/catalog/programs/forms_carbon
Nanotube Models tabletop program: http://www.nisenet.org/catalog/programs/nanotube_models
Exploring Structures – Bucky Balls: http://www.nisenet.org/catalog/programs/exploring_structures_-_buckyballs_nanodays_08_09_10
Balloon Nanotubes (Giant hanging model & tabletop version): http://www.nisenet.org/catalog/programs/balloon_nanotubes_giant_hanging_model
World of Carbon Nanotubes program and presentation: http://www.nisenet.org/catalog/programs/world_carbon_nanotubes