Structure and Properties: Allotropes
adapted from Chemistry in the Community
Allotropes are two or more forms of an element that have distinctly different
physical or chemical properties. The forms must be in the same phase. How
do chemists account for the different properties of allotropes? The explanation
lies in how the atoms of the element are linked and organized – that
is, in the structure of the composition.
Visit the listed web sites and use the structural models to answer the questions
about some properties of carbon allotropes. The first question is answered
for you. Please read the question and answer as an example and clues to how
to respond to the remaining questions. Use Question 4 - which has an answer
provided to guide you in responding to the others.
1. Name three elements that exist as different allotropes.
2. How is does allotropy differ from phase?
3. Which form of carbon is the most stable of the allotropes?
4. What feature of diamond’s structure may account for it property as
a hard rigid substance, one that can scratch most other materials?
Please view the structure from one of the websites above and note how many
different atoms each carbon in the crystal is connected to.
Notice the every carbon atom in the interior of
the diamond model is linked to four other carbon atoms by individual chemical
bonds. These bonds have the effect of holding each carbon atom in place in
a rigid three-dimensional structure. Each carbon-carbon linkage in diamond
is a covalent bond involving the sharing of a pair of electrons. Thus ‘bending’
or ‘denting’ a diamond crystal would be extremely difficult because
it would involve breaking a network of strong chemical bonds that locks each
carbon atom in a fixed position. This accounts for diamond’s rigidity
and hardness.
2. How might the structure of diamond help explain why it is sometimes found in the form of large, single crystals?
3. What feature of graphite’s structure might account for its usefulness as a lubricant? Look at the structure of both diamond and graphite for comparison.
4. Why are fullerenes ‘powdery’ as solids rather than composed of large-scale chunks like diamonds?
5. Describe the structure of the fullerene class known as buckyballs with a comparison to an everyday object.
6. Chemists have demonstrated that it is possible to place an atom of another element inside. What are some of the practical application for ‘carrying’ atoms of another element inside fullerene molecules being considered? http://www.science.org.au/nova/024/024box02.htm