1. Allotropes are each of two or more different physical forms in which an element can exist.
2. Tin and oxygen.
3. A. The first sample is a black solid that feels soft and bit "greasy" to the touch and leaves black marks when you rub it on paper. The second sample is colorless and glasslike (solid). It leaves deep scratch makes when rubbed on a table. The third sample is a fine, powdery solid that is made up of the roundest-shaped molecules. It's been found in prehistoric layers of Earth's crust.
B. The first substance is a useful lubricant and a fairly good conductor of electricity. The second sample is useful as an abrasive and as a coating for saw blades. It's also an electrical insulator, which means that it does not conduct electricity. The third sample was only discovered in 1985.
C. The first sample in one gram sells for less than a penny. The second sample, depending on its quality, can cost $50 per gram or more more than $20,000 per gram. The third sample, currently, costs between $170 and $3000 per gram, depending on the chemical supplier and the amount purchased at one time.
4. Natural materials are mined from the ground and nothing is changed from it, but engineered materials are physically altered.
5. Advantages are hardness and rigidness, but disadvantages are resistances to wear and low chemical reactivity.
6. Polyethylene can be tailored to display relatively soft and pliable properties or crafted to be hard and brittle, almost glasslike in its behavior.
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