The molecular formula of \(n\) -decane is \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{8} \mathrm{CH}_{3}\). Decane is not considered a polymer, whereas polyethylene is. What is the distinction?

First, let's look at the molecular structure of both compounds: n-decane: The molecular formula of n-decane is \(CH_3(CH_2)_8CH_3\). This means that it has a single chain of 10 carbon atoms with the first and last carbons being linked to three hydrogen atoms, and all other carbon atoms linked to two hydrogen atoms. n-decane is an alkane with a fixed molecular structure. Polyethylene: Polyethylene has a repeating unit of \( -CH_2-\) within its structure. While the molecular formula for polyethylene may be written as \((-CH_2-)_n\), its molecular structure consists of long chains of carbon and hydrogen atoms. The subscript 'n' indicates that the number of repeating units in the chain can vary, so polyethylene can have a wide range of molecular weights depending on the number of repeating units.

A polymer is a large molecule, or macromolecule, composed of multiple smaller units called monomers. Polymers can be linear, branched, or cross-linked, and their molecular weights can vary by orders of magnitude. The key feature of a polymer is that it is made up of repeating units, which can be either identical or similar in structure.

Now let's compare the differences between n-decane and polyethylene: n-decane: 1. It has a fixed molecular structure. 2. It is not made up of repeating units. 3. Its molecular weight is fixed and relatively small compared to polymers. Polyethylene: 1. It has a variable molecular structure. 2. It is composed of repeating units \((-CH_2-)_n\). 3. Its molecular weight can vary depending on the number of repeating units, making it much larger than that of n-decane.

The distinction between n-decane and polyethylene is that n-decane has a fixed molecular structure, while polyethylene is a polymer made up of repeating units. Polymers, like polyethylene, are characterized by having large and variable molecular weights, while n-decane is a smaller molecule with a fixed molecular weight. This is why n-decane is not considered a polymer, while polyethylene is.