Answer:
Polythene (polyethene according the IUPAC nomenclature.)
Step-by-step explanation:
Consider the structure of each option:
- Polythene: long chains of atoms that are able to rotate along the bonding axis.
- Graphite: layers of (hexagonal) carbon sheets; each individual sheet is rigid (allows no rotation.)
- Silicon dioxide: three-dimensional (tetrahedral) network of carbon and oxygen atoms; the entire network very rigid (allows no rotation.)
- Diamond: three-dimensional (tetrahedral) network of carbon atoms; likewise, the entire tetrahedral network is very rigid.
Melting each structure requires overcoming the forces that hold the structure rigid:
- In polythene, van der Waal forces hold the chains together and prevents rotations.
- Deshaping graphite requires bending the layers; doing so would require overcoming the covalent bonds within the hexagonal sheets.
- In silicon dioxide and diamond, deshaping the tetrahedral network also requires overcoming covalent bonds.
Van der Waal forces are much easier to break than covalent bonds.
Hence, the melting point of polythene would be the lowest among the options.