Final answer:
Carbon in the form of diamond and silicon from the first 20 elements have a covalent network structure. These materials are characterized by a vast 3D network of strong covalent bonds, contributing to their high melting points and hardness.
Step-by-step explanation:
Among the elements from 1 to 20, the ones that contain a covalent network structure are carbon (in the form of diamond) and silicon. Covalent network solids are materials in which atoms are bonded in a large, interconnected network.
A notable characteristic of these solids is the extended three-dimensional network of covalent bonds that hold the atoms together. For example, diamond is incredibly hard and has a high melting point because of its network structure, which consists of carbon atoms each covalently bonded to four other carbon atoms in a tetrahedral array.