Giant covalent structure determines key properties

Steps:

• Recognize silicon dioxide forms a giant covalent lattice with each silicon bonded to four oxygens.
• Note that strong covalent bonds extend throughout the entire structure.
• Understand that melting requires breaking these numerous bonds, demanding high energy.
• Conclude the dominant property is high melting point, unlike small molecules.

Why B is correct:

• Giant covalent structures have high melting points because extensive networks of strong covalent bonds must be overcome to disrupt the lattice.

Why the others are wrong:

• A: No mobile electrons or ions in covalent network, so poor electricity conductor.
• C: Silicon dioxide is unreactive with dilute acids like hydrochloric acid due to strong Si-O bonds.
• D: Insoluble in water as polar water molecules cannot disrupt the non-polar covalent lattice.

Final answer: B