Which of the following is an example of a giant covalent structure? (A) Water, (B) sodium chloride, (C) diamond, (D) iron, or (E) carbon dioxide.
A covalent or molecular substance is one in which the atoms are bonded together by sharing a pair of electrons. Each atom in the bond donates an electron into the bonding pair between them. And the positively charged nuclei of the atoms are held together electrostatically by the negatively charged bonding pair in the metal.
Generally, nonmetal atoms bond covalently and share bonding electrons. Usually, covalent substances exist as small individual molecules such as water and carbon dioxide. But some covalent substances form giant structures. A classic example of a giant covalent structure is how the carbon in diamond bonds together. Each carbon atom can form four bonds to other carbon atoms, and each of these four atoms can also form four bonds with other carbons.
To draw what this looks like in reality is really quite challenging. This diagram shows some of the bonds to carbon atoms, but in reality every carbon atom will have four bonds to other carbon atoms, filling out in three dimensions into a giant covalent network. Because of the many covalent bonds present, giant covalent structures are very strong. We have seen that diamond is a classic example of a giant covalent structure. Two other common examples are graphite and silicon dioxide.
Generally, when metals and nonmetals bond, covalent bonds are not formed because electrons are not shared between the atoms. In the example of sodium chloride with sodium as the metal and chlorine the nonmetal, we see that the metal forms a positively charged cation and the nonmetal a negatively charged anion. These are held together by a strong electrostatic force, and we call this type of bond an ionic bond. In metals rather than covalent bonds or ionic bonds, rather, a sea of delocalized electrons holds the positively charged nuclei together electrostatically. This type of structure contains metallic bonds.
Because we are asked about giant covalent structures, we can rule out the ionic bond in sodium chloride and the metallic bond in iron. We can also rule out water and carbon dioxide because although they contain covalent bonds, these substances are not examples of giant covalent structures. Finally, the correct example given of a giant covalent structure is diamond.