Provide the principal industrial source of nitrogen and another element in Group VII. Describe the structure and bonding of CO2 and SiO2.

Nitrogen is a major component of the Earth's atmosphere. Industrially, it is obtained from the air.
The principal industrial source of nitrogen is $\boxed{\text{air}}$.

Group VII elements (halogens) include Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), and Astatine (At). The question asks for an element other than iodine, bromine, and astatine.
Another element in Group VII is $\boxed{\text{Chlorine}}$. (Fluorine is also a correct answer).

Carbon dioxide ($\text{CO}_2$) is a simple molecular substance. It consists of discrete molecules where carbon atoms are covalently bonded to oxygen atoms. The forces between these individual $\text{CO}_2$ molecules are weak intermolecular forces (van der Waals forces).

Silicon dioxide ($\text{SiO}_2$) has a giant covalent structure (also known as a macromolecular structure). In this structure, each silicon atom is covalently bonded to four oxygen atoms, and each oxygen atom is covalently bonded to two silicon atoms, forming a continuous, extensive network of strong covalent bonds.

Due to the weak intermolecular forces between $\text{CO}_2$ molecules, very little energy is required to overcome them, which is why carbon dioxide has a low melting point and exists as a gas at room temperature. In contrast, the giant covalent structure of $\text{SiO}_2$ requires a large amount of energy to break the numerous strong covalent bonds throughout the entire network, resulting in a very high melting point and its existence as a solid.