Silicon differs from silica by a group of ?

Silicon (Si) and silica (SiO₂) are chemically related but structurally distinct substances. The primary difference between them lies in the presence of hydroxyl (–OH) groups attached to silicon atoms. Silica is a fully oxidized compound of silicon, while in various silicon derivatives or hydrated forms, Si–OH bonds (also known as silanol groups) are present, making these compounds chemically more reactive.

Silica (SiO₂) has a three-dimensional network structure in which each silicon atom is covalently bonded to four oxygen atoms in a tetrahedral arrangement, forming a continuous Si–O–Si framework. This extended covalent lattice results in silica being a hard, brittle, and chemically inert solid with a very high melting point. It exists naturally as quartz, sand, or opal.

In contrast, silicon compounds that contain Si–OH groups (for example, silicic acids or hydrated silica) are less stable and more reactive. These hydroxyl groups terminate the continuous Si–O–Si network and impart different chemical properties, such as solubility in water and the ability to form esters or condensation products. For example, orthosilicic acid (H₄SiO₄) and metasilicic acid (H₂SiO₃) contain such hydroxyl groups attached to silicon.

The presence of –OH groups allows silicon compounds to undergo condensation reactions, forming polymers like silicones (–Si–O–Si– chains) and various silicates. Thus, hydroxyl substitution significantly alters both the physical and chemical characteristics of silicon compounds compared to pure silica.

In summary, silicon differs from silica by a hydroxyl (–OH) group, as silica represents the oxidized, anhydrous form of silicon, while hydroxylated derivatives represent hydrated or modified forms. Hence, the correct answer is (B) OH.