Which among the following solids in an example of ionic solid?

\(NaCl, MgO, ZnS\)

The correct answer is option 2. \(NaCl, MgO, ZnS\).

Let us look into the explanations for each category of solids mentioned in the options:

1. Metals \((Fe, Cu, Ag, Mg)\):

Metals are characterized by having delocalized electrons that are free to move throughout the solid structure, resulting in high electrical conductivity. They typically have metallic bonding, where metal cations are surrounded by a "sea" of delocalized electrons. They do not form ionic solids.

2. Ionic Solids \((NaCl, MgO, ZnS)\):

Ionic solids consist of positively charged metal cations and negatively charged nonmetal anions held together by electrostatic forces of attraction. In \(NaCl\) (sodium chloride), \(MgO\) (magnesium oxide), and \(ZnS\) (zinc sulfide), each cation is attracted to several surrounding anions, and vice versa, resulting in a three-dimensional lattice structure. They typically have high melting points and are brittle in nature.

3. Molecular Solids \((Ar, CCl_4)\):

Molecular solids are composed of individual molecules held together by intermolecular forces such as London dispersion forces, dipole-dipole interactions, or hydrogen bonding. In \(Ar\) (argon), individual argon atoms form a solid due to weak van der Waals forces. \(CCl_4\) (carbon tetrachloride) consists of discrete \(CCl_4\) molecules held together by van der Waals forces.

4. Covalent Network Solids \((SiO_2, SiC)\):

Covalent network solids are composed of atoms held together by an extensive network of covalent bonds. In \(SiO_2\) (silicon dioxide) and \(SiC\) (silicon carbide), each atom is covalently bonded to several neighboring atoms, forming a three-dimensional network structure. These solids typically have very high melting points and are hard and brittle.

So, among the options provided, only option 2 (\(NaCl, MgO, ZnS\)) represents examples of ionic solids, where positively charged metal cations are attracted to negatively charged nonmetal anions in a lattice structure held together by electrostatic forces.