In which of the following pairs, both the crystals are not of the same type ?

Options

A. Ice , Solid `CO_2`
B. NaCl , BaO
C. Mg,Ar
D. SiC, diamond

Answer

To determine which pair of crystals is not of the same type, we will analyze each pair based on their bonding types and structures.

### Step-by-Step Solution:

1. **Analyze Pair 1: Ice and Solid CO2**
- Ice (H2O) is a molecular solid where water molecules are held together by hydrogen bonds. This is a type of covalent solid.
- Solid CO2 (dry ice) consists of carbon dioxide molecules held together by London dispersion forces. While CO2 is also a molecular solid, the bonding is weaker compared to the hydrogen bonding in ice.
- **Conclusion:** Both are molecular solids, but they have different types of intermolecular forces.

2. **Analyze Pair 2: NaCl and BaO**
- NaCl (sodium chloride) is an ionic solid composed of Na+ and Cl- ions held together by strong electrostatic forces.
- BaO (barium oxide) is also an ionic solid composed of Ba2+ and O2- ions, similarly held together by ionic bonds.
- **Conclusion:** Both are ionic solids and have the same type of bonding.

3. **Analyze Pair 3: Magnesium and Argon**
- Magnesium is a metal, and it exhibits metallic bonding, characterized by a sea of delocalized electrons.
- Argon is a noble gas and forms a solid through weak van der Waals forces, which are very different from metallic bonding.
- **Conclusion:** They have different types of bonding; magnesium is metallic, while argon is a van der Waals solid.

4. **Analyze Pair 4: SiC and Diamond**
- SiC (silicon carbide) has a covalent network structure where silicon and carbon atoms are bonded covalently in a tetrahedral arrangement.
- Diamond is also a covalent network solid, composed entirely of carbon atoms arranged in a tetrahedral lattice.
- **Conclusion:** Both SiC and diamond are covalent solids with similar bonding types.

### Final Answer:
The pair in which both crystals are not of the same type is **Magnesium and Argon (Option C)**.