The melting point of `AlF_(3)` is `104^(@)`C and that of `SiF_(4)` is `-77^(@)`C (it sublimes) because:

Options

A. there is a very large difference in the ionic character of the Al-F and Si-F bonds
B. in `AlF_(3), Al^(3+)` interacts very strongly with the neighbouring `F^(-)` ions to give a three dimenstional structure but in `SiF_(4)` no such interaction is possible
C. the silicon ion in the tetrahedral `SiF_(4)` is not shielded effectively from the flouride ions whereas in ` AlF_(3)`, the `Al^(3+)` ion is shielded on all sides
D. the attractive forces between the `SiF_(4)` molecules are strong whereas those between the `AlF_(3)` molecules are weak

Answer

To explain the difference in melting points between AlF3 and SiF4, we can break down the reasoning into clear steps:

### Step-by-Step Solution:

1. **Identify the Nature of Compounds**:
- AlF3 (Aluminum Fluoride) is an ionic compound.
- SiF4 (Silicon Tetrafluoride) is a covalent compound.

2. **Analyze the Structure of AlF3**:
- AlF3 consists of Al3+ cations and F- anions.
- The Al3+ ion has a high charge density and can form strong electrostatic interactions with the F- ions.
- Al3+ has vacant d-orbitals, allowing it to coordinate with multiple F- ions, typically forming a three-dimensional lattice structure.

3. **Electrostatic Forces in AlF3**:
- The strong electrostatic forces between the Al3+ and F- ions result in a stable and rigid three-dimensional structure.
- This strong ionic bonding leads to a high melting point of 104°C.

4. **Analyze the Structure of SiF4**:
- SiF4 is composed of Si4+ cations and F- anions, but it primarily exhibits covalent bonding.
- The bonding in SiF4 is characterized by covalent bonds between silicon and fluorine, leading to a molecular structure.

5. **Weak Intermolecular Forces in SiF4**:
- In SiF4, the interactions between the molecules are primarily Van der Waals forces, which are much weaker than ionic bonds.
- As a result, SiF4 does not form a strong lattice structure, leading to a low melting point of -77°C.

6. **Conclusion**:
- The significant difference in melting points is due to the nature of bonding and the resulting structural arrangements in AlF3 and SiF4.
- AlF3 has strong ionic interactions leading to a high melting point, while SiF4 has weak Van der Waals forces resulting in a low melting point.

### Final Answer:
The melting point of AlF3 is 104°C due to strong ionic interactions in a three-dimensional structure, whereas SiF4 has a melting point of -77°C because it is a covalent compound with weak Van der Waals forces.

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