To determine which of the statements is not correct, we will analyze each statement one by one.
### Step 1: Analyze the first statement
**Statement 1:** The number of carbon atoms in a unit cell of diamond is 4.
- **Analysis:** Diamond has a zinc-blende structure. In this structure, there are 8 atoms in the unit cell, but when we consider the arrangement, we find that there are actually 4 carbon atoms per unit cell. Therefore, this statement is incorrect.
### Step 2: Analyze the second statement
**Statement 2:** The number of Bravais lattices in which crystals can be categorized is 14.
- **Analysis:** This statement is true. The 14 Bravais lattices are categorized into 7 crystal systems: cubic, orthorhombic, rhombohedral, monoclinic, triclinic, tetragonal, and hexagonal. Each system has specific types of lattices, confirming that there are indeed 14 Bravais lattices.
### Step 3: Analyze the third statement
**Statement 3:** The fraction of the total volume occupied by the atoms in the cubic primitive cell is 0.52.
- **Analysis:** This statement is also true. The volume fraction for different types of cubic lattices is as follows: Simple cubic (0.52), Body-centered cubic (BCC) (0.68), and Face-centered cubic (FCC) (0.74). Thus, this statement is correct.
### Step 4: Analyze the fourth statement
**Statement 4:** Molecular solids are generally volatile.
- **Analysis:** This statement is true. Molecular solids are held together by weak intermolecular forces such as dipole-dipole interactions and Van der Waals forces, which result in lower boiling points and higher volatility. Examples include H2O, HCl, and H2O2.
### Conclusion
After analyzing all the statements, we find that the first statement is incorrect. Therefore, the answer is:
**The correct answer is A.**
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