Which of the following is not a characteristic of a crystalline solid?

Isotropic nature

The correct answer is option 2. Isotropic nature.

Let us delve into each option and discuss why option 2, "Isotropic nature," is not a characteristic of a crystalline solid.

1. Definite and characteristic heat of fusion:
Crystalline solids do indeed have a definite and characteristic heat of fusion. The heat of fusion is the amount of heat energy required to change a substance from a solid to a liquid state at its melting point. This is a characteristic property of crystalline solids.

2. Isotropic nature:
Isotropic materials exhibit the same physical properties in all directions. In the context of crystalline solids, this is not true. Crystalline solids are often anisotropic, meaning their properties vary with direction. For example, the electrical and thermal conductivity, as well as mechanical properties, may differ along different crystallographic directions.

3. A regular periodically repeated pattern of arrangement of constituent particles in the entire crystal:
This is a fundamental characteristic of crystalline solids. In a crystalline solid, the constituent particles (atoms, ions, or molecules) are arranged in a highly ordered and repeating pattern throughout the crystal lattice. This regular arrangement gives rise to the characteristic properties of crystalline solids.

4. True solid:
Crystalline solids are indeed considered true solids. They have a well-defined geometric shape, a definite volume, and a regular structure. This is in contrast to amorphous solids, which lack a long-range order in their atomic arrangement.

In summary, the correct answer is "2. Isotropic nature" because crystalline solids typically exhibit anisotropic properties, contrary to the definition of isotropy. The other options describe characteristics that are consistent with crystalline solids.